ASP.NET Core Razor components
Note
This isn't the latest version of this article. For the current release, see the .NET 8 version of this article.
Warning
This version of ASP.NET Core is no longer supported. For more information, see .NET and .NET Core Support Policy. For the current release, see the .NET 8 version of this article.
Important
This information relates to a pre-release product that may be substantially modified before it's commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
For the current release, see the .NET 8 version of this article.
This article explains how to create and use Razor components in Blazor apps, including guidance on Razor syntax, component naming, namespaces, and component parameters.
Razor components
Blazor apps are built using Razor components, informally known as Blazor components or only components. A component is a self-contained portion of user interface (UI) with processing logic to enable dynamic behavior. Components can be nested, reused, shared among projects, and used in MVC and Razor Pages apps.
Components render into an in-memory representation of the browser's Document Object Model (DOM) called a render tree, which is used to update the UI in a flexible and efficient way.
Although "Razor components" shares some naming with other ASP.NET Core content-rendering technologies, Razor components must be distinguished from the following different features in ASP.NET Core:
- Razor views, which are Razor-based markup pages for MVC apps.
- View components, which are for rendering chunks of content rather than whole responses in Razor Pages and MVC apps.
Important
When using a Blazor Web App, most of the Blazor documentation example components require interactivity to function and demonstrate the concepts covered by the articles. When you test an example component provided by an article, make sure that either the app adopts global interactivity or the component adopts an interactive render mode. More information on this subject is provided by ASP.NET Core Blazor render modes, which is the next article in the table of contents after this article.
Component classes
Components are implemented using a combination of C# and HTML markup in Razor component files with the .razor
file extension.
By default, ComponentBase is the base class for components described by Razor component files. ComponentBase implements the lowest abstraction of components, the IComponent interface. ComponentBase defines component properties and methods for basic functionality, for example, to process a set of built-in component lifecycle events.
ComponentBase
in dotnet/aspnetcore
reference source: The reference source contains additional remarks on the built-in lifecycle events. However, keep in mind that the internal implementations of component features are subject to change at any time without notice.
Note
Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).
Developers typically create Razor components from Razor component files (.razor
) or base their components on ComponentBase, but components can also be built by implementing IComponent. Developer-built components that implement IComponent can take low-level control over rendering at the cost of having to manually trigger rendering with events and lifecycle methods that the developer must create and maintain.
Razor syntax
Components use Razor syntax. Two Razor features are extensively used by components, directives and directive attributes. These are reserved keywords prefixed with @
that appear in Razor markup:
Directives: Change the way component markup is compiled or functions. For example, the
@page
directive specifies a routable component with a route template that can be reached directly by a user's request in the browser at a specific URL.By convention, a component's directives at the top of a component definition (
.razor
file) are placed in a consistent order. For repeated directives, directives are placed alphabetically by namespace or type, except@using
directives, which have special second-level ordering.The following order is adopted by Blazor sample apps and documentation. Components provided by a Blazor project template may differ from the following order and use a different format. For example, Blazor framework Identity components include blank lines between blocks of
@using
directives and blocks of@inject
directives. You're free to use a custom ordering scheme and format in your own apps.Documentation and sample app Razor directive order:
@page
@rendermode
(.NET 8 or later)@using
System
namespaces (alphabetical order)Microsoft
namespaces (alphabetical order)- Third-party API namespaces (alphabetical order)
- App namespaces (alphabetical order)
- Other directives (alphabetical order)
No blank lines appear among the directives. One blank line appears between the directives and the first line of Razor markup.
Example:
@page "/doctor-who-episodes/{season:int}" @rendermode InteractiveWebAssembly @using System.Globalization @using System.Text.Json @using Microsoft.AspNetCore.Localization @using Mandrill @using BlazorSample.Components.Layout @attribute [Authorize] @implements IAsyncDisposable @inject IJSRuntime JS @inject ILogger<DoctorWhoEpisodes> Logger <PageTitle>Doctor Who Episode List</PageTitle> ...
Directive attributes: Change the way a component element is compiled or functions.
Example:
<input @bind="episodeId" />
You can prefix directive attribute values with the at symbol (
@
) for non-explicit Razor expressions (@bind="@episodeId"
), but we don't recommend it, and the docs don't adopt the approach in examples.
Directives and directive attributes used in components are explained further in this article and other articles of the Blazor documentation set. For general information on Razor syntax, see Razor syntax reference for ASP.NET Core.
Component name, class name, and namespace
A component's name must start with an uppercase character:
Supported: ProductDetail.razor
Unsupported: productDetail.razor
Common Blazor naming conventions used throughout the Blazor documentation include:
- File paths and file names use Pascal case† and appear before showing code examples. If a path is present, it indicates the typical folder location. For example,
Components/Pages/ProductDetail.razor
indicates that theProductDetail
component has a file name ofProductDetail.razor
and resides in thePages
folder of theComponents
folder of the app. - Component file paths for routable components match their URLs in kebab case‡ with hyphens appearing between words in a component's route template. For example, a
ProductDetail
component with a route template of/product-detail
(@page "/product-detail"
) is requested in a browser at the relative URL/product-detail
.
†Pascal case (upper camel case) is a naming convention without spaces and punctuation and with the first letter of each word capitalized, including the first word.
‡Kebab case is a naming convention without spaces and punctuation that uses lowercase letters and dashes between words.
Components are ordinary C# classes and can be placed anywhere within a project. Components that produce webpages usually reside in the Components/Pages
folder. Non-page components are frequently placed in the Components
folder or a custom folder added to the project.
Typically, a component's namespace is derived from the app's root namespace and the component's location (folder) within the app. If the app's root namespace is BlazorSample
and the Counter
component resides in the Components/Pages
folder:
- The
Counter
component's namespace isBlazorSample.Components.Pages
. - The fully qualified type name of the component is
BlazorSample.Components.Pages.Counter
.
For custom folders that hold components, add an @using
directive to the parent component or to the app's _Imports.razor
file. The following example makes components in the AdminComponents
folder available:
@using BlazorSample.AdminComponents
Note
@using
directives in the _Imports.razor
file are only applied to Razor files (.razor
), not C# files (.cs
).
Aliased using
statements are supported. In the following example, the public WeatherForecast
class of the GridRendering
component is made available as WeatherForecast
in a component elsewhere in the app:
@using WeatherForecast = Components.Pages.GridRendering.WeatherForecast
Components can also be referenced using their fully qualified names, which doesn't require an @using
directive. The following example directly references the ProductDetail
component in the AdminComponents/Pages
folder of the app:
<BlazorSample.AdminComponents.Pages.ProductDetail />
The namespace of a component authored with Razor is based on the following (in priority order):
- The
@namespace
directive in the Razor file's markup (for example,@namespace BlazorSample.CustomNamespace
). - The project's
RootNamespace
in the project file (for example,<RootNamespace>BlazorSample</RootNamespace>
). - The project namespace and the path from the project root to the component. For example, the framework resolves
{PROJECT NAMESPACE}/Components/Pages/Home.razor
with a project namespace ofBlazorSample
to the namespaceBlazorSample.Components.Pages
for theHome
component.{PROJECT NAMESPACE}
is the project namespace. Components follow C# name binding rules. For theHome
component in this example, the components in scope are all of the components:- In the same folder,
Components/Pages
. - The components in the project's root that don't explicitly specify a different namespace.
- In the same folder,
The following are not supported:
- The
global::
qualification. - Partially-qualified names. For example, you can't add
@using BlazorSample.Components
to a component and then reference theNavMenu
component in the app'sComponents/Layout
folder (Components/Layout/NavMenu.razor
) with<Layout.NavMenu></Layout.NavMenu>
.
A component's name must start with an uppercase character:
Supported: ProductDetail.razor
Unsupported: productDetail.razor
Common Blazor naming conventions used throughout the Blazor documentation include:
- File paths and file names use Pascal case† and appear before showing code examples. If a path is present, it indicates the typical folder location. For example,
Pages/ProductDetail.razor
indicates that theProductDetail
component has a file name ofProductDetail.razor
and resides in thePages
folder of the app. - Component file paths for routable components match their URLs in kebab case‡ with hyphens appearing between words in a component's route template. For example, a
ProductDetail
component with a route template of/product-detail
(@page "/product-detail"
) is requested in a browser at the relative URL/product-detail
.
†Pascal case (upper camel case) is a naming convention without spaces and punctuation and with the first letter of each word capitalized, including the first word.
‡Kebab case is a naming convention without spaces and punctuation that uses lowercase letters and dashes between words.
Components are ordinary C# classes and can be placed anywhere within a project. Components that produce webpages usually reside in the Pages
folder. Non-page components are frequently placed in the Shared
folder or a custom folder added to the project.
Typically, a component's namespace is derived from the app's root namespace and the component's location (folder) within the app. If the app's root namespace is BlazorSample
and the Counter
component resides in the Pages
folder:
- The
Counter
component's namespace isBlazorSample.Pages
. - The fully qualified type name of the component is
BlazorSample.Pages.Counter
.
For custom folders that hold components, add an @using
directive to the parent component or to the app's _Imports.razor
file. The following example makes components in the AdminComponents
folder available:
@using BlazorSample.AdminComponents
Note
@using
directives in the _Imports.razor
file are only applied to Razor files (.razor
), not C# files (.cs
).
Aliased using
statements are supported. In the following example, the public WeatherForecast
class of the GridRendering
component is made available as WeatherForecast
in a component elsewhere in the app:
@using WeatherForecast = Pages.GridRendering.WeatherForecast
Components can also be referenced using their fully qualified names, which doesn't require an @using
directive. The following example directly references the ProductDetail
component in the Components
folder of the app:
<BlazorSample.Components.ProductDetail />
The namespace of a component authored with Razor is based on the following (in priority order):
- The
@namespace
directive in the Razor file's markup (for example,@namespace BlazorSample.CustomNamespace
). - The project's
RootNamespace
in the project file (for example,<RootNamespace>BlazorSample</RootNamespace>
). - The project namespace and the path from the project root to the component. For example, the framework resolves
{PROJECT NAMESPACE}/Pages/Index.razor
with a project namespace ofBlazorSample
to the namespaceBlazorSample.Pages
for theIndex
component.{PROJECT NAMESPACE}
is the project namespace. Components follow C# name binding rules. For theIndex
component in this example, the components in scope are all of the components:- In the same folder,
Pages
. - The components in the project's root that don't explicitly specify a different namespace.
- In the same folder,
The following are not supported:
- The
global::
qualification. - Partially-qualified names. For example, you can't add
@using BlazorSample
to a component and then reference theNavMenu
component in the app'sShared
folder (Shared/NavMenu.razor
) with<Shared.NavMenu></Shared.NavMenu>
.
Partial class support
Components are generated as C# partial classes and are authored using either of the following approaches:
- A single file contains C# code defined in one or more
@code
blocks, HTML markup, and Razor markup. Blazor project templates define their components using this single-file approach. - HTML and Razor markup are placed in a Razor file (
.razor
). C# code is placed in a code-behind file defined as a partial class (.cs
).
Note
A component stylesheet that defines component-specific styles is a separate file (.css
). Blazor CSS isolation is described later in ASP.NET Core Blazor CSS isolation.
The following example shows the default Counter
component with an @code
block in an app generated from a Blazor project template. Markup and C# code are in the same file. This is the most common approach taken in component authoring.
Counter.razor
:
@page "/counter"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
@page "/counter"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
@page "/counter"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
@page "/counter"
<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
@page "/counter"
<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@code {
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
The following Counter
component splits presentation HTML and Razor markup from the C# code using a code-behind file with a partial class. Splitting the markup from the C# code is favored by some organizations and developers to organize their component code to suit how they prefer to work. For example, the organization's UI expert can work on the presentation layer independently of another developer working on the component's C# logic. The approach is also useful when working with automatically-generated code or source generators. For more information, see Partial Classes and Methods (C# Programming Guide).
CounterPartialClass.razor
:
@page "/counter-partial-class"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@page "/counter-partial-class"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@page "/counter-partial-class"
<PageTitle>Counter</PageTitle>
<h1>Counter</h1>
<p role="status">Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@page "/counter-partial-class"
<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
@page "/counter-partial-class"
<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
CounterPartialClass.razor.cs
:
namespace BlazorSample.Components.Pages;
public partial class CounterPartialClass
{
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
namespace BlazorSample.Pages;
public partial class CounterPartialClass
{
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
namespace BlazorSample.Pages
{
public partial class CounterPartialClass
{
private int currentCount = 0;
private void IncrementCount()
{
currentCount++;
}
}
}
@using
directives in the _Imports.razor
file are only applied to Razor files (.razor
), not C# files (.cs
). Add namespaces to a partial class file as needed.
Typical namespaces used by components:
using System.Net.Http;
using System.Net.Http.Json;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Components.Authorization;
using Microsoft.AspNetCore.Components.Forms;
using Microsoft.AspNetCore.Components.Routing;
using Microsoft.AspNetCore.Components.Sections
using Microsoft.AspNetCore.Components.Web;
using static Microsoft.AspNetCore.Components.Web.RenderMode;
using Microsoft.AspNetCore.Components.Web.Virtualization;
using Microsoft.JSInterop;
Typical namespaces also include the namespace of the app and the namespace corresponding to the app's Components
folder:
using BlazorSample;
using BlazorSample.Components;
Additional folders can also be included, such as the Layout
folder:
using BlazorSample.Components.Layout;
using System.Net.Http;
using System.Net.Http.Json;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Components.Authorization;
using Microsoft.AspNetCore.Components.Forms;
using Microsoft.AspNetCore.Components.Routing;
using Microsoft.AspNetCore.Components.Web;
using Microsoft.AspNetCore.Components.Web.Virtualization;
using Microsoft.JSInterop;
Typical namespaces also include the namespace of the app and the namespace corresponding to the app's Shared
folder:
using BlazorSample;
using BlazorSample.Shared;
using System.Net.Http;
using Microsoft.AspNetCore.Components.Forms;
using Microsoft.AspNetCore.Components.Routing;
using Microsoft.AspNetCore.Components.Web;
using Microsoft.JSInterop;
Typical namespaces also include the namespace of the app and the namespace corresponding to the app's Shared
folder:
using BlazorSample;
using BlazorSample.Shared;
Specify a base class
The @inherits
directive is used to specify a base class for a component. Unlike using partial classes, which only split markup from C# logic, using a base class allows you to inherit C# code for use across a group of components that share the base class's properties and methods. Using base classes reduce code redundancy in apps and are useful when supplying base code from class libraries to multiple apps. For more information, see Inheritance in C# and .NET.
In the following example, the BlazorRocksBase1
base class derives from ComponentBase.
BlazorRocks1.razor
:
@page "/blazor-rocks-1"
@inherits BlazorRocksBase1
<PageTitle>Blazor Rocks!</PageTitle>
<h1>Blazor Rocks! Example 1</h1>
<p>
@BlazorRocksText
</p>
@page "/blazor-rocks-1"
@inherits BlazorRocksBase1
<PageTitle>Blazor Rocks!</PageTitle>
<h1>Blazor Rocks! Example 1</h1>
<p>
@BlazorRocksText
</p>
@page "/blazor-rocks-1"
@inherits BlazorRocksBase1
<PageTitle>Blazor Rocks!</PageTitle>
<h1>Blazor Rocks! Example 1</h1>
<p>
@BlazorRocksText
</p>
@page "/blazor-rocks-1"
@inherits BlazorRocksBase1
<h1>Blazor Rocks! Example 1</h1>
<p>
@BlazorRocksText
</p>
@page "/blazor-rocks-1"
@inherits BlazorRocksBase1
<h1>Blazor Rocks! Example 1</h1>
<p>
@BlazorRocksText
</p>
BlazorRocksBase1.cs
:
using Microsoft.AspNetCore.Components;
namespace BlazorSample;
public class BlazorRocksBase1 : ComponentBase
{
public string BlazorRocksText { get; set; } =
"Blazor rocks the browser!";
}
using Microsoft.AspNetCore.Components;
namespace BlazorSample;
public class BlazorRocksBase1 : ComponentBase
{
public string BlazorRocksText { get; set; } =
"Blazor rocks the browser!";
}
using Microsoft.AspNetCore.Components;
namespace BlazorSample;
public class BlazorRocksBase1 : ComponentBase
{
public string BlazorRocksText { get; set; } =
"Blazor rocks the browser!";
}
using Microsoft.AspNetCore.Components;
namespace BlazorSample;
public class BlazorRocksBase1 : ComponentBase
{
public string BlazorRocksText { get; set; } =
"Blazor rocks the browser!";
}
using Microsoft.AspNetCore.Components;
namespace BlazorSample;
public class BlazorRocksBase1 : ComponentBase
{
public string BlazorRocksText { get; set; } =
"Blazor rocks the browser!";
}
Routing
Routing in Blazor is achieved by providing a route template to each accessible component in the app with an @page
directive. When a Razor file with an @page
directive is compiled, the generated class is given a RouteAttribute specifying the route template. At runtime, the router searches for component classes with a RouteAttribute and renders whichever component has a route template that matches the requested URL.
The following HelloWorld
component uses a route template of /hello-world
, and the rendered webpage for the component is reached at the relative URL /hello-world
.
HelloWorld.razor
:
@page "/hello-world"
<PageTitle>Hello World!</PageTitle>
<h1>Hello World!</h1>
@page "/hello-world"
<h1>Hello World!</h1>
@page "/hello-world"
<h1>Hello World!</h1>
@page "/hello-world"
<h1>Hello World!</h1>
@page "/hello-world"
<h1>Hello World!</h1>
The preceding component loads in the browser at /hello-world
regardless of whether or not you add the component to the app's UI navigation. Optionally, components can be added to the NavMenu
component so that a link to the component appears in the app's UI-based navigation.
For the preceding HelloWorld
component, you can add a NavLink
component to the NavMenu
component. For more information, including descriptions of the NavLink
and NavMenu
components, see ASP.NET Core Blazor routing and navigation.
Markup
A component's UI is defined using Razor syntax, which consists of Razor markup, C#, and HTML. When an app is compiled, the HTML markup and C# rendering logic are converted into a component class. The name of the generated class matches the name of the file.
Members of the component class are defined in one or more @code
blocks. In @code
blocks, component state is specified and processed with C#:
- Property and field initializers.
- Parameter values from arguments passed by parent components and route parameters.
- Methods for user event handling, lifecycle events, and custom component logic.
Component members are used in rendering logic using C# expressions that start with the @
symbol. For example, a C# field is rendered by prefixing @
to the field name. The following Markup
component evaluates and renders:
headingFontStyle
for the CSS property valuefont-style
of the heading element.headingText
for the content of the heading element.
Markup.razor
:
@page "/markup"
<PageTitle>Markup</PageTitle>
<h1>Markup Example</h1>
<h2 style="font-style:@headingFontStyle">@headingText</h2>
@code {
private string headingFontStyle = "italic";
private string headingText = "Put on your new Blazor!";
}
@page "/markup"
<h1 style="font-style:@headingFontStyle">@headingText</h1>
@code {
private string headingFontStyle = "italic";
private string headingText = "Put on your new Blazor!";
}
@page "/markup"
<h1 style="font-style:@headingFontStyle">@headingText</h1>
@code {
private string headingFontStyle = "italic";
private string headingText = "Put on your new Blazor!";
}
@page "/markup"
<h1 style="font-style:@headingFontStyle">@headingText</h1>
@code {
private string headingFontStyle = "italic";
private string headingText = "Put on your new Blazor!";
}
@page "/markup"
<h1 style="font-style:@headingFontStyle">@headingText</h1>
@code {
private string headingFontStyle = "italic";
private string headingText = "Put on your new Blazor!";
}
Note
Examples throughout the Blazor documentation specify the private
access modifier for private members. Private members are scoped to a component's class. However, C# assumes the private
access modifier when no access modifier is present, so explicitly marking members "private
" in your own code is optional. For more information on access modifiers, see Access Modifiers (C# Programming Guide).
The Blazor framework processes a component internally as a render tree, which is the combination of a component's DOM and Cascading Style Sheet Object Model (CSSOM). After the component is initially rendered, the component's render tree is regenerated in response to events. Blazor compares the new render tree against the previous render tree and applies any modifications to the browser's DOM for display. For more information, see ASP.NET Core Razor component rendering.
Razor syntax for C# control structures, directives, and directive attributes are lowercase (examples: @if
, @code
, @bind
). Property names are uppercase (example: @Body
for LayoutComponentBase.Body).
Asynchronous methods (async
) don't support returning void
The Blazor framework doesn't track void
-returning asynchronous methods (async
). As a result, exceptions aren't caught if void
is returned. Always return a Task from asynchronous methods.
Nested components
Components can include other components by declaring them using HTML syntax. The markup for using a component looks like an HTML tag where the name of the tag is the component type.
Consider the following Heading
component, which can be used by other components to display a heading.
Heading.razor
:
<h1 style="font-style:@headingFontStyle">Heading Example</h1>
@code {
private string headingFontStyle = "italic";
}
<h1 style="font-style:@headingFontStyle">Heading Example</h1>
@code {
private string headingFontStyle = "italic";
}
<h1 style="font-style:@headingFontStyle">Heading Example</h1>
@code {
private string headingFontStyle = "italic";
}
<h1 style="font-style:@headingFontStyle">Heading Example</h1>
@code {
private string headingFontStyle = "italic";
}
<h1 style="font-style:@headingFontStyle">Heading Example</h1>
@code {
private string headingFontStyle = "italic";
}
The following markup in the HeadingExample
component renders the preceding Heading
component at the location where the <Heading />
tag appears.
HeadingExample.razor
:
@page "/heading-example"
<PageTitle>Heading</PageTitle>
<h1>Heading Example</h1>
<Heading />
@page "/heading-example"
<Heading />
@page "/heading-example"
<Heading />
@page "/heading-example"
<Heading />
@page "/heading-example"
<Heading />
If a component contains an HTML element with an uppercase first letter that doesn't match a component name within the same namespace, a warning is emitted indicating that the element has an unexpected name. Adding an @using
directive for the component's namespace makes the component available, which resolves the warning. For more information, see the Component name, class name, and namespace section.
The Heading
component example shown in this section doesn't have an @page
directive, so the Heading
component isn't directly accessible to a user via a direct request in the browser. However, any component with an @page
directive can be nested in another component. If the Heading
component was directly accessible by including @page "/heading"
at the top of its Razor file, then the component would be rendered for browser requests at both /heading
and /heading-example
.
Component parameters
Component parameters pass data to components and are defined using public C# properties on the component class with the [Parameter]
attribute. In the following example, a built-in reference type (System.String) and a user-defined reference type (PanelBody
) are passed as component parameters.
PanelBody.cs
:
namespace BlazorSample;
public class PanelBody
{
public string? Text { get; set; }
public string? Style { get; set; }
}
public class PanelBody
{
public string? Text { get; set; }
public string? Style { get; set; }
}
public class PanelBody
{
public string? Text { get; set; }
public string? Style { get; set; }
}
public class PanelBody
{
public string Text { get; set; }
public string Style { get; set; }
}
public class PanelBody
{
public string Text { get; set; }
public string Style { get; set; }
}
ParameterChild.razor
:
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">@Title</div>
<div class="card-body" style="font-style:@Body.Style">
@Body.Text
</div>
</div>
@code {
[Parameter]
public string Title { get; set; } = "Set By Child";
[Parameter]
public PanelBody Body { get; set; } =
new()
{
Text = "Card content set by child.",
Style = "normal"
};
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">@Title</div>
<div class="card-body" style="font-style:@Body.Style">
@Body.Text
</div>
</div>
@code {
[Parameter]
public string Title { get; set; } = "Set By Child";
[Parameter]
public PanelBody Body { get; set; } =
new()
{
Text = "Set by child.",
Style = "normal"
};
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">@Title</div>
<div class="card-body" style="font-style:@Body.Style">
@Body.Text
</div>
</div>
@code {
[Parameter]
public string Title { get; set; } = "Set By Child";
[Parameter]
public PanelBody Body { get; set; } =
new()
{
Text = "Set by child.",
Style = "normal"
};
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">@Title</div>
<div class="card-body" style="font-style:@Body.Style">
@Body.Text
</div>
</div>
@code {
[Parameter]
public string Title { get; set; } = "Set By Child";
[Parameter]
public PanelBody Body { get; set; } =
new()
{
Text = "Set by child.",
Style = "normal"
};
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">@Title</div>
<div class="card-body" style="font-style:@Body.Style">
@Body.Text
</div>
</div>
@code {
[Parameter]
public string Title { get; set; } = "Set By Child";
[Parameter]
public PanelBody Body { get; set; } =
new PanelBody()
{
Text = "Set by child.",
Style = "normal"
};
}
Warning
Providing initial values for component parameters is supported, but don't create a component that writes to its own parameters after the component is rendered for the first time. For more information, see Avoid overwriting parameters in ASP.NET Core Blazor.
The Title
and Body
component parameters of the ParameterChild
component are set by arguments in the HTML tag that renders the instance of the component. The following ParameterParent
component renders two ParameterChild
components:
- The first
ParameterChild
component is rendered without supplying parameter arguments. - The second
ParameterChild
component receives values forTitle
andBody
from theParameterParent
component, which uses an explicit C# expression to set the values of thePanelBody
's properties.
Parameter1.razor
:
@page "/parameter-1"
<PageTitle>Parameter 1</PageTitle>
<h1>Parameter Example 1</h1>
<h1>Child component (without attribute values)</h1>
<ParameterChild />
<h1>Child component (with attribute values)</h1>
<ParameterChild Title="Set by Parent"
Body="@(new PanelBody() { Text = "Set by parent.", Style = "italic" })" />
ParameterParent.razor
:
@page "/parameter-parent"
<h1>Child component (without attribute values)</h1>
<ParameterChild />
<h1>Child component (with attribute values)</h1>
<ParameterChild Title="Set by Parent"
Body="@(new PanelBody() { Text = "Set by parent.", Style = "italic" })" />
ParameterParent.razor
:
@page "/parameter-parent"
<h1>Child component (without attribute values)</h1>
<ParameterChild />
<h1>Child component (with attribute values)</h1>
<ParameterChild Title="Set by Parent"
Body="@(new PanelBody() { Text = "Set by parent.", Style = "italic" })" />
ParameterParent.razor
:
@page "/parameter-parent"
<h1>Child component (without attribute values)</h1>
<ParameterChild />
<h1>Child component (with attribute values)</h1>
<ParameterChild Title="Set by Parent"
Body="@(new PanelBody() { Text = "Set by parent.", Style = "italic" })" />
ParameterParent.razor
:
@page "/parameter-parent"
<h1>Child component (without attribute values)</h1>
<ParameterChild />
<h1>Child component (with attribute values)</h1>
<ParameterChild Title="Set by Parent"
Body="@(new PanelBody() { Text = "Set by parent.", Style = "italic" })" />
The following rendered HTML markup from the ParameterParent
component shows ParameterChild
component default values when the ParameterParent
component doesn't supply component parameter values. When the ParameterParent
component provides component parameter values, they replace the ParameterChild
component's default values.
Note
For clarity, rendered CSS style classes aren't shown in the following rendered HTML markup.
<h1>Child component (without attribute values)</h1>
<div>
<div>Set By Child</div>
<div>Set by child.</div>
</div>
<h1>Child component (with attribute values)</h1>
<div>
<div>Set by Parent</div>
<div>Set by parent.</div>
</div>
Assign a C# field, property, or result of a method to a component parameter as an HTML attribute value. The value of the attribute can typically be any C# expression that matches the type of the parameter. The value of the attribute can optionally lead with a Razor reserved @
symbol, but it isn't required.
If the component parameter is of type string, then the attribute value is instead treated as a C# string literal by default. If you want to specify a C# expression instead, then use the @
prefix.
The following ParameterParent2
component displays four instances of the preceding ParameterChild
component and sets their Title
parameter values to:
- The value of the
title
field. - The result of the
GetTitle
C# method. - The current local date in long format with ToLongDateString, which uses an implicit C# expression.
- The
panelData
object'sTitle
property.
Quotes around parameter attribute values are optional in most cases per the HTML5 specification. For example, Value=this
is supported, instead of Value="this"
. However, we recommend using quotes because it's easier to remember and widely adopted across web-based technologies.
Throughout the documentation, code examples:
- Always use quotes. Example:
Value="this"
. - Don't use the
@
prefix with nonliterals unless required. Example:Count="ct"
, wherect
is a number-typed variable.Count="@ct"
is a valid stylistic approach, but the documentation and examples don't adopt the convention. - Always avoid
@
for literals, outside of Razor expressions. Example:IsFixed="true"
. This includes keywords (for example,this
) andnull
, but you can choose to use them if you wish. For example,IsFixed="@true"
is uncommon but supported.
Parameter2.razor
:
@page "/parameter-2"
<PageTitle>Parameter 2</PageTitle>
<h1>Parameter Example 2</h1>
<ParameterChild Title="@title" />
<ParameterChild Title="@GetTitle()" />
<ParameterChild Title="@DateTime.Now.ToLongDateString()" />
<ParameterChild Title="@panelData.Title" />
@code {
private string title = "From Parent field";
private PanelData panelData = new();
private string GetTitle()
{
return "From Parent method";
}
private class PanelData
{
public string Title { get; set; } = "From Parent object";
}
}
ParameterParent2.razor
:
@page "/parameter-parent-2"
<ParameterChild Title="@title" />
<ParameterChild Title="@GetTitle()" />
<ParameterChild Title="@DateTime.Now.ToLongDateString()" />
<ParameterChild Title="@panelData.Title" />
@code {
private string title = "From Parent field";
private PanelData panelData = new();
private string GetTitle()
{
return "From Parent method";
}
private class PanelData
{
public string Title { get; set; } = "From Parent object";
}
}
ParameterParent2.razor
:
@page "/parameter-parent-2"
<ParameterChild Title="@title" />
<ParameterChild Title="@GetTitle()" />
<ParameterChild Title="@DateTime.Now.ToLongDateString()" />
<ParameterChild Title="@panelData.Title" />
@code {
private string title = "From Parent field";
private PanelData panelData = new();
private string GetTitle()
{
return "From Parent method";
}
private class PanelData
{
public string Title { get; set; } = "From Parent object";
}
}
ParameterParent2.razor
:
@page "/parameter-parent-2"
<ParameterChild Title="@title" />
<ParameterChild Title="@GetTitle()" />
<ParameterChild Title="@DateTime.Now.ToLongDateString()" />
<ParameterChild Title="@panelData.Title" />
@code {
private string title = "From Parent field";
private PanelData panelData = new PanelData();
private string GetTitle()
{
return "From Parent method";
}
private class PanelData
{
public string Title { get; set; } = "From Parent object";
}
}
Note
When assigning a C# member to a component parameter, don't prefix the parameter's HTML attribute with @
.
Correct (Title
is a string parameter, Count
is a number-typed parameter):
<ParameterChild Title="@title" Count="ct" />
<ParameterChild Title="@title" Count="@ct" />
Incorrect:
<ParameterChild @Title="@title" @Count="ct" />
<ParameterChild @Title="@title" @Count="@ct" />
Unlike in Razor pages (.cshtml
), Blazor can't perform asynchronous work in a Razor expression while rendering a component. This is because Blazor is designed for rendering interactive UIs. In an interactive UI, the screen must always display something, so it doesn't make sense to block the rendering flow. Instead, asynchronous work is performed during one of the asynchronous lifecycle events. After each asynchronous lifecycle event, the component may render again. The following Razor syntax is not supported:
<ParameterChild Title="await ..." />
<ParameterChild Title="@await ..." />
The code in the preceding example generates a compiler error when the app is built:
The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'.
To obtain a value for the Title
parameter in the preceding example asynchronously, the component can use the OnInitializedAsync
lifecycle event, as the following example demonstrates:
<ParameterChild Title="@title" />
@code {
private string? title;
protected override async Task OnInitializedAsync()
{
title = await ...;
}
}
For more information, see ASP.NET Core Razor component lifecycle.
Use of an explicit Razor expression to concatenate text with an expression result for assignment to a parameter is not supported. The following example seeks to concatenate the text "Set by
" with an object's property value. Although this syntax is supported in a Razor page (.cshtml
), it isn't valid for assignment to the child's Title
parameter in a component. The following Razor syntax is not supported:
<ParameterChild Title="Set by @(panelData.Title)" />
The code in the preceding example generates a compiler error when the app is built:
Component attributes do not support complex content (mixed C# and markup).
To support the assignment of a composed value, use a method, field, or property. The following example performs the concatenation of "Set by
" and an object's property value in the C# method GetTitle
:
Parameter3.razor
:
@page "/parameter-3"
<PageTitle>Parameter 3</PageTitle>
<h1>Parameter Example 3</h1>
<ParameterChild Title="@GetTitle()" />
@code {
private PanelData panelData = new();
private string GetTitle() => $"Set by {panelData.Title}";
private class PanelData
{
public string Title { get; set; } = "Parent";
}
}
ParameterParent3.razor
:
@page "/parameter-parent-3"
<ParameterChild Title="@GetTitle()" />
@code {
private PanelData panelData = new();
private string GetTitle() => $"Set by {panelData.Title}";
private class PanelData
{
public string Title { get; set; } = "Parent";
}
}
ParameterParent3.razor
:
@page "/parameter-parent-3"
<ParameterChild Title="@GetTitle()" />
@code {
private PanelData panelData = new();
private string GetTitle() => $"Set by {panelData.Title}";
private class PanelData
{
public string Title { get; set; } = "Parent";
}
}
ParameterParent3.razor
:
@page "/parameter-parent-3"
<ParameterChild Title="@GetTitle()" />
@code {
private PanelData panelData = new();
private string GetTitle() => $"Set by {panelData.Title}";
private class PanelData
{
public string Title { get; set; } = "Parent";
}
}
ParameterParent3.razor
:
@page "/parameter-parent-3"
<ParameterChild Title="@GetTitle()" />
@code {
private PanelData panelData = new PanelData();
private string GetTitle() => $"Set by {panelData.Title}";
private class PanelData
{
public string Title { get; set; } = "Parent";
}
}
For more information, see Razor syntax reference for ASP.NET Core.
Warning
Providing initial values for component parameters is supported, but don't create a component that writes to its own parameters after the component is rendered for the first time. For more information, see Avoid overwriting parameters in ASP.NET Core Blazor.
Component parameters should be declared as auto-properties, meaning that they shouldn't contain custom logic in their get
or set
accessors. For example, the following StartData
property is an auto-property:
[Parameter]
public DateTime StartData { get; set; }
Don't place custom logic in the get
or set
accessor because component parameters are purely intended for use as a channel for a parent component to flow information to a child component. If a set
accessor of a child component property contains logic that causes rerendering of the parent component, an infinite rendering loop results.
To transform a received parameter value:
- Leave the parameter property as an auto-property to represent the supplied raw data.
- Create a different property or method to supply the transformed data based on the parameter property.
Override OnParametersSetAsync
to transform a received parameter each time new data is received.
Writing an initial value to a component parameter is supported because initial value assignments don't interfere with the Blazor's automatic component rendering. The following assignment of the current local DateTime with DateTime.Now to StartData
is valid syntax in a component:
[Parameter]
public DateTime StartData { get; set; } = DateTime.Now;
After the initial assignment of DateTime.Now, do not assign a value to StartData
in developer code. For more information, see Avoid overwriting parameters in ASP.NET Core Blazor.
Apply the [EditorRequired]
attribute to specify a required component parameter. If a parameter value isn't provided, editors or build tools may display warnings to the user. This attribute is only valid on properties also marked with the [Parameter]
attribute. The EditorRequiredAttribute is enforced at design-time and when the app is built. The attribute isn't enforced at runtime, and it doesn't guarantee a non-null
parameter value.
[Parameter]
[EditorRequired]
public string? Title { get; set; }
Single-line attribute lists are also supported:
[Parameter, EditorRequired]
public string? Title { get; set; }
Don't use the required
modifier or init
accessor on component parameter properties. Components are usually instantiated and assigned parameter values using reflection, which bypasses the guarantees that init
and required
are designed to make. Instead, use the [EditorRequired]
attribute to specify a required component parameter.
Don't use the init
accessor on component parameter properties because setting component parameter values with ParameterView.SetParameterProperties uses reflection, which bypasses the init-only setter restriction. Use the [EditorRequired]
attribute to specify a required component parameter.
Don't use the init
accessor on component parameter properties because setting component parameter values with ParameterView.SetParameterProperties uses reflection, which bypasses the init-only setter restriction.
Tuples
(API documentation) are supported for component parameters and RenderFragment
types. The following component parameter example passes three values in a Tuple
:
RenderTupleChild.razor
:
<div class="card w-50" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Tuple Card</div>
<div class="card-body">
<ul>
<li>Integer: @Data?.Item1</li>
<li>String: @Data?.Item2</li>
<li>Boolean: @Data?.Item3</li>
</ul>
</div>
</div>
@code {
[Parameter]
public (int, string, bool)? Data { get; set; }
}
RenderTupleParent.razor
:
@page "/render-tuple-parent"
<PageTitle>Render Tuple Parent</PageTitle>
<h1>Render Tuple Parent Example</h1>
<RenderTupleChild Data="data" />
@code {
private (int, string, bool) data = new(999, "I aim to misbehave.", true);
}
Named tuples are supported, as seen in the following example:
NamedTupleChild.razor
:
<div class="card w-50" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Tuple Card</div>
<div class="card-body">
<ul>
<li>Integer: @Data?.TheInteger</li>
<li>String: @Data?.TheString</li>
<li>Boolean: @Data?.TheBoolean</li>
</ul>
</div>
</div>
@code {
[Parameter]
public (int TheInteger, string TheString, bool TheBoolean)? Data { get; set; }
}
NamedTuples.razor
:
@page "/named-tuples"
<PageTitle>Named Tuples</PageTitle>
<h1>Named Tuples Example</h1>
<NamedTupleChild Data="data" />
@code {
private (int TheInteger, string TheString, bool TheBoolean) data =
new(999, "I aim to misbehave.", true);
}
Quote ©2005 Universal Pictures: Serenity (Nathan Fillion)
Tuples
(API documentation) are supported for component parameters and RenderFragment
types. The following component parameter example passes three values in a Tuple
:
RenderTupleChild.razor
:
<div class="card w-50" style="margin-bottom:15px">
<div class="card-header font-weight-bold"><code>Tuple</code> Card</div>
<div class="card-body">
<ul>
<li>Integer: @Data?.Item1</li>
<li>String: @Data?.Item2</li>
<li>Boolean: @Data?.Item3</li>
</ul>
</div>
</div>
@code {
[Parameter]
public (int, string, bool)? Data { get; set; }
}
RenderTupleParent.razor
:
@page "/render-tuple-parent"
<h1>Render Tuple Parent</h1>
<RenderTupleChild Data="data" />
@code {
private (int, string, bool) data = new(999, "I aim to misbehave.", true);
}
Named tuples are supported, as seen in the following example:
RenderNamedTupleChild.razor
:
<div class="card w-50" style="margin-bottom:15px">
<div class="card-header font-weight-bold"><code>Tuple</code> Card</div>
<div class="card-body">
<ul>
<li>Integer: @Data?.TheInteger</li>
<li>String: @Data?.TheString</li>
<li>Boolean: @Data?.TheBoolean</li>
</ul>
</div>
</div>
@code {
[Parameter]
public (int TheInteger, string TheString, bool TheBoolean)? Data { get; set; }
}
RenderNamedTupleParent.razor
:
@page "/render-named-tuple-parent"
<h1>Render Named Tuple Parent</h1>
<RenderNamedTupleChild Data="data" />
@code {
private (int TheInteger, string TheString, bool TheBoolean) data =
new(999, "I aim to misbehave.", true);
}
Quote ©2005 Universal Pictures: Serenity (Nathan Fillion)
Route parameters
Components can specify route parameters in the route template of the @page
directive. The Blazor router uses route parameters to populate corresponding component parameters.
RouteParameter1.razor
:
@page "/route-parameter-1/{text}"
<PageTitle>Route Parameter 1</PageTitle>
<h1>Route Parameter Example 1</h1>
<p>Blazor is @Text!</p>
@code {
[Parameter]
public string? Text { get; set; }
}
@page "/route-parameter-1/{text}"
<h1>Blazor is @Text!</h1>
@code {
[Parameter]
public string? Text { get; set; }
}
@page "/route-parameter-1/{text}"
<h1>Blazor is @Text!</h1>
@code {
[Parameter]
public string? Text { get; set; }
}
@page "/route-parameter-1/{text}"
<h1>Blazor is @Text!</h1>
@code {
[Parameter]
public string Text { get; set; }
}
@page "/route-parameter-1/{text}"
<h1>Blazor is @Text!</h1>
@code {
[Parameter]
public string Text { get; set; }
}
For more information, see the Route parameters section of ASP.NET Core Blazor routing and navigation. Optional route parameters are also supported and covered in the same section. For information on catch-all route parameters ({*pageRoute}
), which capture paths across multiple folder boundaries, see the Catch-all route parameters section of ASP.NET Core Blazor routing and navigation.
For more information, see the Route parameters section of ASP.NET Core Blazor routing and navigation. Optional route parameters aren't supported, so two @page
directives are required (see the Route parameters section for more information). For information on catch-all route parameters ({*pageRoute}
), which capture paths across multiple folder boundaries, see the Catch-all route parameters section of ASP.NET Core Blazor routing and navigation.
Warning
With compression, which is enabled by default, avoid creating secure (authenticated/authorized) interactive server-side components that render data from untrusted sources. Untrusted sources include route parameters, query strings, data from JS interop, and any other source of data that a third-party user can control (databases, external services). For more information, see ASP.NET Core Blazor SignalR guidance and Threat mitigation guidance for ASP.NET Core Blazor interactive server-side rendering.
Child content render fragments
Components can set the content of another component. The assigning component provides the content between the child component's opening and closing tags.
In the following example, the RenderFragmentChild
component has a ChildContent
component parameter that represents a segment of the UI to render as a RenderFragment. The position of ChildContent
in the component's Razor markup is where the content is rendered in the final HTML output.
RenderFragmentChild.razor
:
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Child content</div>
<div class="card-body">@ChildContent</div>
</div>
@code {
[Parameter]
public RenderFragment? ChildContent { get; set; }
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Child content</div>
<div class="card-body">@ChildContent</div>
</div>
@code {
[Parameter]
public RenderFragment? ChildContent { get; set; }
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Child content</div>
<div class="card-body">@ChildContent</div>
</div>
@code {
[Parameter]
public RenderFragment? ChildContent { get; set; }
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Child content</div>
<div class="card-body">@ChildContent</div>
</div>
@code {
[Parameter]
public RenderFragment ChildContent { get; set; }
}
<div class="card w-25" style="margin-bottom:15px">
<div class="card-header font-weight-bold">Child content</div>
<div class="card-body">@ChildContent</div>
</div>
@code {
[Parameter]
public RenderFragment ChildContent { get; set; }
}
Important
The property receiving the RenderFragment content must be named ChildContent
by convention.
Event callbacks aren't supported for RenderFragment.
The following component provides content for rendering the RenderFragmentChild
by placing the content inside the child component's opening and closing tags.
RenderFragments.razor
:
@page "/render-fragments"
<PageTitle>Render Fragments</PageTitle>
<h1>Render Fragments Example</h1>
<RenderFragmentChild>
Content of the child component is supplied
by the parent component.
</RenderFragmentChild>
RenderFragmentParent.razor
:
@page "/render-fragment-parent"
<h1>Render child content</h1>
<RenderFragmentChild>
Content of the child component is supplied
by the parent component.
</RenderFragmentChild>
RenderFragmentParent.razor
:
@page "/render-fragment-parent"
<h1>Render child content</h1>
<RenderFragmentChild>
Content of the child component is supplied
by the parent component.
</RenderFragmentChild>
RenderFragmentParent.razor
:
@page "/render-fragment-parent"
<h1>Render child content</h1>
<RenderFragmentChild>
Content of the child component is supplied
by the parent component.
</RenderFragmentChild>
RenderFragmentParent.razor
:
@page "/render-fragment-parent"
<h1>Render child content</h1>
<RenderFragmentChild>
Content of the child component is supplied
by the parent component.
</RenderFragmentChild>
Due to the way that Blazor renders child content, rendering components inside a for
loop requires a local index variable if the incrementing loop variable is used in the RenderFragmentChild
component's content. The following example can be added to the preceding parent component:
<h1>Three children with an index variable</h1>
@for (int c = 0; c < 3; c++)
{
var current = c;
<RenderFragmentChild>
Count: @current
</RenderFragmentChild>
}
Alternatively, use a foreach
loop with Enumerable.Range instead of a for
loop. The following example can be added to the preceding parent component:
<h1>Second example of three children with an index variable</h1>
@foreach (var c in Enumerable.Range(0,3))
{
<RenderFragmentChild>
Count: @c
</RenderFragmentChild>
}
Render fragments are used to render child content throughout Blazor apps and are described with examples in the following articles and article sections:
- Blazor layouts
- Pass data across a component hierarchy
- Templated components
- Global exception handling
Note
Blazor framework's built-in Razor components use the same ChildContent
component parameter convention to set their content. You can see the components that set child content by searching for the component parameter property name ChildContent
in the API documentation (filters API with the search term "ChildContent").
Render fragments for reusable rendering logic
You can factor out child components purely as a way of reusing rendering logic. In any component's @code
block, define a RenderFragment and render the fragment from any location as many times as needed:
@RenderWelcomeInfo
<p>Render the welcome info a second time:</p>
@RenderWelcomeInfo
@code {
private RenderFragment RenderWelcomeInfo = @<p>Welcome to your new app!</p>;
}
For more information, see Reuse rendering logic.
Capture references to components
Component references provide a way to reference a component instance for issuing commands. To capture a component reference:
- Add an
@ref
attribute to the child component. - Define a field with the same type as the child component.
When the component is rendered, the field is populated with the component instance. You can then invoke .NET methods on the instance.
Consider the following ReferenceChild
component that logs a message when its ChildMethod
is called.
ReferenceChild.razor
:
@inject ILogger<ReferenceChild> Logger
@if (value > 0)
{
<p>
<code>value</code>: @value
</p>
}
@code {
private int value;
public void ChildMethod(int value)
{
Logger.LogInformation("Received {Value} in ChildMethod", value);
this.value = value;
StateHasChanged();
}
}
@using Microsoft.Extensions.Logging
@inject ILogger<ReferenceChild> Logger
@code {
public void ChildMethod(int value)
{
Logger.LogInformation("Received {Value} in ChildMethod", value);
}
}
@using Microsoft.Extensions.Logging
@inject ILogger<ReferenceChild> Logger
@code {
public void ChildMethod(int value)
{
Logger.LogInformation("Received {Value} in ChildMethod", value);
}
}
@using Microsoft.Extensions.Logging
@inject ILogger<ReferenceChild> Logger
@code {
public void ChildMethod(int value)
{
Logger.LogInformation("Received {Value} in ChildMethod", value);
}
}
@using Microsoft.Extensions.Logging
@inject ILogger<ReferenceChild> Logger
@code {
public void ChildMethod(int value)
{
Logger.LogInformation("Received {Value} in ChildMethod", value);
}
}
A component reference is only populated after the component is rendered and its output includes ReferenceChild
's element. Until the component is rendered, there's nothing to reference. Don't attempt to call a referenced component method to an event handler directly (for example, @onclick="childComponent!.ChildMethod(5)"
) because the reference variable may not be assigned at the time the click event is assigned.
To manipulate component references after the component has finished rendering, use the OnAfterRender
or OnAfterRenderAsync
methods.
The following example uses the preceding ReferenceChild
component.
ReferenceParent.razor
:
@page "/reference-parent"
<div>
<button @onclick="@(() => childComponent1!.ChildMethod(5))">
Call <code>ReferenceChild.ChildMethod</code> (first instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent1" />
</div>
<div>
<button @onclick="CallChildMethod">
Call <code>ReferenceChild.ChildMethod</code> (second instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent2" />
</div>
@code {
private ReferenceChild? childComponent1;
private ReferenceChild? childComponent2;
private void CallChildMethod()
{
childComponent2!.ChildMethod(5);
}
}
@page "/reference-parent"
<div>
<button @onclick="@(() => childComponent1!.ChildMethod(5))">
Call <code>ReferenceChild.ChildMethod</code> (first instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent1" />
</div>
<div>
<button @onclick="CallChildMethod">
Call <code>ReferenceChild.ChildMethod</code> (second instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent2" />
</div>
@code {
private ReferenceChild? childComponent1;
private ReferenceChild? childComponent2;
private void CallChildMethod()
{
childComponent2!.ChildMethod(5);
}
}
@page "/reference-parent"
<div>
<button @onclick="@(() => childComponent1!.ChildMethod(5))">
Call <code>ReferenceChild.ChildMethod</code> (first instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent1" />
</div>
<div>
<button @onclick="CallChildMethod">
Call <code>ReferenceChild.ChildMethod</code> (second instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent2" />
</div>
@code {
private ReferenceChild? childComponent1;
private ReferenceChild? childComponent2;
private void CallChildMethod()
{
childComponent2!.ChildMethod(5);
}
}
@page "/reference-parent"
<div>
<button @onclick="@(() => childComponent1!.ChildMethod(5))">
Call <code>ReferenceChild.ChildMethod</code> (first instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent1" />
</div>
<div>
<button @onclick="CallChildMethod">
Call <code>ReferenceChild.ChildMethod</code> (second instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent2" />
</div>
@code {
private ReferenceChild childComponent1;
private ReferenceChild childComponent2;
private void CallChildMethod()
{
childComponent2!.ChildMethod(5);
}
}
@page "/reference-parent"
<div>
<button @onclick="@(() => childComponent1!.ChildMethod(5))">
Call <code>ReferenceChild.ChildMethod</code> (first instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent1" />
</div>
<div>
<button @onclick="CallChildMethod">
Call <code>ReferenceChild.ChildMethod</code> (second instance)
with an argument of 5
</button>
<ReferenceChild @ref="childComponent2" />
</div>
@code {
private ReferenceChild childComponent1;
private ReferenceChild childComponent2;
private void CallChildMethod()
{
childComponent2!.ChildMethod(5);
}
}
While capturing component references use a similar syntax to capturing element references, capturing component references isn't a JavaScript interop feature. Component references aren't passed to JavaScript code. Component references are only used in .NET code.
Important
Do not use component references to mutate the state of child components. Instead, use normal declarative component parameters to pass data to child components. Use of component parameters result in child components that rerender at the correct times automatically. For more information, see the component parameters section and the ASP.NET Core Blazor data binding article.
Apply an attribute
Attributes can be applied to components with the @attribute
directive. The following example applies the [Authorize]
attribute to the component's class:
@page "/"
@attribute [Authorize]
Conditional HTML element attributes and DOM properties
Blazor adopts the following general behaviors:
- For HTML attributes, Blazor sets or removes the attribute conditionally based on the .NET value. If the .NET value is
false
ornull
, the attribute isn't set or is removed if it was previously set. - For DOM properties, such as
checked
orvalue
, Blazor sets the DOM property based on the .NET value. If the .NET value isfalse
ornull
, the DOM property is reset to a default value.
Which Razor syntax attributes correspond to HTML attributes and which ones correspond to DOM properties remains undocumented because it's an implementation detail that might change without notice.
Warning
Some HTML attributes, such as aria-pressed
, must have a string value of either "true" or "false". Since they require a string value and not a boolean, you must use a .NET string
and not a bool
for their value. This is a requirement set by browser DOM APIs.
Raw HTML
Strings are normally rendered using DOM text nodes, which means that any markup they may contain is ignored and treated as literal text. To render raw HTML, wrap the HTML content in a MarkupString value. The value is parsed as HTML or SVG and inserted into the DOM.
Warning
Rendering raw HTML constructed from any untrusted source is a security risk and should always be avoided.
The following example shows using the MarkupString type to add a block of static HTML content to the rendered output of a component.
MarkupStrings.razor
:
@page "/markup-strings"
<PageTitle>Markup Strings</PageTitle>
<h1>Markup Strings Example</h1>
@((MarkupString)myMarkup)
@code {
private string myMarkup =
"<p class=\"text-danger\">This is a dangerous <em>markup string</em>.</p>";
}
MarkupStringExample.razor
:
@page "/markup-string-example"
@((MarkupString)myMarkup)
@code {
private string myMarkup =
"<p class=\"text-danger\">This is a dangerous <em>markup string</em>.</p>";
}
MarkupStringExample.razor
:
@page "/markup-string-example"
@((MarkupString)myMarkup)
@code {
private string myMarkup =
"<p class=\"text-danger\">This is a dangerous <em>markup string</em>.</p>";
}
MarkupStringExample.razor
:
@page "/markup-string-example"
@((MarkupString)myMarkup)
@code {
private string myMarkup =
"<p class=\"text-danger\">This is a dangerous <em>markup string</em>.</p>";
}
MarkupStringExample.razor
:
@page "/markup-string-example"
@((MarkupString)myMarkup)
@code {
private string myMarkup =
"<p class=\"text-danger\">This is a dangerous <em>markup string</em>.</p>";
}
Razor templates
Render fragments can be defined using Razor template syntax to define a UI snippet. Razor templates use the following format:
@<{HTML tag}>...</{HTML tag}>
The following example illustrates how to specify RenderFragment and RenderFragment<TValue> values and render templates directly in a component. Render fragments can also be passed as arguments to templated components.
RazorTemplate.razor
:
@page "/razor-template"
<PageTitle>Razor Template</PageTitle>
<h1>Razor Template Example</h1>
@timeTemplate
@petTemplate(new Pet { Name = "Nutty Rex" })
@code {
private RenderFragment timeTemplate = @<p>The time is @DateTime.Now.</p>;
private RenderFragment<Pet> petTemplate = (pet) => @<p>Pet: @pet.Name</p>;
private class Pet
{
public string? Name { get; set; }
}
}
@page "/razor-template"
@timeTemplate
@petTemplate(new Pet { Name = "Nutty Rex" })
@code {
private RenderFragment timeTemplate = @<p>The time is @DateTime.Now.</p>;
private RenderFragment<Pet> petTemplate = (pet) => @<p>Pet: @pet.Name</p>;
private class Pet
{
public string? Name { get; set; }
}
}
@page "/razor-template"
@timeTemplate
@petTemplate(new Pet { Name = "Nutty Rex" })
@code {
private RenderFragment timeTemplate = @<p>The time is @DateTime.Now.</p>;
private RenderFragment<Pet> petTemplate = (pet) => @<p>Pet: @pet.Name</p>;
private class Pet
{
public string? Name { get; set; }
}
}
@page "/razor-template"
@timeTemplate
@petTemplate(new Pet { Name = "Nutty Rex" })
@code {
private RenderFragment timeTemplate = @<p>The time is @DateTime.Now.</p>;
private RenderFragment<Pet> petTemplate = (pet) => @<p>Pet: @pet.Name</p>;
private class Pet
{
public string Name { get; set; }
}
}
@page "/razor-template"
@timeTemplate
@petTemplate(new Pet { Name = "Nutty Rex" })
@code {
private RenderFragment timeTemplate = @<p>The time is @DateTime.Now.</p>;
private RenderFragment<Pet> petTemplate = (pet) => @<p>Pet: @pet.Name</p>;
private class Pet
{
public string Name { get; set; }
}
}
Rendered output of the preceding code:
<p>The time is 4/19/2021 8:54:46 AM.</p>
<p>Pet: Nutty Rex</p>
Static assets
Blazor follows the convention of ASP.NET Core apps for static assets. Static assets are located in the project's web root
(wwwroot
) folder or folders under the wwwroot
folder.
Use a base-relative path (/
) to refer to the web root for a static asset. In the following example, logo.png
is physically located in the {PROJECT ROOT}/wwwroot/images
folder. {PROJECT ROOT}
is the app's project root.
<img alt="Company logo" src="/images/logo.png" />
Components do not support tilde-slash notation (~/
).
For information on setting an app's base path, see Host and deploy ASP.NET Core Blazor.
Tag Helpers aren't supported in components
Tag Helpers
aren't supported in components. To provide Tag Helper-like functionality in Blazor, create a component with the same functionality as the Tag Helper and use the component instead.
Scalable Vector Graphics (SVG) images
Since Blazor renders HTML, browser-supported images, including Scalable Vector Graphics (SVG) images (.svg
), are supported via the <img>
tag:
<img alt="Example image" src="image.svg" />
Similarly, SVG images are supported in the CSS rules of a stylesheet file (.css
):
.element-class {
background-image: url("image.svg");
}
Blazor supports the <foreignObject>
element to display arbitrary HTML within an SVG. The markup can represent arbitrary HTML, a RenderFragment, or a Razor component.
The following example demonstrates:
- Display of a
string
(@message
). - Two-way binding with an
<input>
element and avalue
field. - A
Robot
component.
<svg width="200" height="200" xmlns="http://www.w3.org/2000/svg">
<rect x="0" y="0" rx="10" ry="10" width="200" height="200" stroke="black"
fill="none" />
<foreignObject x="20" y="20" width="160" height="160">
<p>@message</p>
</foreignObject>
</svg>
<svg xmlns="http://www.w3.org/2000/svg">
<foreignObject width="200" height="200">
<label>
Two-way binding:
<input @bind="value" @bind:event="oninput" />
</label>
</foreignObject>
</svg>
<svg xmlns="http://www.w3.org/2000/svg">
<foreignObject>
<Robot />
</foreignObject>
</svg>
@code {
private string message = "Lorem ipsum dolor sit amet, consectetur adipiscing " +
"elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.";
private string? value;
}
Whitespace rendering behavior
Unless the @preservewhitespace
directive is used with a value of true
, extra whitespace is removed by default if:
- Leading or trailing within an element.
- Leading or trailing within a RenderFragment/RenderFragment<TValue> parameter (for example, child content passed to another component).
- It precedes or follows a C# code block, such as
@if
or@foreach
.
Whitespace removal might affect the rendered output when using a CSS rule, such as white-space: pre
. To disable this performance optimization and preserve the whitespace, take one of the following actions:
- Add the
@preservewhitespace true
directive at the top of the Razor file (.razor
) to apply the preference to a specific component. - Add the
@preservewhitespace true
directive inside an_Imports.razor
file to apply the preference to a subdirectory or to the entire project.
In most cases, no action is required, as apps typically continue to behave normally (but faster). If stripping whitespace causes a rendering problem for a particular component, use @preservewhitespace true
in that component to disable this optimization.
Whitespace is retained in a component's source markup. Whitespace-only text renders in the browser's DOM even when there's no visual effect.
Consider the following component markup:
<ul>
@foreach (var item in Items)
{
<li>
@item.Text
</li>
}
</ul>
The preceding example renders the following unnecessary whitespace:
- Outside of the
@foreach
code block. - Around the
<li>
element. - Around the
@item.Text
output.
A list of 100 items results in over 400 areas of whitespace. None of the extra whitespace visually affects the rendered output.
When rendering static HTML for components, whitespace inside a tag isn't preserved. For example, view the rendered output of the following <img>
tag in a component Razor file (.razor
):
<img alt="Example image" src="img.png" />
Whitespace isn't preserved from the preceding markup:
<img alt="Example image" src="img.png" />
Root component
A root Razor component (root component) is the first component loaded of any component hierarchy created by the app.
In an app created from the Blazor Web App project template, the App
component (App.razor
) is specified as the default root component by the type parameter declared for the call to MapRazorComponents<TRootComponent>
in the server-side Program
file. The following example shows the use of the App
component as the root component, which is the default for an app created from the Blazor project template:
app.MapRazorComponents<App>();
Note
Making a root component interactive, such as the App
component, isn't supported.
In an app created from the Blazor Server project template, the App
component (App.razor
) is specified as the default root component in Pages/_Host.cshtml
using the Component Tag Helper:
<component type="typeof(App)" render-mode="ServerPrerendered" />
In an app created from the Blazor WebAssembly project template, the App
component (App.razor
) is specified as the default root component in the Program
file:
builder.RootComponents.Add<App>("#app");
In the preceding code, the CSS selector, #app
, indicates that the App
component is specified for the <div>
in wwwroot/index.html
with an id
of app
:
<div id="app">...</app>
MVC and Razor Pages apps can also use the Component Tag Helper to register statically-rendered Blazor WebAssembly root components:
<component type="typeof(App)" render-mode="WebAssemblyPrerendered" />
Statically-rendered components can only be added to the app. They can't be removed or updated afterwards.
For more information, see the following resources: