ASP.NET Core Blazor layouts

This article explains how to create reusable layout components for Blazor apps.

Usefulness of Blazor layouts

Some app elements, such as menus, copyright messages, and company logos, are usually part of app's overall presentation. Placing a copy of the markup for these elements into all of the components of an app isn't efficient. Every time that one of these elements is updated, every component that uses the element must be updated. This approach is costly to maintain and can lead to inconsistent content if an update is missed. Layouts solve these problems.

A Blazor layout is a Razor component that shares markup with components that reference it. Layouts can use data binding, dependency injection, and other features of components.

Layout components

Create a layout component

To create a layout component:

• Create a Razor component defined by a Razor template or C# code. Layout components based on a Razor template use the .razor file extension just like ordinary Razor components. Because layout components are shared across an app's components, they're usually placed in the app's Shared or Layout folder. However, layouts can be placed in any location accessible to the components that use it. For example, a layout can be placed in the same folder as the components that use it.
• Inherit the component from LayoutComponentBase. The LayoutComponentBase defines a Body property (RenderFragment type) for the rendered content inside the layout.
• Use the Razor syntax @Body to specify the location in the layout markup where the content is rendered.

Note

For more information on RenderFragment, see ASP.NET Core Razor components.

The following DoctorWhoLayout component shows the Razor template of a layout component. The layout inherits LayoutComponentBase and sets the @Body between the navigation bar (<nav>...</nav>) and the footer (<footer>...</footer>).

DoctorWhoLayout.razor:

@inherits LayoutComponentBase

<PageTitle>Doctor Who® Database</PageTitle>

<header>
    <h1>Doctor Who® Database</h1>
</header>

<nav>
    <a href="main-list">Main Episode List</a>
    <a href="search">Search</a>
    <a href="new">Add Episode</a>
</nav>

@Body

<footer>
    @TrademarkMessage
</footer>

@code {
    public string TrademarkMessage { get; set; } =
        "Doctor Who is a registered trademark of the BBC. " +
        "https://www.doctorwho.tv/ https://www.bbc.com";
}

MainLayout component

In an app created from a Blazor project template, the MainLayout component is the app's default layout. Blazor's layout adopts the Flexbox layout model (MDN documentation) (W3C specification).

Blazor's CSS isolation feature applies isolated CSS styles to the MainLayout component. By convention, the styles are provided by the accompanying stylesheet of the same name, MainLayout.razor.css. The ASP.NET Core framework implementation of the stylesheet is available for inspection in the ASP.NET Core reference source (dotnet/aspnetcore GitHub repository):

• Blazor Web App MainLayout.razor.css
• Blazor WebAssembly MainLayout.razor.css

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).

Apply a layout

Make the layout namespace available

Layout file locations and namespaces changed over time for the Blazor framework. Depending on the version of Blazor and type of Blazor app that you're building, you may need to indicate the layout's namespace when using it. When referencing a layout implementation and the layout isn't found without indicating the layout's namespace, take any of the following approaches:

• Add an @using directive to the _Imports.razor file for the location of the layouts. In the following example, a folder of layouts with the name Layout is inside a Components folder, and the app's namespace is BlazorSample:

  @using BlazorSample.Components.Layout
  

• Add an @using directive at the top of the component definition where the layout is used:

  @using BlazorSample.Components.Layout
  @layout DoctorWhoLayout
  

• Fully qualify the namespace of the layout where it's used:

  @layout BlazorSample.Components.Layout.DoctorWhoLayout
  

Apply a layout to a component

Use the @layout Razor directive to apply a layout to a routable Razor component that has an @page directive. The compiler converts @layout into a LayoutAttribute and applies the attribute to the component class.

The content of the following Episodes component is inserted into the DoctorWhoLayout at the position of @Body.

Episodes.razor:

@page "/episodes"
@layout DoctorWhoLayout

<h2>Doctor Who® Episodes</h2>

<ul>
    <li>
        <a href="https://www.bbc.co.uk/programmes/p00vfknq">
            <em>The Ribos Operation</em>
        </a>
    </li>
    <li>
        <a href="https://www.bbc.co.uk/programmes/p00vfdsb">
            <em>The Sunmakers</em>
        </a>
    </li>
    <li>
        <a href="https://www.bbc.co.uk/programmes/p00vhc26">
            <em>Nightmare of Eden</em>
        </a>
    </li>
</ul>

The following rendered HTML markup is produced by the preceding DoctorWhoLayout and Episodes component. Extraneous markup doesn't appear in order to focus on the content provided by the two components involved:

• The H1 "database" heading (<h1>...</h1>) in the header (<header>...</header>), navigation bar (<nav>...</nav>), and trademark information in the footer (<footer>...</footer>) come from the DoctorWhoLayout component.
• The H2 "episodes" heading (<h2>...</h2>) and episode list (<ul>...</ul>) come from the Episodes component.

<header>
    <h1 ...>...</h1>
</header>

<nav>
    ...
</nav>

<h2>...</h2>

<ul>
    <li>...</li>
    <li>...</li>
    <li>...</li>
</ul>

<footer>
    ...
</footer>

Specifying the layout directly in a component overrides a default layout:

• Set by an @layout directive imported from an _Imports.razor file, as described in the following Apply a layout to a folder of components section.
• Set as the app's default layout, as described in the Apply a default layout to an app section later in this article.

Apply a layout to a folder of components

Every folder of an app can optionally contain a template file named _Imports.razor. The compiler includes the directives specified in the imports file in all of the Razor templates in the same folder and recursively in all of its subfolders. Therefore, an _Imports.razor file containing @layout DoctorWhoLayout ensures that all of the components in a folder use the DoctorWhoLayout component. There's no need to repeatedly add @layout DoctorWhoLayout to all of the Razor components (.razor) within the folder and subfolders.

_Imports.razor:

@layout DoctorWhoLayout
...

The _Imports.razor file is similar to the _ViewImports.cshtml file for Razor views and pages but applied specifically to Razor component files.

Specifying a layout in _Imports.razor overrides a layout specified as the router's default app layout, which is described in the following section.

Warning

Do not add a Razor @layout directive to the root _Imports.razor file, which results in an infinite loop of layouts. To control the default app layout, specify the layout in the Router component. For more information, see the following Apply a default layout to an app section.

Note

The @layout Razor directive only applies a layout to routable Razor components with an @page directive.

Apply a default layout to an app

Specify the default app layout in the Router component's RouteView component. Use the DefaultLayout parameter to set the layout type:

<RouteView RouteData="routeData" DefaultLayout="typeof({LAYOUT})" />

In the preceding example, the {LAYOUT} placeholder is the layout (for example, DoctorWhoLayout if the layout file name is DoctorWhoLayout.razor). You may need to idenfity the layout's namespace depending on the .NET version and type of Blazor app. For more information, see the Make the layout namespace available section.

Specifying the layout as a default layout in the Router component's RouteView is a useful practice because you can override the layout on a per-component or per-folder basis, as described in the preceding sections of this article. We recommend using the Router component to set the app's default layout because it's the most general and flexible approach for using layouts.

Apply a layout to arbitrary content (LayoutView component)

To set a layout for arbitrary Razor template content, specify the layout with a LayoutView component. You can use a LayoutView in any Razor component. The following example sets a layout component named ErrorLayout for the MainLayout component's NotFound template (<NotFound>...</NotFound>).

<Router ...>
    <Found ...>
        ...
    </Found>
    <NotFound>
        <LayoutView Layout="typeof(ErrorLayout)">
            <h1>Page not found</h1>
            <p>Sorry, there's nothing at this address.</p>
        </LayoutView>
    </NotFound>
</Router>

You may need to identity the layout's namespace depending on the .NET version and type of Blazor app. For more information, see the Make the layout namespace available section.

Important

Blazor Web Apps don't use the NotFound parameter (<NotFound>...</NotFound> markup), but the parameter is supported for backward compatibility to avoid a breaking change in the framework. The server-side ASP.NET Core middleware pipeline processes requests on the server. Use server-side techniques to handle bad requests. For more information, see ASP.NET Core Blazor render modes.

Nested layouts

A component can reference a layout that in turn references another layout. For example, nested layouts are used to create a multi-level menu structures.

The following example shows how to use nested layouts. The Episodes component shown in the Apply a layout to a component section is the component to display. The component references the DoctorWhoLayout component.

The following DoctorWhoLayout component is a modified version of the example shown earlier in this article. The header and footer elements are removed, and the layout references another layout, ProductionsLayout. The Episodes component is rendered where @Body appears in the DoctorWhoLayout.

DoctorWhoLayout.razor:

@inherits LayoutComponentBase
@layout ProductionsLayout

<PageTitle>Doctor Who® Database</PageTitle>

<h1>Doctor Who® Database</h1>

<nav>
    <a href="main-episode-list">Main Episode List</a>
    <a href="episode-search">Search</a>
    <a href="new-episode">Add Episode</a>
</nav>

@Body

<div>
    @TrademarkMessage
</div>

@code {
    public string TrademarkMessage { get; set; } =
        "Doctor Who is a registered trademark of the BBC. " +
        "https://www.doctorwho.tv/ https://www.bbc.com";
}

The ProductionsLayout component contains the top-level layout elements, where the header (<header>...</header>) and footer (<footer>...</footer>) elements now reside. The DoctorWhoLayout with the Episodes component is rendered where @Body appears.

ProductionsLayout.razor:

@inherits LayoutComponentBase

<header>
    <h1>Productions</h1>
</header>

<nav>
    <a href="main-production-list">Main Production List</a>
    <a href="production-search">Search</a>
    <a href="new-production">Add Production</a>
</nav>

@Body

<footer>
    Footer of Productions Layout
</footer>

The following rendered HTML markup is produced by the preceding nested layout. Extraneous markup doesn't appear in order to focus on the nested content provided by the three components involved:

• The header (<header>...</header>), production navigation bar (<nav>...</nav>), and footer (<footer>...</footer>) elements and their content come from the ProductionsLayout component.
• The H1 "database" heading (<h1>...</h1>), episode navigation bar (<nav>...</nav>), and trademark information (<div>...</div>) come from the DoctorWhoLayout component.
• The H2 "episodes" heading (<h2>...</h2>) and episode list (<ul>...</ul>) come from the Episodes component.

<header>
    ...
</header>

<nav>
    <a href="main-production-list">Main Production List</a>
    <a href="production-search">Search</a>
    <a href="new-production">Add Production</a>
</nav>

<h1>...</h1>

<nav>
    <a href="main-episode-list">Main Episode List</a>
    <a href="episode-search">Search</a>
    <a href="new-episode">Add Episode</a>
</nav>

<h2>...</h2>

<ul>
    <li>...</li>
    <li>...</li>
    <li>...</li>
</ul>

<div>
    ...
</div>

<footer>
    ...
</footer>

Share a Razor Pages layout with integrated components

When routable components are integrated into a Razor Pages app, the app's shared layout can be used with the components. For more information, see Integrate ASP.NET Core Razor components into ASP.NET Core apps.

Sections

To control the content in a layout from a child Razor component, see ASP.NET Core Blazor sections.

Additional resources

• Layout in ASP.NET Core
• Blazor samples GitHub repository (dotnet/blazor-samples) (how to download)