Consume ASP.NET Core Razor components from a Razor class library (RCL)

Components can be shared in a Razor class library (RCL) across projects. Include components and static assets in an app from:

• Another project in the solution.
• A referenced .NET library.
• A NuGet package.

Just as components are regular .NET types, components provided by an RCL are normal .NET assemblies.

Create an RCL

Visual Studio

1. Create a new project.
2. In the Create a new project dialog, select Razor Class Library from the list of ASP.NET Core project templates. Select Next.
3. In the Configure your new project dialog, provide a project name in the Project name field or accept the default project name. Examples in this topic use the project name ComponentLibrary. Select Create.
4. In the Create a new Razor class library dialog, select Create.
5. Add the RCL to a solution:
   1. Open the solution.
   2. Right-click the solution in Solution Explorer. Select Add > Existing Project.
   3. Navigate to the RCL's project file.
   4. Select the RCL's project file (.csproj).
6. Add a reference to the RCL from the app:
   1. Right-click the app project. Select Add > Project Reference.
   2. Select the RCL project. Select OK.

If the Support pages and views checkbox is selected to support pages and views when generating the RCL from the template:

• Add an _Imports.razor file to root of the generated RCL project with the following contents to enable Razor component authoring:

  @using Microsoft.AspNetCore.Components.Web
  

• Add the following SupportedPlatform item to the project file (.csproj):

  <ItemGroup>
    <SupportedPlatform Include="browser" />
  </ItemGroup>
  

  For more information on the SupportedPlatform item, see the client-side browser compatibility analyzer section.

Visual Studio Code / .NET Core CLI

1. Use the Razor Class Library project template (razorclasslib) with the dotnet new command in a command shell. In the following example, an RCL is created and named ComponentLibrary using the -o|--output option. The folder that holds ComponentLibrary is created automatically when the command is executed:

   dotnet new razorclasslib -o ComponentLibrary
   

2. To add the library to an existing project, use the dotnet add reference command in a command shell. In the following command, the {PATH TO LIBRARY} placeholder is the path to the library's project folder:

   dotnet add reference {PATH TO LIBRARY}
   

If the -s|--support-pages-and-views option is used to support pages and views when generating the RCL from the template:

• Add an _Imports.razor file to root of the generated RCL project with the following contents to enable Razor component authoring:

  @using Microsoft.AspNetCore.Components.Web
  

• Add the following SupportedPlatform item to the project file (.csproj):

  <ItemGroup>
    <SupportedPlatform Include="browser" />
  </ItemGroup>
  

  For more information on the SupportedPlatform item, see the client-side browser compatibility analyzer section.

Consume a Razor component from an RCL

To consume components from an RCL in another project, use either of the following approaches:

• Use the full component type name, which includes the RCL's namespace.
• Individual components can be added by name without the RCL's namespace if Razor's @using directive declares the RCL's namespace. Use the following approaches:
  • Add the @using directive to individual components.
  • include the @using directive in the top-level _Imports.razor file to make the library's components available to an entire project. Add the directive to an _Imports.razor file at any level to apply the namespace to a single component or set of components within a folder. When an _Imports.razor file is used, individual components don't require an @using directive for the RCL's namespace.

In the following examples, ComponentLibrary is an RCL containing the Component1 component. The Component1 component is an example component automatically added to an RCL created from the RCL project template that isn't created to support pages and views.

Note

If the RCL is created to support pages and views, manually add the Component1 component and its static assets to the RCL if you plan to follow the examples in this article. The component and static assets are shown in this section.

Component1.razor in the ComponentLibrary RCL:

<div class="my-component">
    This component is defined in the <strong>ComponentLibrary</strong> package.
</div>

In the app that consumes the RCL, reference the Component1 component using its namespace, as the following example shows.

ConsumeComponent1.razor:

@page "/consume-component-1"

<h1>Consume component (full namespace example)</h1>

<ComponentLibrary.Component1 />

Alternatively, add a @using directive and use the component without its namespace. The following @using directive can also appear in any _Imports.razor file in or above the current folder.

ConsumeComponent2.razor:

@page "/consume-component-2"
@using ComponentLibrary

<h1>Consume component (<code>@@using</code> example)</h1>

<Component1 />

For library components that use CSS isolation, the component styles are automatically made available to the consuming app. There's no need to manually link or import the library's individual component stylesheets or its bundled CSS file in the app that consumes the library. The app uses CSS imports to reference the RCL's bundled styles. The bundled styles aren't published as a static web asset of the app that consumes the library. For a class library named ClassLib and a Blazor app with a BlazorSample.styles.css stylesheet, the RCL's stylesheet is imported at the top of the app's stylesheet automatically at build time:

@import '_content/ClassLib/ClassLib.bundle.scp.css';

For the preceding examples, Component1's stylesheet (Component1.razor.css) is bundled automatically.

Component1.razor.css in the ComponentLibrary RCL:

.my-component {
    border: 2px dashed red;
    padding: 1em;
    margin: 1em 0;
    background-image: url('background.png');
}

The background image is also included from the RCL project template and resides in the wwwroot folder of the RCL.

wwwroot/background.png in the ComponentLibrary RCL:

To provide additional library component styles from stylesheets in the library's wwwroot folder, add stylesheet <link> tags to the RCL's consumer, as the next example demonstrates.

Important

Generally, library components use CSS isolation to bundle and provide component styles. Component styles that rely upon CSS isolation are automatically made available to the app that uses the RCL. There's no need to manually link or import the library's individual component stylesheets or its bundled CSS file in the app that consumes the library. The following example is for providing global stylesheets outside of CSS isolation, which usually isn't a requirement for typical apps that consume RCLs.

The following background image is used in the next example. If you implement the example shown in this section, right-click the image to save it locally.

wwwroot/extra-background.png in the ComponentLibrary RCL:

Add a new stylesheet to the RCL with an extra-style class.

wwwroot/additionalStyles.css in the ComponentLibrary RCL:

.extra-style {
    border: 2px dashed blue;
    padding: 1em;
    margin: 1em 0;
    background-image: url('extra-background.png');
}

Add a component to the RCL that uses the extra-style class.

ExtraStyles.razor in the ComponentLibrary RCL:

<div class="extra-style">
    <p>
        This component is defined in the <strong>ComponentLibrary</strong> package.
    </p>
</div>

Add a page to the app that uses the ExtraStyles component from the RCL.

ConsumeComponent3.razor:

@page "/consume-component-3"
@using ComponentLibrary

<h1>Consume component (<code>additionalStyles.css</code> example)</h1>

<ExtraStyles />

Link to the library's stylesheet in the app's <head> markup (location of <head> content).

<link href="_content/ComponentLibrary/additionalStyles.css" rel="stylesheet" />

Make routable components available from the RCL

To make routable components in the RCL available for direct requests, the RCL's assembly must be disclosed to the app's router.

Open the app's App component (App.razor). Add or update the AdditionalAssemblies parameter of the <Router> tag to include the RCL's assembly. In the following example, the ComponentLibrary.Component1 component is used to discover the RCL's assembly.

AdditionalAssemblies="new[] { typeof(ComponentLibrary.Component1).Assembly }"

For more information, see ASP.NET Core Blazor routing and navigation.

Create an RCL with static assets in the wwwroot folder

An RCL's static assets are available to any app that consumes the library.

Place static assets in the wwwroot folder of the RCL and reference the static assets with the following path in the app: _content/{PACKAGE ID}/{PATH AND FILE NAME}. The {PACKAGE ID} placeholder is the library's package ID. The package ID defaults to the project's assembly name if <PackageId> isn't specified in the project file. The {PATH AND FILE NAME} placeholder is path and file name under wwwroot. This path format is also used in the app for static assets supplied by NuGet packages added to the RCL.

The following example demonstrates the use of RCL static assets with an RCL named ComponentLibrary and a Blazor app that consumes the RCL. The app has a project reference for the ComponentLibrary RCL.

The following Jeep® image is used in this section's example. If you implement the example shown in this section, right-click the image to save it locally.

wwwroot/jeep-yj.png in the ComponentLibrary RCL:

Add the following JeepYJ component to the RCL.

JeepYJ.razor in the ComponentLibrary RCL:

<h3>ComponentLibrary.JeepYJ</h3>

<p>
    <img alt="Jeep YJ&reg;" src="_content/ComponentLibrary/jeep-yj.png" />
</p>

Add the following Jeep component to the app that consumes the ComponentLibrary RCL. The Jeep component uses:

• The Jeep YJ® image from the ComponentLibrary RCL's wwwroot folder.
• The JeepYJ component from the RCL.

Jeep.razor:

@page "/jeep"
@using ComponentLibrary

<div style="float:left;margin-right:10px">
    <h3>Direct use</h3>

    <p>
        <img alt="Jeep YJ&reg;" src="_content/ComponentLibrary/jeep-yj.png" />
    </p>
</div>

<JeepYJ />

<p>
    <em>Jeep</em> and <em>Jeep YJ</em> are registered trademarks of 
    <a href="https://www.stellantis.com">FCA US LLC (Stellantis NV)</a>.
</p>

Rendered Jeep component:

For more information, see Reusable Razor UI in class libraries with ASP.NET Core.

Create an RCL with JavaScript files collocated with components

Collocation of JavaScript (JS) files for pages, views, and Razor components is a convenient way to organize scripts in an app.

Collocate JS files using the following filename extension conventions:

• Pages of Razor Pages apps and views of MVC apps: .cshtml.js. Examples:
  • Pages/Index.cshtml.js for the Index page of a Razor Pages app at Pages/Index.cshtml.
  • Views/Home/Index.cshtml.js for the Index view of an MVC app at Views/Home/Index.cshtml.
• Razor components of Blazor apps: .razor.js. Example: Index.razor.js for the Index component.

Collocated JS files are publicly addressable using the path to the file in the project:

• Pages, views, and components from a collocated scripts file in the app:

  {PATH}/{PAGE, VIEW, OR COMPONENT}.{EXTENSION}.js

  • The {PATH} placeholder is the path to the page, view, or component.
  • The {PAGE, VIEW, OR COMPONENT} placeholder is the page, view, or component.
  • The {EXTENSION} placeholder matches the extension of the page, view, or component, either razor or cshtml.

  Razor Pages example:

  A JS file for the Index page is placed in the Pages folder (Pages/Index.cshtml.js) next to the Index page (Pages/Index.cshtml). In the Index page, the script is referenced at the path in the Pages folder:

  @section Scripts {
    <script src="~/Pages/Index.cshtml.js"></script>
  }
  

  When the app is published, the framework automatically moves the script to the web root. In the preceding example, the script is moved to bin\Release\{TARGET FRAMEWORK MONIKER}\publish\wwwroot\Pages\Index.cshtml.js, where the {TARGET FRAMEWORK MONIKER} placeholder is the Target Framework Moniker (TFM). No change is required to the script's relative URL in the Index page.

  Blazor example:

  A JS file for the Index component is placed next to the Index component (Index.razor). In the Index component, the script is referenced at its path.

  Index.razor.js:

  export function showPrompt(message) {
    return prompt(message, 'Type anything here');
  }
  

  In the OnAfterRenderAsync method of the Index component (Index.razor):

  module = await JS.InvokeAsync<IJSObjectReference>(
      "import", "./Components/Pages/Index.razor.js");
  

  When the app is published, the framework automatically moves the script to the web root. In the preceding example, the script is moved to bin\Release\{TARGET FRAMEWORK MONIKER}\publish\wwwroot\Components\Pages\Index.razor.js, where the {TARGET FRAMEWORK MONIKER} placeholder is the Target Framework Moniker (TFM). No change is required to the script's relative URL in the Index component.

• For scripts provided by a Razor class library (RCL):

  _content/{PACKAGE ID}/{PATH}/{PAGE, VIEW, OR COMPONENT}.{EXTENSION}.js

  • The {PACKAGE ID} placeholder is the RCL's package identifier (or library name for a class library referenced by the app).
  • The {PATH} placeholder is the path to the page, view, or component. If a Razor component is located at the root of the RCL, the path segment isn't included.
  • The {PAGE, VIEW, OR COMPONENT} placeholder is the page, view, or component.
  • The {EXTENSION} placeholder matches the extension of page, view, or component, either razor or cshtml.

  In the following Blazor app example:

  • The RCL's package identifier is AppJS.
  • A module's scripts are loaded for the Index component (Index.razor).
  • The Index component is in the Pages folder of the Components folder of the RCL.

  var module = await JS.InvokeAsync<IJSObjectReference>("import", 
      "./_content/AppJS/Components/Pages/Index.razor.js");
  

Client-side browser compatibility analyzer

Client-side apps target the full .NET API surface area, but not all .NET APIs are supported on WebAssembly due to browser sandbox constraints. Unsupported APIs throw PlatformNotSupportedException when running on WebAssembly. A platform compatibility analyzer warns the developer when the app uses APIs that aren't supported by the app's target platforms. For client-side apps, this means checking that APIs are supported in browsers. Annotating .NET framework APIs for the compatibility analyzer is an on-going process, so not all .NET framework API is currently annotated.

Blazor Web Apps that enable Interactive WebAssembly components, Blazor WebAssembly apps, and RCL projects automatically enable browser compatibility checks by adding browser as a supported platform with the SupportedPlatform MSBuild item. Library developers can manually add the SupportedPlatform item to a library's project file to enable the feature:

<ItemGroup>
  <SupportedPlatform Include="browser" />
</ItemGroup>

When authoring a library, indicate that a particular API isn't supported in browsers by specifying browser to UnsupportedOSPlatformAttribute:

using System.Runtime.Versioning;

...

[UnsupportedOSPlatform("browser")]
private static string GetLoggingDirectory()
{
    ...
}

For more information, see Annotating APIs as unsupported on specific platforms (dotnet/designs GitHub repository.

JavaScript isolation in JavaScript modules

Blazor enables JavaScript isolation in standard JavaScript modules. JavaScript isolation provides the following benefits:

• Imported JavaScript no longer pollutes the global namespace.
• Consumers of the library and components aren't required to manually import the related JavaScript.

For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Avoid trimming JavaScript-invokable .NET methods

Runtime relinking trims class instance JavaScript-invokable .NET methods unless they're explicitly preserved. For more information, see Call .NET methods from JavaScript functions in ASP.NET Core Blazor.

Build, pack, and ship to NuGet

Because Razor class libraries that contain Razor components are standard .NET libraries, packing and shipping them to NuGet is no different from packing and shipping any library to NuGet. Packing is performed using the dotnet pack command in a command shell:

dotnet pack

Upload the package to NuGet using the dotnet nuget push command in a command shell.

Trademarks

Jeep and Jeep YJ are registered trademarks of FCA US LLC (Stellantis NV).

Additional resources

• Reusable Razor UI in class libraries with ASP.NET Core
• Use ASP.NET Core APIs in a class library
• Add an XML Intermediate Language (IL) Trimmer configuration file to a library
• CSS isolation support with Razor class libraries