class-libraries.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Consume ASP.NET Core Razor components from a Razor class library (RCL)	guardrex	Discover how components can be included in Blazor apps from an external Razor class library (RCL).	>= aspnetcore-3.1	riande	mvc	02/09/2024	blazor/components/class-libraries

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

[!INCLUDE]

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.

:::moniker range=">= aspnetcore-5.0"

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 and package reference for Microsoft.AspNetCore.Components.Web to the project file (.csproj):

  <ItemGroup>
    <SupportedPlatform Include="browser" />
  </ItemGroup>
  
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.Web" Version="{VERSION}" />
  </ItemGroup>

  In the preceding example, the {VERSION} placeholder is the package version.

  [!IMPORTANT] When a Razor class library is added to a solution with the Support pages and views checkbox selected, the library also references the Microsoft.AspNetCore.App ASP.NET Core shared framework:

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

  The shared framework isn't supported on Blazor WebAssembly, so libraries that target client-side apps should remove the framework reference. When the library targets both server-side and client-side apps, control the dependency with an MSBuild condition:

  <ItemGroup Condition="'$(IsWasmProject)' != 'true'">
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

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

:::moniker-end

:::moniker range="< aspnetcore-5.0"

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

:::moniker-end

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}
   

:::moniker range=">= aspnetcore-5.0"

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 and package reference for Microsoft.AspNetCore.Components.Web to the project file (.csproj):

  <ItemGroup>
    <SupportedPlatform Include="browser" />
  </ItemGroup>
  
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.Web" Version="{VERSION}" />
  </ItemGroup>

  In the preceding example, the {VERSION} placeholder is the package version.

  [!IMPORTANT] When a Razor class library is added to a solution with the Support pages and views checkbox selected, the library also references the Microsoft.AspNetCore.App ASP.NET Core shared framework:

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

  The shared framework isn't supported on Blazor WebAssembly, so libraries that target client-side apps should remove the framework reference. When the library targets both server-side and client-side apps, control the dependency with an MSBuild condition:

  <ItemGroup Condition="'$(IsWasmProject)' != 'true'">
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

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

:::moniker-end

:::moniker range="< aspnetcore-5.0"

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

:::moniker-end

---

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 />

:::moniker range=">= aspnetcore-6.0"

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" />

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

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:

:::moniker-end

:::moniker range="< aspnetcore-5.0"

The following background image and stylesheet are used by the RCL's Component1 example component. There's no need to add these static assets to a new RCL created from the RCL project template, as they're added automatically by the project template.

wwwroot/background.png in the ComponentLibrary RCL:

wwwroot/styles.css in the ComponentLibrary RCL:

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

To provide Component1's my-component CSS class, link to the library's stylesheet in the app's <head> markup (location of <head> content):

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

:::moniker-end

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). Assign an xref:System.Reflection.Assembly collection to the xref:Microsoft.AspNetCore.Components.Routing.Router.AdditionalAssemblies%2A parameter of the xref:Microsoft.AspNetCore.Components.Routing.Router component 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 xref:blazor/fundamentals/routing#route-to-components-from-multiple-assemblies.

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 xref:razor-pages/ui-class#create-an-rcl-with-static-assets.

:::moniker range=">= aspnetcore-6.0"

Create an RCL with JavaScript files collocated with components

[!INCLUDE]

:::moniker-end

:::moniker range="< aspnetcore-8.0"

Supply components and static assets to multiple hosted Blazor apps

For more information, see xref:blazor/host-and-deploy/multiple-hosted-webassembly.

:::moniker-end

:::moniker range=">= aspnetcore-5.0"

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 xref:System.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 xref:System.Runtime.Versioning.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.

:::moniker-end

:::moniker range=">= aspnetcore-5.0"

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 xref:blazor/js-interop/call-javascript-from-dotnet#javascript-isolation-in-javascript-modules.

:::moniker-end

:::moniker range=">= aspnetcore-6.0"

Avoid trimming JavaScript-invokable .NET methods

Runtime relinking trims class instance JavaScript-invokable .NET methods unless they're explicitly preserved. For more information, see xref:blazor/js-interop/call-dotnet-from-javascript#avoid-trimming-javascript-invokable-net-methods.

:::moniker-end

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

:::moniker range=">= aspnetcore-5.0"

• xref:razor-pages/ui-class
• xref:fundamentals/target-aspnetcore
• Add an XML Intermediate Language (IL) Trimmer configuration file to a library
• CSS isolation support with Razor class libraries

:::moniker-end

:::moniker range="< aspnetcore-5.0"

• xref:razor-pages/ui-class
• xref:fundamentals/target-aspnetcore
• Add an XML Intermediate Language (IL) Linker configuration file to a library

:::moniker-end