Create reusable UI using the Razor class library project in ASP.NET Core

By Rick Anderson

Razor views, pages, controllers, page models, Razor components, View components, and data models can be built into a Razor class library (RCL). The RCL can be packaged and reused. Applications can include the RCL and override the views and pages it contains. When a view, partial view, or Razor Page is found in both the web app and the RCL, the Razor markup (.cshtml file) in the web app takes precedence.

This feature requires .NET Core 2.1 SDK or later

View or download sample code (how to download)

Create a class library containing Razor UI

  • From the Visual Studio File menu, select New > Project.
  • Select ASP.NET Core Web Application.
  • Name the library (for example, "RazorClassLib") > OK. To avoid a file name collision with the generated view library, ensure the library name doesn't end in .Views.
  • Verify ASP.NET Core 2.1 or later is selected.
  • Select Razor Class Library > OK.

An RCL has the following project file:

<Project Sdk="Microsoft.NET.Sdk.Razor">

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.1.2" />
  </ItemGroup>

</Project>

Add Razor files to the RCL.

The ASP.NET Core templates assume the RCL content is in the Areas folder. See RCL Pages layout to create an RCL that exposes content in ~/Pages rather than ~/Areas/Pages.

Referencing RCL content

The RCL can be referenced by:

Walkthrough: Create an RCL project and use from a Razor Pages project

You can download the complete project and test it rather than creating it. The sample download contains additional code and links that make the project easy to test. You can leave feedback in this GitHub issue with your comments on download samples versus step-by-step instructions.

Test the download app

If you haven't downloaded the completed app and would rather create the walkthrough project, skip to the next section.

Open the .sln file in Visual Studio. Run the app.

Follow the instructions in Test WebApp1

Create an RCL

In this section, an RCL is created. Razor files are added to the RCL.

Create the RCL project:

  • From the Visual Studio File menu, select New > Project.
  • Select ASP.NET Core Web Application.
  • Name the app RazorUIClassLib > OK.
  • Verify ASP.NET Core 2.1 or later is selected.
  • Select Razor Class Library > OK.
  • Add a Razor partial view file named RazorUIClassLib/Areas/MyFeature/Pages/Shared/_Message.cshtml.

Add Razor files and folders to the project

  • Replace the markup in RazorUIClassLib/Areas/MyFeature/Pages/Shared/_Message.cshtml with the following code:
<h3>_Message.cshtml partial view.</h3>

<p>RazorUIClassLib\Areas\MyFeature\Pages\Shared\_Message.cshtml</p>
  • Replace the markup in RazorUIClassLib/Areas/MyFeature/Pages/Page1.cshtml with the following code:
@page
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

<h2>Hello from a Razor UI class library!</h2>
<p> From  RazorUIClassLib\Areas\MyFeature\Pages\Page1.cshtml</p>

<partial name="_Message" />

@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers is required to use the partial view (<partial name="_Message" />). Rather than including the @addTagHelper directive, you can add a _ViewImports.cshtml file. For example:

dotnet new viewimports -o RazorUIClassLib/Areas/MyFeature/Pages

For more information on _ViewImports.cshtml, see Importing Shared Directives

  • Build the class library to verify there are no compiler errors:
dotnet build RazorUIClassLib

The build output contains RazorUIClassLib.dll and RazorUIClassLib.Views.dll. RazorUIClassLib.Views.dll contains the compiled Razor content.

Use the Razor UI library from a Razor Pages project

Create the Razor Pages web app:

  • From Solution Explorer, right-click the solution > Add > New Project.

  • Select ASP.NET Core Web Application.

  • Name the app WebApp1.

  • Verify ASP.NET Core 2.1 or later is selected.

  • Select Web Application > OK.

  • From Solution Explorer, right-click on WebApp1 and select Set as StartUp Project.

  • From Solution Explorer, right-click on WebApp1 and select Build Dependencies > Project Dependencies.

  • Check RazorUIClassLib as a dependency of WebApp1.

  • From Solution Explorer, right-click on WebApp1 and select Add > Reference.

  • In the Reference Manager dialog, check RazorUIClassLib > OK.

Run the app.

Test WebApp1

Verify the Razor UI class library is in use:

  • Browse to /MyFeature/Page1.

Override views, partial views, and pages

When a view, partial view, or Razor Page is found in both the web app and the RCL, the Razor markup (.cshtml file) in the web app takes precedence. For example, add WebApp1/Areas/MyFeature/Pages/Page1.cshtml to WebApp1, and Page1 in the WebApp1 will take precedence over Page1 in the RCL.

In the sample download, rename WebApp1/Areas/MyFeature2 to WebApp1/Areas/MyFeature to test precedence.

Copy the RazorUIClassLib/Areas/MyFeature/Pages/Shared/_Message.cshtml partial view to WebApp1/Areas/MyFeature/Pages/Shared/_Message.cshtml. Update the markup to indicate the new location. Build and run the app to verify the app's version of the partial is being used.

RCL Pages layout

To reference RCL content as though it is part of the web app's Pages folder, create the RCL project with the following file structure:

  • RazorUIClassLib/Pages
  • RazorUIClassLib/Pages/Shared

Suppose RazorUIClassLib/Pages/Shared contains two partial files: _Header.cshtml and _Footer.cshtml. The <partial> tags could be added to _Layout.cshtml file:

<body>
  <partial name="_Header">
  @RenderBody()
  <partial name="_Footer">
</body>

Create an RCL with static assets

An RCL may require companion static assets that can be referenced by the consuming app of the RCL. ASP.NET Core allows creating RCLs that include static assets that are available to a consuming app.

To include companion assets as part of an RCL, create a wwwroot folder in the class library and include any required files in that folder.

When packing an RCL, all companion assets in the wwwroot folder are automatically included in the package.

Exclude static assets

To exclude static assets, add the desired exclusion path to the $(DefaultItemExcludes) property group in the project file. Separate entries with a semicolon (;).

In the following example, the lib.css stylesheet in the wwwroot folder isn't considered a static asset and isn't included in the published RCL:

<PropertyGroup>
  <DefaultItemExcludes>$(DefaultItemExcludes);wwwroot\lib.css</DefaultItemExcludes>
</PropertyGroup>

Typescript integration

To include TypeScript files in an RCL:

  1. Place the TypeScript files (.ts) outside of the wwwroot folder. For example, place the files in a Client folder.

  2. Configure the TypeScript build output for the wwwroot folder. Set the TypescriptOutDir property inside of a PropertyGroup in the project file:

    <TypescriptOutDir>wwwroot</TypescriptOutDir>
    
  3. Include the TypeScript target as a dependency of the ResolveCurrentProjectStaticWebAssets target by adding the following target inside of a PropertyGroup in the project file:

    <ResolveCurrentProjectStaticWebAssetsInputsDependsOn>
      TypeScriptCompile;
      $(ResolveCurrentProjectStaticWebAssetsInputs)
    </ResolveCurrentProjectStaticWebAssetsInputsDependsOn>
    

Consume content from a referenced RCL

The files included in the wwwroot folder of the RCL are exposed to the consuming app under the prefix _content/{LIBRARY NAME}/. For example, a library named Razor.Class.Lib results in a path to static content at _content/Razor.Class.Lib/.

The consuming app references static assets provided by the library with <script>, <style>, <img>, and other HTML tags. The consuming app must have static file support enabled in Startup.Configure:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    ...

    app.UseStaticFiles();

    ...
}

When running the consuming app from build output (dotnet run), static web assets are enabled by default in the Development environment. To support assets in other environments when running from build output, call UseStaticWebAssets on the host builder in Program.cs:

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStaticWebAssets();
                webBuilder.UseStartup<Startup>();
            });
}

Calling UseStaticWebAssets isn't required when running an app from published output (dotnet publish).

Multi-project development flow

When the consuming app runs:

  • The assets in the RCL stay in their original folders. The assets aren't moved to the consuming app.
  • Any change within the RCL's wwwroot folder is reflected in the consuming app after the RCL is rebuilt and without rebuilding the consuming app.

When the RCL is built, a manifest is produced that describes the static web asset locations. The consuming app reads the manifest at runtime to consume the assets from referenced projects and packages. When a new asset is added to an RCL, the RCL must be rebuilt to update its manifest before a consuming app can access the new asset.

Publish

When the app is published, the companion assets from all referenced projects and packages are copied into the wwwroot folder of the published app under _content/{LIBRARY NAME}/.