Razor views, pages, controllers, page models, [Razor components](xref:blazor/class-libraries), [View components](xref:mvc/views/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.
[View or download sample code](https://github.com/dotnet/AspNetCore.Docs/tree/master/aspnetcore/razor-pages/ui-class/samples) ([how to download](xref:index#how-to-download-a-sample))
* From Visual Studio select **Create new a new project**.
* Select **Razor Class Library** > **Next**.
* Name the library (for example, "RazorClassLib"), > **Create**. To avoid a file name collision with the generated view library, ensure the library name doesn't end in `.Views`.
* Select **Support pages and views** if you need to support views. By default, only Razor Pages are supported. Select **Create**.
The Razor class library (RCL) template defaults to Razor component development by default. The **Support pages and views** option supports pages and views.
The Razor class library (RCL) template defaults to Razor component development by default. Pass the `--support-pages-and-views` option (`dotnet new razorclasslib --support-pages-and-views`) to provide support for pages and views.
For more information, see [dotnet new](/dotnet/core/tools/dotnet-new). To avoid a file name collision with the generated view library, ensure the library name doesn't end in `.Views`.
---
Add Razor files to the RCL.
The ASP.NET Core templates assume the RCL content is in the *Areas* folder. See [RCL Pages layout](#rcl-pages-layout) to create an RCL that exposes content in `~/Pages` rather than `~/Areas/Pages`.
## Reference RCL content
The RCL can be referenced by:
* NuGet package. See [Creating NuGet packages](/nuget/create-packages/creating-a-package) and [dotnet add package](/dotnet/core/tools/dotnet-add-package) and [Create and publish a NuGet package](/nuget/quickstart/create-and-publish-a-package-using-visual-studio).
* *{ProjectName}.csproj*. See [dotnet-add reference](/dotnet/core/tools/dotnet-add-reference).
## 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:
An RCL may require companion static assets that can be referenced by either the RCL or 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:
1. Place the TypeScript files (*.ts*) outside of the *wwwroot* folder. For example, place the files in a *Client* folder.
1. Configure the TypeScript build output for the *wwwroot* folder. Set the `TypescriptOutDir` property inside of a `PropertyGroup` in the project file:
```xml
<TypescriptOutDir>wwwroot</TypescriptOutDir>
```
1. Include the TypeScript target as a dependency of the `ResolveCurrentProjectStaticWebAssets` target by adding the following target inside of a `PropertyGroup` in the project file:
The files included in the *wwwroot* folder of the RCL are exposed to either the RCL or 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/`. When producing a NuGet package and the assembly name isn't the same as the package ID, use the package ID for `{LIBRARY NAME}`.
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](xref:fundamentals/static-files) enabled in `Startup.Configure`:
```csharp
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*:
```csharp
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}/`.
::: moniker-end
::: moniker range="<aspnetcore-3.0"
Razor views, pages, controllers, page models, [Razor components](xref:blazor/class-libraries), [View components](xref:mvc/views/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.
[View or download sample code](https://github.com/dotnet/AspNetCore.Docs/tree/master/aspnetcore/razor-pages/ui-class/samples) ([how to download](xref:index#how-to-download-a-sample))
* 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`.
For more information, see [dotnet new](/dotnet/core/tools/dotnet-new). To avoid a file name collision with the generated view library, ensure the library name doesn't end in `.Views`.
The ASP.NET Core templates assume the RCL content is in the *Areas* folder. See [RCL Pages layout](#rcl-pages-layout) to create an RCL that exposes content in `~/Pages` rather than `~/Areas/Pages`.
* NuGet package. See [Creating NuGet packages](/nuget/create-packages/creating-a-package) and [dotnet add package](/dotnet/core/tools/dotnet-add-package) and [Create and publish a NuGet package](/nuget/quickstart/create-and-publish-a-package-using-visual-studio).
* *{ProjectName}.csproj*. See [dotnet-add reference](/dotnet/core/tools/dotnet-add-reference).
You can download the [complete project](https://github.com/dotnet/AspNetCore.Docs/tree/master/aspnetcore/razor-pages/ui-class/samples) 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](https://github.com/dotnet/AspNetCore.Docs/issues/6098) with your comments on download samples versus step-by-step instructions.
`@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers` is required to use the partial view (`<partialname="_Message"/>`). Rather than including the `@addTagHelper` directive, you can add a *_ViewImports.cshtml* file. For example:
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.
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.
Suppose *RazorUIClassLib/Pages/Shared* contains two partial files: *_Header.cshtml* and *_Footer.cshtml*. The `<partial>` tags could be added to *_Layout.cshtml* file:
