Razor files with a *.cshtml* extension are compiled at both build and publish time using the [Razor SDK](xref:razor-pages/sdk). Runtime compilation may be optionally enabled by configuring your project.
## Razor compilation
Build-time and publish-time compilation of Razor files is enabled by default by the Razor SDK. When enabled, runtime compilation complements build-time compilation, allowing Razor files to be updated if they're edited.
## Enable runtime compilation at project creation
The Razor Pages and MVC project templates include an option to enable runtime compilation when the project is created. This option is supported in ASP.NET Core 3.1 and later.
# [Visual Studio](#tab/visual-studio)
In the **Create a new ASP.NET Core web application** dialog:
1. Select either the **Web Application** or the **Web Application (Model-View-Controller)** project template.
1. Select the **Enable Razor runtime compilation** check box.
# [.NET Core CLI](#tab/netcore-cli)
Use the `-rrc` or `--razor-runtime-compilation` template option. For example, the following command creates a new Razor Pages project with runtime compilation enabled:
```dotnetcli
dotnet new webapp --razor-runtime-compilation
```
---
## Enable runtime compilation in an existing project
To enable runtime compilation for all environments in an existing project:
1. Install the [Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation](https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation/) NuGet package.
1. Update the project's `Startup.ConfigureServices` method to include a call to <xref:Microsoft.Extensions.DependencyInjection.RazorRuntimeCompilationMvcBuilderExtensions.AddRazorRuntimeCompilation*>. For example:
```csharp
public void ConfigureServices(IServiceCollection services)
{
services.AddRazorPages()
.AddRazorRuntimeCompilation();
// code omitted for brevity
}
```
## Conditionally enable runtime compilation in an existing project
Runtime compilation can be enabled such that it's only available for local development. Conditionally enabling in this manner ensures that the published output:
* Uses compiled views.
* Doesn't enable file watchers in production.
To enable runtime compilation only in the Development environment:
1. Install the [Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation](https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation/) NuGet package.
1. Modify the launch profile `environmentVariables` section in *launchSettings.json*:
* Verify `ASPNETCORE_ENVIRONMENT` is set to `"Development"`.
* Set `ASPNETCORE_HOSTINGSTARTUPASSEMBLIES` to `"Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation"`.
In the following example, runtime compilation is enabled in the Development environment for the `IIS Express` and `RazorPagesApp` launch profiles:
No code changes are needed in the project's `Startup` class. At runtime, ASP.NET Core searches for an [assembly-level HostingStartup attribute](xref:fundamentals/configuration/platform-specific-configuration#hostingstartup-attribute) in `Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation`. The `HostingStartup` attribute specifies the app startup code to execute. That startup code enables runtime compilation.
## Enable runtime compilation for a Razor Class Library
Consider a scenario in which a Razor Pages project references a [Razor Class Library (RCL)](xref:razor-pages/ui-class) named *MyClassLib*. The RCL contains a *_Layout.cshtml* file that all of your team's MVC and Razor Pages projects consume. You want to enable runtime compilation for the *_Layout.cshtml* file in that RCL. Make the following changes in the Razor Pages project:
1. Enable runtime compilation with the instructions at [Conditionally enable runtime compilation in an existing project](#conditionally-enable-runtime-compilation-in-an-existing-project).
1. Configure the runtime compilation options in `Startup.ConfigureServices`:
In the preceding code, an absolute path to the *MyClassLib* RCL is constructed. The [PhysicalFileProvider API](xref:fundamentals/file-providers#physicalfileprovider) is used to locate directories and files at that absolute path. Finally, the `PhysicalFileProvider` instance is added to a file providers collection, which allows access to the RCL's *.cshtml* files.
Razor files with a *.cshtml* extension are compiled at both build and publish time using the [Razor SDK](xref:razor-pages/sdk). Runtime compilation may be optionally enabled by configuring your application.
Build-time and publish-time compilation of Razor files is enabled by default by the Razor SDK. When enabled, runtime compilation complements build-time compilation, allowing Razor files to be updated if they're edited.
1. Update the project's `Startup.ConfigureServices` method to include a call to <xref:Microsoft.Extensions.DependencyInjection.RazorRuntimeCompilationMvcBuilderExtensions.AddRazorRuntimeCompilation*>. For example:
public void ConfigureServices(IServiceCollection services)
{
services.AddRazorPages()
.AddRazorRuntimeCompilation();
// code omitted for brevity
}
```
### Conditionally enable runtime compilation
Runtime compilation can be enabled such that it's only available for local development. Conditionally enabling in this manner ensures that the published output:
* Uses compiled views.
* Is smaller in size.
* Doesn't enable file watchers in production.
To enable runtime compilation based on the environment and configuration mode:
1. Conditionally reference the [Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation](https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation/) package based on the active `Configuration` value:
1. Update the project's `Startup.ConfigureServices` method to include a call to `AddRazorRuntimeCompilation`. Conditionally execute `AddRazorRuntimeCompilation` such that it only runs in Debug mode when the `ASPNETCORE_ENVIRONMENT` variable is set to `Development`:
* See the [runtime compilation sample on GitHub](https://github.com/aspnet/samples/tree/master/samples/aspnetcore/mvc/runtimecompilation) for a sample that shows making runtime compilation work across projects.
A Razor file is compiled at runtime, when the associated Razor Page or MVC view is invoked. Razor files are compiled at both build and publish time using the [Razor SDK](xref:razor-pages/sdk).
Build- and publish-time compilation of Razor files is enabled by default by the Razor SDK. Editing Razor files after they're updated is supported at build time. By default, only the compiled *Views.dll* and no *.cshtml* files or references assemblies required to compile Razor files are deployed with your app.
> The precompilation tool has been deprecated, and will be removed in ASP.NET Core 3.0. We recommend migrating to [Razor Sdk](xref:razor-pages/sdk).
>
> The Razor SDK is effective only when no precompilation-specific properties are set in the project file. For instance, setting the *.csproj* file's `MvcRazorCompileOnPublish` property to `true` disables the Razor SDK.
## Runtime compilation
Build-time compilation is supplemented by runtime compilation of Razor files. ASP.NET Core MVC will recompile Razor files when the contents of a *.cshtml* file change.
