Razor file compilation in ASP.NET Core

By Rick Anderson

A Razor file is compiled at runtime, when the associated MVC view is invoked. Build-time Razor file publishing is unsupported. Razor files can optionally be compiled at publish time and deployed with the app—using the precompilation tool.

A Razor file is compiled at runtime, when the associated Razor Page or MVC view is invoked. Build-time Razor file publishing is unsupported. Razor files can optionally be compiled at publish time and deployed with the app—using the precompilation tool.

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.

Precompilation considerations

The following are side effects of precompiling Razor files:

  • A smaller published bundle
  • A faster startup time
  • You can't edit Razor files—the associated content is absent from the published bundle.

Deploy precompiled files

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 are deployed with your app.

Important

The precompilation tool will be removed in ASP.NET Core 3.0. We recommend migrating to Razor 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.

If your project targets .NET Framework, install the Microsoft.AspNetCore.Mvc.Razor.ViewCompilation NuGet package:

<PackageReference Include="Microsoft.AspNetCore.Mvc.Razor.ViewCompilation"
                  Version="2.0.4"
                  PrivateAssets="All" />

If your project targets .NET Core, no changes are necessary.

The ASP.NET Core 2.x project templates implicitly set the MvcRazorCompileOnPublish property to true by default. Consequently, this element can be safely removed from the .csproj file.

Important

The precompilation tool will be removed in ASP.NET Core 3.0. We recommend migrating to Razor Sdk.

Razor file precompilation is unavailable when performing a self-contained deployment (SCD) in ASP.NET Core 2.0.

Set the MvcRazorCompileOnPublish property to true, and install the Microsoft.AspNetCore.Mvc.Razor.ViewCompilation NuGet package. The following .csproj sample highlights these settings:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework>
    <MvcRazorCompileOnPublish>true</MvcRazorCompileOnPublish>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore" Version="1.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="1.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="1.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Razor.ViewCompilation" Version="1.1.0-*" />
  </ItemGroup>
</Project>

Prepare the app for a framework-dependent deployment with the .NET Core CLI publish command. For example, execute the following command at the project root:

dotnet publish -c Release

A <project_name>.PrecompiledViews.dll file, containing the compiled Razor files, is produced when precompilation succeeds. For example, the screenshot below depicts the contents of Index.cshtml within WebApplication1.PrecompiledViews.dll:

Razor views inside DLL

Recompile Razor files on change

The RazorViewEngineOptions AllowRecompilingViewsOnFileChange gets or sets a value that determines if Razor files (Razor views and Razor Pages) are recompiled and updated if files change on disk.

When set to true, IFileProvider.Watch watches for changes to Razor files in configured IFileProvider instances.

The default value is true for:

  • ASP.NET Core 2.1 or earlier apps.
  • ASP.NET Core 2.2 or later apps in the Development environment.

AllowRecompilingViewsOnFileChange is associated with a compatibility switch and can provide different behavior depending on the configured compatibility version for the app. Configuring the app by setting AllowRecompilingViewsOnFileChange takes precedence over the value implied by the app's compatibility version.

If the app's compatibility version is set to Version_2_1 or earlier, AllowRecompilingViewsOnFileChange is set to true unless explicitly configured.

If the app's compatibility version is set to Version_2_2 or later, AllowRecompilingViewsOnFileChange is set to false unless the environment is Development or the value is explicitly configured.

For guidance and examples of setting the app's compatibility version, see Compatibility version for ASP.NET Core MVC.

Additional resources