Razor file compilation in ASP.NET Core

Razor files with a .cshtml extension are compiled at both build and publish time using the Razor SDK. Runtime compilation may be optionally enabled by configuring the project.

Note

Runtime compilation isn't supported for Razor components of Blazor apps.

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.

In addition to build-time compilation, updating Razor views and Razor Pages is supported using .NET Hot Reload support for ASP.NET Core.

Note

When enabled, runtime compilation currently disables .NET Hot Reload. Runtime compilation with Hot Reload is planned for a future release.

Enable runtime compilation for all environments

To enable runtime compilation for all environments:

  1. Install the Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation NuGet package.

  2. Call AddRazorRuntimeCompilation in Program.cs:

    var builder = WebApplication.CreateBuilder(args);
    
    builder.Services.AddRazorPages()
        .AddRazorRuntimeCompilation();
    

Enable runtime compilation conditionally

Runtime compilation can be enabled conditionally, which ensures that the published output:

  • Uses compiled views.
  • Doesn't enable file watchers in production.

To enable runtime compilation only for the Development environment:

  1. Install the Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation NuGet package.

  2. Call AddRazorRuntimeCompilation in Program.cs when the current environment is set to Development:

    var builder = WebApplication.CreateBuilder(args);
    
    var mvcBuilder = builder.Services.AddRazorPages();
    
    if (builder.Environment.IsDevelopment())
    {
        mvcBuilder.AddRazorRuntimeCompilation();
    }
    

Runtime compilation can also be enabled with a hosting startup assembly. To enable runtime compilation in the Development environment for specific launch profiles:

  1. Install the Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation NuGet package.
  2. Modify the launch profile's environmentVariables section in launchSettings.json:
    • Verify that ASPNETCORE_ENVIRONMENT is set to "Development".

    • Set ASPNETCORE_HOSTINGSTARTUPASSEMBLIES to "Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation". For example, the following launchSettings.json enables runtime compilation for the ViewCompilationSample and IIS Express launch profiles:

      {
        "iisSettings": {
          "windowsAuthentication": false,
          "anonymousAuthentication": true,
          "iisExpress": {
            "applicationUrl": "http://localhost:7098",
            "sslPort": 44332
          }
        },
        "profiles": {
          "ViewCompilationSample": {
            "commandName": "Project",
            "dotnetRunMessages": true,
            "launchBrowser": true,
            "applicationUrl": "https://localhost:7173;http://localhost:5251",
            "environmentVariables": {
              "ASPNETCORE_ENVIRONMENT": "Development",
              "ASPNETCORE_HOSTINGSTARTUPASSEMBLIES": "Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation"
            }
          },
          "IIS Express": {
            "commandName": "IISExpress",
            "launchBrowser": true,
            "environmentVariables": {
              "ASPNETCORE_ENVIRONMENT": "Development",
              "ASPNETCORE_HOSTINGSTARTUPASSEMBLIES": "Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation"
            }
          }
        }
      }
      

With this approach, no code changes are needed in Program.cs. At runtime, ASP.NET Core searches for an assembly-level HostingStartup attribute in Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation. The HostingStartup attribute specifies the app startup code to execute and 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) named MyClassLib. The RCL contains a _Layout.cshtml file consumed by MVC and Razor Pages projects. 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 Enable runtime compilation conditionally.

  2. Configure MvcRazorRuntimeCompilationOptions in Program.cs:

    var builder = WebApplication.CreateBuilder(args);
    
    builder.Services.AddRazorPages();
    
    builder.Services.Configure<MvcRazorRuntimeCompilationOptions>(options =>
    {
        var libraryPath = Path.GetFullPath(
            Path.Combine(builder.Environment.ContentRootPath, "..", "MyClassLib"));
    
        options.FileProviders.Add(new PhysicalFileProvider(libraryPath));
    });
    

    The preceding code builds an absolute path to the MyClassLib RCL. The PhysicalFileProvider API 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.

Additional resources

Razor files with a .cshtml extension are compiled at both build and publish time using the Razor 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.

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.
  2. Select the Enable Razor runtime compilation checkbox.

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 NuGet package.

  2. Update the project's Startup.ConfigureServices method to include a call to AddRazorRuntimeCompilation. For example:

    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 NuGet package.
  2. 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:

{
    "iisSettings": {
      "windowsAuthentication": false,
      "anonymousAuthentication": true,
      "iisExpress": {
        "applicationUrl": "http://localhost:57676",
        "sslPort": 44364
      }
    },
    "profiles": {
      "IIS Express": {
        "commandName": "IISExpress",
        "launchBrowser": true,
        "environmentVariables": {
          "ASPNETCORE_ENVIRONMENT": "Development",
          "ASPNETCORE_HOSTINGSTARTUPASSEMBLIES": "Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation"
        }
      },
      "RazorPagesApp": {
        "commandName": "Project",
        "launchBrowser": true,
        "applicationUrl": "https://localhost:5001;http://localhost:5000",
        "environmentVariables": {
          "ASPNETCORE_ENVIRONMENT": "Development",
          "ASPNETCORE_HOSTINGSTARTUPASSEMBLIES": "Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation"
        }
      }
    }
  }

No code changes are needed in the project's Startup class. At runtime, ASP.NET Core searches for an assembly-level 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) 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.

  2. Configure the runtime compilation options in Startup.ConfigureServices:

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();
    
        services.Configure<MvcRazorRuntimeCompilationOptions>(options =>
        {
            var libraryPath = Path.GetFullPath(
                Path.Combine(HostEnvironment.ContentRootPath, "..", "MyClassLib"));
            options.FileProviders.Add(new PhysicalFileProvider(libraryPath));
        });
    }
    

    In the preceding code, an absolute path to the MyClassLib RCL is constructed. The PhysicalFileProvider API 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.

Additional resources