Show Blazor namespaces (#20637)

aspnetcore/migration/31-to-50.md

@@ -80,6 +80,38 @@ If updating a Blazor WebAssembly project, skip to the [Update Blazor WebAssembly
     +<link href="BlazorSample.Client.styles.css" rel="stylesheet" />
     ```
 
+1. Include a new namespace in the app's `_Imports.razor` file for [component virtualization](xref:blazor/components/virtualization), <xref:Microsoft.AspNetCore.Components.Web.Virtualization?displayProperty=fullName>. The following `_Imports.razor` files show the default namespaces in apps generated from the Blazor project templates. The placeholder `{ASSEMBLY NAME}` is the app's assembly name.
+
+   Blazor WebAssembly (`_Imports.razor`):
+   
+   ```razor
+   @using System.Net.Http
+   @using System.Net.Http.Json
+   @using Microsoft.AspNetCore.Components.Forms
+   @using Microsoft.AspNetCore.Components.Routing
+   @using Microsoft.AspNetCore.Components.Web
+   @using Microsoft.AspNetCore.Components.Web.Virtualization
+   @using Microsoft.AspNetCore.Components.WebAssembly.Http
+   @using Microsoft.JSInterop
+   @using {ASSEMBLY NAME}
+   @using {ASSEMBLY NAME}.Shared
+   ```
+
+   Blazor Server (`_Imports.razor`):
+   
+   ```razor
+   @using System.Net.Http
+   @using Microsoft.AspNetCore.Authorization
+   @using Microsoft.AspNetCore.Components.Authorization
+   @using Microsoft.AspNetCore.Components.Forms
+   @using Microsoft.AspNetCore.Components.Routing
+   @using Microsoft.AspNetCore.Components.Web
+   @using Microsoft.AspNetCore.Components.Web.Virtualization
+   @using Microsoft.JSInterop
+   @using {ASSEMBLY NAME}
+   @using {ASSEMBLY NAME}.Shared
+   ```
+   
 1. In the `MainLayout` component (`Shared/MainLayout.razor`), surround the component's HTML markup with a `<div>` element that has a `class` attribute set to `page`:
 
     ```razor