Convert Blazor Server App to Blazor Web App

This skill helps an agent convert a pre-.NET 8 Blazor Server app into a .NET 8+ Blazor Web App. The old hosting model uses AddServerSideBlazor/MapBlazorHub with a _Host.cshtml Razor Page as the entry point. The new Blazor Web App model uses AddRazorComponents/MapRazorComponents with an App.razor root component, enabling per-component render modes, enhanced navigation, streaming rendering, and other .NET 8+ features. The converted app uses InteractiveServer render mode to preserve existing interactive behavior.

When to Use

• Migrating a Blazor Server app from .NET 6 or .NET 7 to .NET 8+
• App currently uses AddServerSideBlazor() and MapBlazorHub() in Program.cs (or Startup.cs)
• App uses Pages/_Host.cshtml (or _Host.razor) as the host page with Component Tag Helpers
• Want to adopt new Blazor Web App features while keeping interactive server rendering

When Not to Use

• The app already uses AddRazorComponents and MapRazorComponents. It is already a Blazor Web App — no conversion is needed. Stop here and tell the user the app is already using the Blazor Web App model.
• Blazor WebAssembly or hosted Blazor WebAssembly app — these have a different migration path
• The app should stay on the legacy Blazor Server hosting model (just update TFM and packages)
• The app targets .NET Framework — it must be migrated to .NET first

Inputs

Workflow

Commit strategy: Commit after each logical step so the migration is reviewable and bisectable.

Step 1: Update the project file

Update the .csproj file:

1. Change the Target Framework Moniker (TFM) to the target version:

   <TargetFramework>net8.0</TargetFramework>
   

2. Update all Microsoft.AspNetCore.*, Microsoft.EntityFrameworkCore.*, Microsoft.Extensions.*, and System.Net.Http.Json package references to the matching version.

For non-Blazor project file changes (nullable reference types, implicit usings, HTTP/3 support, etc.), see the general ASP.NET Core migration guide.

Step 2: Create Routes.razor from App.razor

The old App.razor contains the <Router> component. This content moves to a new Routes.razor file so that App.razor can become the root HTML document component.

1. Create a new file Routes.razor in the project root.
2. Move the entire content of App.razor into Routes.razor.
3. If the content is wrapped in <CascadingAuthenticationState>, remove that wrapper (it will be replaced by a service in Step 5).
4. Leave App.razor empty for the next step.

The resulting Routes.razor should look similar to:

<Router AppAssembly="@typeof(Program).Assembly">
    <Found Context="routeData">
        <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
        <FocusOnNavigate RouteData="@routeData" Selector="h1" />
    </Found>
    <NotFound>
        <LayoutView Layout="@typeof(MainLayout)">
            <p>Sorry, there's nothing at this address.</p>
        </LayoutView>
    </NotFound>
</Router>

If the app uses <AuthorizeRouteView> instead of <RouteView>, keep it — it works the same way in Blazor Web Apps.

Step 3: Convert _Host.cshtml to App.razor

Move the HTML shell from Pages/_Host.cshtml into the now-empty App.razor and transform it from a Razor Page into a Razor component:

1. Remove Razor Page directives — delete @page "/", @using Microsoft.AspNetCore.Components.Web, @namespace, and @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers.

2. Add component injection — if using environment-conditional error UI, add:

   @inject IHostEnvironment Env
   

3. Fix the base tag — replace <base href="~/" /> with <base href="/" />.

4. Replace HeadOutlet Component Tag Helper — replace:

   <component type="typeof(HeadOutlet)" render-mode="ServerPrerendered" />
   

   with:

   <HeadOutlet @rendermode="InteractiveServer" />
   

5. Replace App Component Tag Helper with Routes — replace:

   <component type="typeof(App)" render-mode="ServerPrerendered" />
   

   with:

   <Routes @rendermode="InteractiveServer" />
   

6. Replace Environment Tag Helpers — replace:

   <environment include="Staging,Production">
       An error has occurred. This application may no longer respond until reloaded.
   </environment>
   <environment include="Development">
       An unhandled exception has occurred. See browser dev tools for details.
   </environment>
   

   with:

   @if (Env.IsDevelopment())
   {
       <text>
           An unhandled exception has occurred. See browser dev tools for details.
       </text>
   }
   else
   {
       <text>
           An error has occurred. This app may no longer respond until reloaded.
       </text>
   }
   

7. Update the Blazor script — replace:

   <script src="_framework/blazor.server.js"></script>
   

   with:

   <script src="_framework/blazor.web.js"></script>
   

8. Add render mode import — add to _Imports.razor:

   @using static Microsoft.AspNetCore.Components.Web.RenderMode
   

9. Delete Pages/_Host.cshtml (and Pages/_Host.cshtml.cs if it exists).

Prerendering note: If the original app used render-mode="Server" (not "ServerPrerendered"), prerendering was disabled. Preserve this by using new InteractiveServerRenderMode(prerender: false) instead of InteractiveServer for both HeadOutlet and Routes.

Step 4: Update Program.cs

Make the following changes to Program.cs (or Startup.cs if the app uses the older hosting pattern):

1. Replace Blazor Server services — replace:

   builder.Services.AddServerSideBlazor();
   

   with:

   builder.Services.AddRazorComponents()
       .AddInteractiveServerComponents();
   

   If AddServerSideBlazor had options configured (e.g., circuit options, hub options, detailed errors), migrate them to AddInteractiveServerComponents:

   // Old:
   builder.Services.AddServerSideBlazor(options =>
   {
       options.DetailedErrors = true;
       options.DisconnectedCircuitRetentionPeriod = TimeSpan.FromMinutes(10);
   });
   
   // New:
   builder.Services.AddRazorComponents()
       .AddInteractiveServerComponents(options =>
       {
           options.DetailedErrors = true;
           options.DisconnectedCircuitRetentionPeriod = TimeSpan.FromMinutes(10);
       });
   

2. Replace Blazor endpoint mapping — replace:

   app.MapBlazorHub();
   

   with:

   app.MapRazorComponents<App>()
       .AddInteractiveServerRenderMode();
   

   Ensure there is a using statement for the project's root namespace so that App resolves to the App.razor component.

3. Remove the fallback route — delete:

   app.MapFallbackToPage("/_Host");
   

4. Remove explicit routing middleware — delete if present:

   app.UseRouting();
   

   Endpoint routing is the default and explicit UseRouting() is no longer needed.

5. Add antiforgery middleware — add after UseAuthentication/UseAuthorization if present:

   app.UseAntiforgery();
   

   AddRazorComponents registers antiforgery services automatically, but the middleware must be explicitly added to the pipeline. Without it, form POST requests fail with 400 errors.

Step 5: Migrate CascadingAuthenticationState (if present)

If the app used <CascadingAuthenticationState> to wrap the router:

1. Remove the <CascadingAuthenticationState> component wrapper (already done in Step 2 if following this workflow).
2. Add the cascading authentication state service in Program.cs:

   builder.Services.AddCascadingAuthenticationState();
   

The component wrapper approach does not work across render mode boundaries in Blazor Web Apps. The service-based approach provides Task<AuthenticationState> as a cascading value to all components regardless of render mode.

Step 6: Recommended improvements (optional)

These are optional modernization improvements — not required for the conversion to work. If you suggest any of these, state explicitly that they are optional.

• Replace UseStaticFiles with MapStaticAssets (.NET 9+): app.MapStaticAssets() provides optimized static file serving with fingerprinting, pre-compression, and content-based ETags. See MapStaticAssets documentation.
• Add @attribute [StreamRendering] to pages with async data loading (OnInitializedAsync) for improved perceived performance. The page renders its initial synchronous content immediately and re-renders when async data arrives.
• Update CSS isolation bundle reference if the <link> tag referenced a _Host assembly name; ensure it matches the project's actual assembly name: <link href="{AssemblyName}.styles.css" rel="stylesheet" />.
• For other non-Blazor improvements (minimal hosting, HTTP/3, output caching, etc.), see the general ASP.NET Core migration guide.

Step 7: Verify the migration

1. Build the project targeting the new framework. Confirm no compile errors.
2. Search for remaining references to removed APIs:
   • AddServerSideBlazor
   • MapBlazorHub
   • MapFallbackToPage
   • blazor.server.js
   • _Host.cshtml
3. Run the app and verify:
   • Pages load and render correctly
   • Interactive features work (forms, event handlers, SignalR circuits)
   • Navigation between pages works
   • Authentication and authorization flows work if present
4. Run existing tests.

Validation

• No references to AddServerSideBlazor remain
• No references to MapBlazorHub remain
• No references to MapFallbackToPage("/_Host") remain
• No references to blazor.server.js remain
• Pages/_Host.cshtml has been deleted
• App.razor serves as the root component with a full HTML document structure
• Routes.razor contains the <Router> configuration
• Program.cs uses AddRazorComponents().AddInteractiveServerComponents()
• Program.cs uses MapRazorComponents<App>().AddInteractiveServerRenderMode()
• app.UseAntiforgery() is present in the middleware pipeline
• If the app used <CascadingAuthenticationState>, it has been replaced with AddCascadingAuthenticationState() service registration
• App builds and runs successfully on the target framework

Common Pitfalls

More Info

• Convert a Blazor Server app into a Blazor Web App — the official step-by-step migration guide
• ASP.NET Core Blazor render modes — understanding InteractiveServer, InteractiveWebAssembly, and InteractiveAuto
• Migrate CascadingAuthenticationState to services — replacing the component wrapper with a service
• MapStaticAssets — optimized static file serving in .NET 9+
• Migrate from ASP.NET Core 7.0 to 8.0 — general migration guide for all ASP.NET Core changes
• Stream rendering with Blazor — @attribute [StreamRendering] for async data loading
• Cascading values and render mode boundaries — why cascading parameters do not cross render mode boundaries