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

| Input | Required | Description | |-------|----------|-------------| | Blazor Server project | Yes | The .csproj and source files of the Blazor Server app | | Target framework | Yes | .NET 8 or later (e.g., net8.0, net9.0, net10.0) | | Program.cs or Startup.cs | Yes | The app's service and middleware configuration | | _Host.cshtml location | Recommended | Usually Pages/_Host.cshtml; may be _Host.razor in some projects |

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:

xml <TargetFramework>net8.0</TargetFramework>

1. 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:

razor
<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.

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

razor @inject IHostEnvironment Env

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

1. Replace HeadOutlet Component Tag Helper — replace:

html <component type="typeof(HeadOutlet)" render-mode="ServerPrerendered" /> with: razor <HeadOutlet @rendermode="InteractiveServer" />

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

html <component type="typeof(App)" render-mode="ServerPrerendered" /> with: razor <Routes @rendermode="InteractiveServer" />

1. Replace Environment Tag Helpers — replace:

html <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: razor @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> }

1. Update the Blazor script — replace:

html <script src="_framework/blazor.server.js"></script> with: html <script src="_framework/blazor.web.js"></script>

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

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

1. 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:

csharp builder.Services.AddServerSideBlazor(); with: csharp builder.Services.AddRazorComponents() .AddInteractiveServerComponents();

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

1. Replace Blazor endpoint mapping — replace:

csharp app.MapBlazorHub(); with: csharp 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.

1. Remove the fallback route — delete:

csharp app.MapFallbackToPage("/_Host");

1. Remove explicit routing middleware — delete if present:

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

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

csharp 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:

csharp 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

1. 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

1. 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

| Pitfall | Solution | |---------|----------| | Missing UseAntiforgery() middleware | AddRazorComponents registers antiforgery services, but the middleware must be explicitly added. Place app.UseAntiforgery() after UseAuthentication/UseAuthorization. Without it, form POST requests fail with 400 errors. | | Forgetting to replace blazor.server.js with blazor.web.js | The old script does not work with the Blazor Web App model. Replace all references to _framework/blazor.server.js with _framework/blazor.web.js. | | Not removing <CascadingAuthenticationState> wrapper | The component wrapper does not work across render mode boundaries in Blazor Web Apps. Use builder.Services.AddCascadingAuthenticationState() instead. | | Leaving app.UseRouting() in the pipeline | Explicit UseRouting() is no longer needed and can interfere with endpoint routing. Remove it unless other middleware specifically requires it. | | Using InteractiveServer when prerendering was disabled | If the original app used render-mode="Server" (not "ServerPrerendered"), use new InteractiveServerRenderMode(prerender: false) to preserve the same behavior. Using InteractiveServer enables prerendering which can cause unexpected issues with components that depend on JS interop during initialization. | | Not migrating AddServerSideBlazor circuit options | If circuit options, hub options, or detailed error settings were configured, migrate them to AddInteractiveServerComponents(options => { ... }). Otherwise those settings are silently lost. | | UseAntiforgery() placed before authentication middleware | The antiforgery middleware must be placed after UseAuthentication and UseAuthorization. Placing it before causes antiforgery validation to run before the user identity is established. | | CSS isolation bundle link has wrong assembly name | If the <link href="{Name}.styles.css"> tag referenced the old project name, update it to match the current assembly name. |

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