/convert-blazor-server-to-webapp

name: convert-blazor-server-to-webapp
license: MIT
description: >
  Guides conversion of a pre-.NET 8 Blazor Server app into a .NET 8+ Blazor Web App.
  USE FOR: migrating apps that use AddServerSideBlazor and MapBlazorHub to the
  AddRazorComponents/MapRazorComponents model, converting _Host.cshtml to an App.razor
  root component, replacing blazor.server.js with blazor.web.js, migrating
  CascadingAuthenticationState to a service, adopting new Blazor Web App features
  like enhanced navigation and streaming rendering.
  DO NOT USE FOR: apps that are already Blazor Web Apps (already use AddRazorComponents
  and MapRazorComponents), Blazor WebAssembly or hosted Blazor WebAssembly apps
  (different migration path), apps that should stay on the Blazor Server hosting
  model without converting, or apps still targeting .NET Framework.

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:

   <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](https://learn.microsoft.com/aspnet/core/migration/70-to-80).

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())
   {