MapStaticAssets/MapRazorComponents not mapping expected URL when used with UsePathBase

[dougclutter]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

We have an existing .NET 8 application which has the following projects types:

• Microsoft.NET.Sdk.BlazorWebAssembly: Form 1 UI
• Microsoft.NET.Sdk.Web: API which also hosts the Form 1 UI project
• Microsoft.NET.Sdk.Razor: Razor Component Library (RCL) used by the Form 1 UI
• Microsoft.NET.Sdk: Services used by API project
• Microsoft.NET.Sdk: Class Library shared by Services, API, and the Form 1 UI projects

It has the following URLs:

• /forms/api/myService - provides APIs used by the UI
• /forms/form1/ - serves the Form 1 UI

We will soon be adding the following URLs:

• /forms/form2/ - serves Form 2 UI
• /forms/form3/ - serves Form 3 UI

These new projects will use the existing RCL and Class Library projects.

We have been unable to follow the guidance provide in the
Migration Docs
to update UseStaticFiles to MapStaticAssets.

In this solution, we have provided a minimal reproduction of the problems we are having.
This solution contains the following projects which were created using the current .NET 9 templates:

• MyApp: equivalent to our API project
• MyApp.Client: equivalent to our Form 1 UI project

We have updated these projects as follows:

• MyApp -> Program.cs: added app.UsePathBase("/forms")
• MyApp -> Program.cs: added options.PathPrefix = "/form1" to AddInteractiveWebAssemblyRenderMode per Prefix Docs
• MyApp -> App.razor: added <base href="/form1/" />
• MyApp.Client.csproj: tried adding <StaticWebAssetBasePath>form1</StaticWebAssetBasePath> but it didn't help

Expected Behavior

URLs are not returning expected values:

URL	Expected	Actual
/forms/form1	Form 1 UI	HTTP 404
/forms	HTTP 404	static HTML of App.razor
/	HTTP 404	static HTML of App.razor

Steps To Reproduce

Repo available at https://github.com/dougclutter/MapStaticAssetsIssues

Exceptions (if any)

No response

.NET Version

9.0.303

Anything else?

ASP.NET Core v9.0.7
VS Enterprise v17.14.9

dotnet --info
.NET SDK:
Version: 9.0.303
Commit: 5d97611193
Workload version: 9.0.300-manifests.183aaee6
MSBuild version: 17.14.13+65391c53b

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.303\

.NET workloads installed:
[android]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 35.0.78/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.android\35.0.78\WorkloadManifest.json
Install Type: Msi

[aspire]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 8.2.2/8.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.2\WorkloadManifest.json
Install Type: Msi

[ios]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 18.5.9207/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.ios\18.5.9207\WorkloadManifest.json
Install Type: Msi

[maccatalyst]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 18.5.9207/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maccatalyst\18.5.9207\WorkloadManifest.json
Install Type: Msi

[maui-windows]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 9.0.51/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maui\9.0.51\WorkloadManifest.json
Install Type: Msi

[wasm-tools]
Installation Source: SDK 9.0.300, VS 17.14.36310.24
Manifest Version: 9.0.7/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.current\9.0.7\WorkloadManifest.json
Install Type: Msi

Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.7
Architecture: x64
Commit: 3c298d9f00

.NET SDKs installed:
6.0.428 [C:\Program Files\dotnet\sdk]
7.0.410 [C:\Program Files\dotnet\sdk]
8.0.412 [C:\Program Files\dotnet\sdk]
9.0.302 [C:\Program Files\dotnet\sdk]
9.0.303 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

[byron-grogan]

I am hitting this issue as well on my blazor web apps (interactive server) hosted on IIS under a subpath.

When using app.UsePathBase("/CoolApp"), routing and base path behavior only works correctly if the path ends with a trailing slash Without the trailing slash, some requests fail or assets aren't resolved correctly.

• Works: mysite.com/CoolApp/
• Fails: mysite.com/CoolApp

[dougclutter]

@byron-grogan - that's weird. I tried accessing my sample app with a trailing slash and it still gives me a 404. We've given up on trying to convert our Blazor Hosted WASM into a Blazor Web App. .NET 9 just doesn't seem to allow this. Hopefully, .NET 10 will be better. :)

[javiercn]

@dougclutter thanks for contacting us.

When using UsePathBase you can't use the implicit routing feature. Essentially UsePathBase is being applied too late. You need to explicitly call app.UseRouting() after app.UsePathBase() and app.UseEndpoints(endpoints => ...) at the end of the pipeline so that the path base applies.

[dougclutter]

Hi @javiercn - thanks for the feedback.

I tried what you suggested (...at least I think I did), but it didn't seem to work. I added app.UseRouting() right after app.UseBasePath(), and I wrapped my calls to MapStaticAssets() and MapRazorComponents() in a app.UseEndpoints(). You can see my changes here.

I also tried not wrapping my calls to MapStaticAssets() and MapRazorComponents() in the app.UseEndpoints(), but that didn't work either.

I'm sure I'm missing something really simple here, but I'm just not seeing it. Do I need to wrap the UseEndpoints() call in a MapWhen as described here?

[javiercn]

@dougclutter Yes.

I asked copilot to create a simplified guide based on one of our tests. I think this should do it.

Guide: Setting Up a Blazor Application on a Subpath in ASP.NET Core

This guide walks you through configuring an ASP.NET Core Blazor app to run under a subpath (such as /subdir). The instructions are based on patterns implemented in the RazorComponentEndpointsStartup test server from the ASP.NET Core source code.

---

Minimal Sample

Below is a minimal sample of how to configure your Blazor app to run under a subpath and serve static assets:

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorComponents();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.Map("/subdir", subdirApp =>
        {
            subdirApp.UseRouting();
            subdirApp.UseEndpoints(endpoints =>
            {
                endpoints.MapStaticAssets();
                endpoints.MapRazorComponents<App>();
            });
        });
    }
}

• Replace App with your root Blazor component.
• All requests to /subdir will serve your Blazor app and static files.

---

Essential Steps

1. Map the Subpath:
   Use app.Map("/subdir", ...) to scope routing and endpoints to your desired subpath.

2. Serve Static Assets:
   Call endpoints.MapStaticAssets(); inside your subpath mapping to make static files (CSS, JS, images) available under /subdir.

3. Set the Base Tag:
   For client-side Blazor, set <base href="/subdir/"> in your index.html or _Host.cshtml.

4. Access Your App:
   After starting your host, your Blazor app will be accessible at https://yourdomain/subdir/.

---

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