ASP.NET Core - MapFallbackToFile() with condition based on subdomain

Question

I have a YARP gateway that serves static SPA files.

//...
app.MapReverseProxy();
app.MapFallbackToFile("index.html");
//...

Now I need to serve another bundle of static SPA files for a specific subdomain.

So if hostname starts with for example admin. (never mind the domain) I need to serve the index.html inside wwwroot/admin, otherwise I need to serve the index.html inside wwwroot (which I can move to another folder as well).

How can I achieve that?

Answer 1

This is what worked for me, it's based on the MapFallbackToFile() source code.

app.MapFallback(CreateRequestDelegate(app, "index.html"))
   .WithMetadata(new HttpMethodMetadata(new[] { HttpMethods.Get, HttpMethods.Head }));

static RequestDelegate CreateRequestDelegate(
    IEndpointRouteBuilder endpoints,
    string filePath,
    StaticFileOptions? options = null)
{
    var app = endpoints.CreateApplicationBuilder();
    app.Use(next => context =>
    {
        if (context.Request.Host.Host.StartsWith("admin", StringComparison.InvariantCultureIgnoreCase))
        {
            filePath = "admin/" + filePath;
        }

        context.Request.Path = "/" + filePath;

        // Set endpoint to null so the static files middleware will handle the request.
        context.SetEndpoint(null);

        return next(context);
    });

    if (options == null)
    {
        app.UseStaticFiles();
    }
    else
    {
        app.UseStaticFiles(options);
    }

    return app.Build();
}

The problem with @Guru's answer was that it made the browser to download the HTML file instead of rendering it.