Changing RouteData in Asp.Net Core 3.1 in Middleware - asp.net-core-3.1

Question

I've recently updated my .Net Core 2.1 application to 3.1 and there is one part that has not upgraded as expected.
I'd written code to map subdomains to areas as described here
I now realise that the method of using app.UseMvc() should be replaced with app.UseEndpoints() but I can't find anywhere in the 3.1 framework that will allow me to write to the RouteData before app.UseEndpoints()
//update RouteData before this
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
"admin", "{area:exists}/{controller=Home}/{action=Index}/{id?}");
endpoints.MapControllerRoute(
"default", "{controller=Home}/{action=Index}/{id?}");
});
Is there a way to write to RouteData using Middleware?
I've tried going back to app.UseMvc() as it's still in the framework but MvcRouteHandler doesn't seem to exist anymore
app.UseMvc(routes =>
{
routes.DefaultHandler = new MvcRouteHandler(); //The type of namespace could not be found
routes.MapRoute(
"admin",
"{area:exists}/{controller=Home}/{action=Index}/{id?}");
routes.MapRoute(
"default",
"{controller=Home}/{action=Index}/{id?}");
});

Answer 1

Try to use custom middleware.
Add a reference to using Microsoft.AspNetCore.Routing and use Httpcontext.GetRouteData() method to achieve RouteData
app.UseRouting();
app.Use(async (context, next) =>
{
string url = context.Request.Headers["HOST"];
var splittedUrl = url.Split('.');
if (splittedUrl != null && (splittedUrl.Length > 0 && splittedUrl[0] == "admin"))
{
context.GetRouteData().Values.Add("area", "Admin");
}
// Call the next delegate/middleware in the pipeline
await next();
});
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
"admin", "{area:exists}/{controller=Home}/{action=Index}/{id?}");
endpoints.MapControllerRoute(
"default", "{controller=Home}/{action=Index}/{id?}");
});