Add test for multiple ConfigureWebHost calls with UseStartup

Co-authored-by: BrennanConroy <[email protected]>

Author: Copilot

src/Hosting/Hosting/test/GenericWebHostBuilderTests.cs

@@ -2,8 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Linq;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
@@ -128,6 +130,59 @@ public void ReadsUrlsOrPorts(string urls, string httpPorts, string httpsPorts, s
         Assert.Equal(expected, string.Join(';', server.Addresses));
     }
 
+    [Fact]
+    public async Task MultipleConfigureWebHostCallsWithUseStartupLastWins()
+    {
+        var server = new TestServer();
+        
+        using var host = new HostBuilder()
+            .ConfigureWebHost(webHostBuilder =>
+            {
+                webHostBuilder
+                    .UseServer(server)
+                    .UseStartup<FirstStartup>();
+            })
+            .ConfigureWebHost(webHostBuilder =>
+            {
+                webHostBuilder
+                    .UseStartup<SecondStartup>();
+            })
+            .Build();
+
+        await host.StartAsync();
+        await AssertResponseContains(server.RequestDelegate, "SecondStartup");
+    }
+
+    private async Task AssertResponseContains(RequestDelegate app, string expectedText)
+    {
+        var httpContext = new DefaultHttpContext();
+        httpContext.Response.Body = new MemoryStream();
+        await app(httpContext);
+        httpContext.Response.Body.Seek(0, SeekOrigin.Begin);
+        var bodyText = new StreamReader(httpContext.Response.Body).ReadToEnd();
+        Assert.Contains(expectedText, bodyText);
+    }
+
+    private class FirstStartup
+    {
+        public void ConfigureServices(IServiceCollection services) { }
+        
+        public void Configure(IApplicationBuilder app)
+        {
+            app.Run(context => context.Response.WriteAsync("FirstStartup"));
+        }
+    }
+
+    private class SecondStartup
+    {
+        public void ConfigureServices(IServiceCollection services) { }
+        
+        public void Configure(IApplicationBuilder app)
+        {
+            app.Run(context => context.Response.WriteAsync("SecondStartup"));
+        }
+    }
+
     private class TestServer : IServer, IServerAddressesFeature
     {
         public TestServer()
@@ -139,9 +194,31 @@ public TestServer()
 
         public ICollection<string> Addresses { get; } = new List<string>();
         public bool PreferHostingUrls { get; set; }
+        public RequestDelegate RequestDelegate { get; private set; }
 
-        public Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) => Task.CompletedTask;
-        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
         public void Dispose() { }
+
+        public Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken)
+        {
+            // For testing that uses RequestDelegate
+            RequestDelegate = async ctx =>
+            {
+                var httpContext = application.CreateContext(ctx.Features);
+                try
+                {
+                    await application.ProcessRequestAsync(httpContext);
+                }
+                catch (Exception ex)
+                {
+                    application.DisposeContext(httpContext, ex);
+                    throw;
+                }
+                application.DisposeContext(httpContext, null);
+            };
+
+            return Task.CompletedTask;
+        }
+
+        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
     }
 }