[release/9.0] Add .slnx fallback for TestHost content root discovery

src/Hosting/TestHost/src/WebHostBuilderExtensions.cs

@@ -163,6 +163,23 @@ public static IWebHostBuilder UseSolutionRelativeContentRoot(
         }
         while (directoryInfo is not null);
 
+        if (string.Equals(solutionName, "*.sln", StringComparison.OrdinalIgnoreCase))
+        {
+            directoryInfo = new DirectoryInfo(applicationBasePath);
+            do
+            {
+                var solutionPath = Directory.EnumerateFiles(directoryInfo.FullName, "*.slnx").FirstOrDefault();
+                if (solutionPath != null)
+                {
+                    builder.UseContentRoot(Path.GetFullPath(Path.Combine(directoryInfo.FullName, solutionRelativePath)));
+                    return builder;
+                }
+
+                directoryInfo = directoryInfo.Parent;
+            }
+            while (directoryInfo is not null);
+        }
+
         throw new InvalidOperationException($"Solution root could not be located using application root {applicationBasePath}.");
     }
 

src/Hosting/TestHost/test/WebHostBuilderExtensionsTests.cs

@@ -0,0 +1,67 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.IO;
+using Microsoft.AspNetCore.Hosting;
+using Xunit;
+
+namespace Microsoft.AspNetCore.TestHost;
+
+public class WebHostBuilderExtensionsTests
+{
+    [Fact]
+    public void UseSolutionRelativeContentRoot_FallsBackToSlnx()
+    {
+        var rootPath = Path.Combine(Path.GetTempPath(), "slnx-" + Guid.NewGuid().ToString("n"));
+        var applicationBasePath = Path.Combine(rootPath, "src", "MyApp", "bin");
+
+        try
+        {
+            Directory.CreateDirectory(applicationBasePath);
+            File.WriteAllText(Path.Combine(rootPath, "Test.slnx"), string.Empty);
+            Directory.CreateDirectory(Path.Combine(rootPath, "src", "MyApp"));
+
+            var builder = new WebHostBuilder();
+
+            builder.UseSolutionRelativeContentRoot("src/MyApp", applicationBasePath, "*.sln");
+
+            Assert.Equal(Path.GetFullPath(Path.Combine(rootPath, "src", "MyApp")), builder.GetSetting(WebHostDefaults.ContentRootKey));
+        }
+        finally
+        {
+            if (Directory.Exists(rootPath))
+            {
+                Directory.Delete(rootPath, recursive: true);
+            }
+        }
+    }
+
+    [Fact]
+    public void UseSolutionRelativeContentRoot_PrefersSlnOverSlnx()
+    {
+        var rootPath = Path.Combine(Path.GetTempPath(), "sln-" + Guid.NewGuid().ToString("n"));
+        var applicationBasePath = Path.Combine(rootPath, "child", "content", "bin");
+
+        try
+        {
+            Directory.CreateDirectory(applicationBasePath);
+            File.WriteAllText(Path.Combine(rootPath, "Parent.sln"), string.Empty);
+            File.WriteAllText(Path.Combine(rootPath, "child", "Child.slnx"), string.Empty);
+            Directory.CreateDirectory(Path.Combine(rootPath, "parentContent"));
+
+            var builder = new WebHostBuilder();
+
+            builder.UseSolutionRelativeContentRoot("parentContent", applicationBasePath, "*.sln");
+
+            Assert.Equal(Path.GetFullPath(Path.Combine(rootPath, "parentContent")), builder.GetSetting(WebHostDefaults.ContentRootKey));
+        }
+        finally
+        {
+            if (Directory.Exists(rootPath))
+            {
+                Directory.Delete(rootPath, recursive: true);
+            }
+        }
+    }
+}