Fix DirectoryInfo.CreateSubdirectory for root directories

src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs

@@ -88,7 +88,9 @@ public DirectoryInfo CreateSubdirectory(string path)
             // We want to make sure the requested directory is actually under the subdirectory.
             if (trimmedNewPath.StartsWith(trimmedCurrentPath, PathInternal.StringComparison)
                 // Allow the exact same path, but prevent allowing "..\FooBar" through when the directory is "Foo"
-                && ((trimmedNewPath.Length == trimmedCurrentPath.Length) || PathInternal.IsDirectorySeparator(newPath[trimmedCurrentPath.Length])))
+                && ((trimmedNewPath.Length == trimmedCurrentPath.Length)
+                    || PathInternal.IsDirectorySeparator(newPath[trimmedCurrentPath.Length])
+                    || PathInternal.IsRoot(trimmedCurrentPath)))
             {
                 FileSystem.CreateDirectory(newPath);
                 return new DirectoryInfo(newPath);

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DirectoryInfo/CreateSubdirectory.cs

@@ -235,6 +235,50 @@ public void ParentDirectoryNameAsPrefixShouldThrow()
             Assert.Throws<ArgumentException>(() => di.CreateSubdirectory(Path.Combine("..", randomName + "abc", GetTestFileName())));
         }
 
+        [Fact]
+        public void CreateSubdirectoryFromRootDirectory()
+        {
+            string rootPath = Path.GetPathRoot(TestDirectory);
+            if (rootPath != null)
+            {
+                DirectoryInfo rootDir = new DirectoryInfo(rootPath);
+                string subDirName = GetTestFileName();
+
+                // For the actual root directory, we expect permission issues, but the validation should pass
+                // So we can't test actual creation, but we can ensure it doesn't throw ArgumentException
+                try
+                {
+                    DirectoryInfo result = rootDir.CreateSubdirectory(subDirName);
+
+                    // If we get here without ArgumentException, the validation passed
+                    Assert.NotNull(result);
+                    Assert.Equal(Path.Combine(rootPath, subDirName), result.FullName);
+
+                    // Clean up if somehow we managed to create it
+                    try
+                    {
+                        if (result.Exists)
+                            result.Delete();
+                    }
+                    catch
+                    {
+                        // Ignore cleanup errors
+                    }
+                }
+                catch (UnauthorizedAccessException)
+                {
+                    // This is expected for root directories - the validation passed but creation failed due to permissions
+                    // This is the correct behavior and indicates our fix worked
+                }
+                catch (IOException ex) when (ex.Message.Contains("Permission denied"))
+                {
+                    // This is also expected for root directories - the validation passed but creation failed due to permissions
+                    // This is the correct behavior and indicates our fix worked
+                }
+                // ArgumentException should NOT be thrown - if it is, the test will fail
+            }
+        }
+
         #endregion
     }
 }