WhiteSpace and PrefixParent subdirectory names throw exception (#27810)

Author: Anipik

src/System.IO.FileSystem/src/System/IO/DirectoryInfo.cs

@@ -56,14 +56,16 @@ public DirectoryInfo CreateSubdirectory(string path)
         {
             if (path == null)
                 throw new ArgumentNullException(nameof(path));
-            if (path.Length == 0)
+            if (PathInternal.IsEffectivelyEmpty(path))
                 throw new ArgumentException(SR.Argument_PathEmpty, nameof(path));
             if (Path.IsPathRooted(path))
                 throw new ArgumentException(SR.Arg_Path2IsRooted, nameof(path));
 
             string fullPath = Path.GetFullPath(Path.Combine(FullPath, path));
 
-            if (0 != string.Compare(FullPath, 0, fullPath, 0, FullPath.Length, PathInternal.StringComparison))
+            if (fullPath.Length < FullPath.Length 
+                || (fullPath.Length > FullPath.Length && !PathInternal.IsDirectorySeparator(fullPath[FullPath.Length]))
+                || string.Compare(FullPath, 0, fullPath, 0, FullPath.Length, PathInternal.StringComparison) != 0)
             {
                 throw new ArgumentException(SR.Format(SR.Argument_InvalidSubPath, path, FullPath), nameof(path));
             }

src/System.IO.FileSystem/tests/DirectoryInfo/CreateSubdirectory.cs

@@ -159,7 +159,16 @@ public void WindowsControlWhiteSpace_Core(string component)
 
         [Theory,
             MemberData(nameof(SimpleWhiteSpace))]
-        [PlatformSpecific(TestPlatforms.Windows)]  // Simple whitespace is trimmed in path
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void WindowsSimpleWhiteSpaceThrowsException(string component)
+        {
+            Assert.Throws<ArgumentException>(() => new DirectoryInfo(TestDirectory).CreateSubdirectory(component));
+        }
+
+        [Theory,
+            MemberData(nameof(SimpleWhiteSpace))]
+        [SkipOnTargetFramework(~TargetFrameworkMonikers.NetFramework)] // Simple whitespace is trimmed in path
         public void WindowsSimpleWhiteSpace(string component)
         {
             DirectoryInfo result = new DirectoryInfo(TestDirectory).CreateSubdirectory(component);
@@ -211,6 +220,16 @@ public void UNCPathWithOnlySlashes()
             Assert.Throws<ArgumentException>(() => testDir.CreateSubdirectory("//"));
         }
 
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        public void ParentDirectoryNameAsPrefixShouldThrow()
+        {
+            string randomName = GetTestFileName();
+            DirectoryInfo di = Directory.CreateDirectory(Path.Combine(TestDirectory, randomName));
+
+            Assert.Throws<ArgumentException>(() => di.CreateSubdirectory(Path.Combine("..", randomName + "abc", GetTestFileName())));
+        }
+
         #endregion
     }
 }