Track disposal of NativeImageList

src/System.Windows.Forms/src/System/Windows/Forms/ImageList.NativeImageList.cs

@@ -1,9 +1,8 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
+using System.Diagnostics;
 using static Interop;
 
 namespace System.Windows.Forms
@@ -12,20 +11,19 @@ namespace System.Windows.Forms
     {
         internal class NativeImageList : IDisposable, IHandle
         {
-            private IntPtr _himl;
 #if DEBUG
             private readonly string _callStack;
 #endif
 
             internal NativeImageList(IntPtr himl)
             {
-                _himl = himl;
+                Handle = himl;
 #if DEBUG
                 _callStack = Environment.StackTrace;
 #endif
             }
 
-            public IntPtr Handle => _himl;
+            public IntPtr Handle { get; private set; }
 
             public void Dispose()
             {
@@ -35,14 +33,25 @@ namespace System.Windows.Forms
 
             public void Dispose(bool disposing)
             {
-                if (_himl != IntPtr.Zero)
+                if (Handle != IntPtr.Zero)
                 {
-                    ComCtl32.ImageList.Destroy(_himl);
-                    _himl = IntPtr.Zero;
+                    ComCtl32.ImageList.Destroy(Handle);
+                    Handle = IntPtr.Zero;
                 }
+
+#if DEBUG
+                GC.SuppressFinalize(this);
+#endif
             }
 
-            ~NativeImageList() => Dispose(false);
+            ~NativeImageList()
+            {
+#if DEBUG
+                Debug.Fail($"{nameof(NativeImageList)} was not disposed properly. Originating stack:\n{_callStack}");
+#endif
+
+                Dispose(false);
+            }
         }
     }
 }