Changes after review

src/Avalonia.MicroCom/MicroComProxyBase.cs

@@ -61,6 +61,22 @@ namespace Avalonia.MicroCom
             throw new COMException("QueryInterface failed", rv);
         }
 
+        public bool TryQueryInterface<T>(out T result) where T : IUnknown
+        {
+            var guid = MicroComRuntime.GetGuidFor(typeof(T));
+            var rv = QueryInterface(guid, out var ppv);
+            if (rv == 0)
+            {
+                result = (T)MicroComRuntime.CreateProxyFor(typeof(T), ppv, true);
+                return true;
+            }
+            else
+            {
+                result = default;
+                return false;
+            }
+        }
+
         public bool IsDisposed => _nativePointer == IntPtr.Zero;
 
         protected virtual void Dispose(bool disposing)

src/Windows/Avalonia.Win32/ClipboardImpl.cs

@@ -83,7 +83,7 @@ namespace Avalonia.Win32
 
             while (true)
             {
-                var ptr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDataObject>(wrapper, true);
+                var ptr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDataObject>(wrapper);
                 var hr = UnmanagedMethods.OleSetClipboard(ptr);
 
                 if (hr == 0)
@@ -107,8 +107,8 @@ namespace Avalonia.Win32
 
                 if (hr == 0)
                 {
-                    using var proxy = MicroCom.MicroComRuntime.CreateProxyFor<Win32Com.IDataObject>(dataObject, false);
-                    var wrapper = new OleDataObject(proxy);
+                    using var proxy = MicroCom.MicroComRuntime.CreateProxyFor<Win32Com.IDataObject>(dataObject, true);
+                    using var wrapper = new OleDataObject(proxy);
                     var formats = wrapper.GetDataFormats().ToArray();
                     return formats;
                 }
@@ -131,8 +131,8 @@ namespace Avalonia.Win32
 
                 if (hr == 0)
                 {
-                    using var proxy = MicroCom.MicroComRuntime.CreateProxyFor<Win32Com.IDataObject>(dataObject, false);
-                    var wrapper = new OleDataObject(proxy);
+                    using var proxy = MicroCom.MicroComRuntime.CreateProxyFor<Win32Com.IDataObject>(dataObject, true);
+                    using var wrapper = new OleDataObject(proxy);
                     var rv = wrapper.Get(format);
                     return rv;
                 }

src/Windows/Avalonia.Win32/DataObject.cs

@@ -16,33 +16,43 @@ using IDataObject = Avalonia.Input.IDataObject;
 
 namespace Avalonia.Win32
 {
+    public interface IDisposableDataObject : IDataObject, IDisposable { }
+
     internal static class DataObjectEx
     {
-        public static unsafe IDataObject GetAvaloniaObject(this Win32Com.IDataObject pDataObj)
+        public static unsafe IDisposableDataObject GetAvaloniaObjectFromCOM(this Win32Com.IDataObject pDataObj)
         {
-            switch (pDataObj)
+            if (pDataObj is null)
+            {
+                throw new ArgumentNullException(nameof(pDataObj));
+            }
+            if (pDataObj is IDisposableDataObject disposableDataObject)
+            {
+                return disposableDataObject;
+            }
+
+            // If DataObject was created on the our side (drag'n'drop initiated by Avalonia),
+            // then pDataObj will implement IAvnDataObject interface as well. So we can safely case it back to DataObject.
+            if (((MicroComProxyBase)pDataObj).TryQueryInterface<Win32Com.IAvnDataObject>(out var avnInterface))
+            {
+                using (avnInterface)
+                {
+                    var ppv = MicroComRuntime.GetNativeIntPtr(avnInterface);
+                    return MicroComRuntime.GetObjectFromCcw(ppv) as DataObject;
+                }
+            }
+            // Otherwise wrap pDataObj into OleDataObject.
+            else
             {
-                case null:
-                    throw new ArgumentNullException(nameof(pDataObj));
-                case DataObject avnObj:
-                    return avnObj;
-                case OleDataObject oleAvnObj:
-                    return oleAvnObj;
-                default:
-                    var guid = MicroComRuntime.GetGuidFor(typeof(Win32Com.IAvnDataObject));
-                    var rv = ((MicroComProxyBase)pDataObj).QueryInterface(guid, out var ppv);
-                    return rv == 0
-                        ? MicroComRuntime.GetObjectFromCcw(ppv) as DataObject
-                        : new OleDataObject(pDataObj);
+                return new OleDataObject(pDataObj);
             }
         }
     }
 
-    internal class DataObject : IDataObject, Win32Com.IAvnDataObject, IMicroComShadowContainer
+    internal class DataObject : IDisposableDataObject, Win32Com.IAvnDataObject, IMicroComShadowContainer
     {
         // Compatibility with WinForms + WPF...
         internal static readonly byte[] SerializedObjectGUID = new Guid("FD9EA796-3B13-4370-A679-56106BB288FB").ToByteArray();
-        private static Dictionary<IntPtr, IDataObject> _avnDataObject = new Dictionary<IntPtr, IDataObject>();
 
         class FormatEnumerator : Win32Com.IEnumFORMATETC, IMicroComShadowContainer
         {
@@ -143,7 +153,7 @@ namespace Avalonia.Win32
 
         public DataObject(IDataObject wrapped)
         {
-            if (_wrapped is DataObject)
+            if (_wrapped is DataObject || _wrapped is OleDataObject)
             {
                 throw new InvalidOperationException();
             }

src/Windows/Avalonia.Win32/DragSource.cs

@@ -15,12 +15,12 @@ namespace Avalonia.Win32
 
             triggerEvent.Pointer.Capture(null);
             
-            var dataObject = new DataObject(data);
-            var src = new OleDragSource();
+            using var dataObject = new DataObject(data);
+            using var src = new OleDragSource();
             var allowed = OleDropTarget.ConvertDropEffect(allowedEffects);
             
-            var objPtr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDataObject>(dataObject, true);
-            var srcPtr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDropSource>(src, true);
+            var objPtr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDataObject>(dataObject);
+            var srcPtr = MicroCom.MicroComRuntime.GetNativeIntPtr<Win32Com.IDropSource>(src);
 
             UnmanagedMethods.DoDragDrop(objPtr, srcPtr, (int)allowed, out var finalEffect);
             return Task.FromResult(OleDropTarget.ConvertDropEffect((Win32Com.DropEffect)finalEffect));

src/Windows/Avalonia.Win32/OleContext.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Win32
                 return false;
             }
 
-            var trgPtr = MicroCom.MicroComRuntime.GetNativeIntPtr(target, false);
+            var trgPtr = MicroCom.MicroComRuntime.GetNativeIntPtr(target);
             return UnmanagedMethods.RegisterDragDrop(hwnd.Handle, trgPtr) == UnmanagedMethods.HRESULT.S_OK;
         }
 

src/Windows/Avalonia.Win32/OleDataObject.cs

@@ -8,17 +8,18 @@ using System.Runtime.InteropServices.ComTypes;
 using System.Runtime.Serialization.Formatters.Binary;
 using System.Text;
 using Avalonia.Input;
+using Avalonia.MicroCom;
 using Avalonia.Win32.Interop;
 
 namespace Avalonia.Win32
 {
-    internal class OleDataObject : Avalonia.Input.IDataObject
+    internal class OleDataObject : IDisposableDataObject
     {
         private readonly Win32Com.IDataObject _wrapped;
 
         public OleDataObject(Win32Com.IDataObject wrapped)
         {
-            _wrapped = wrapped;
+            _wrapped = wrapped.CloneReference();
         }
 
         public bool Contains(string dataFormat)
@@ -183,5 +184,10 @@ namespace Avalonia.Win32
             }
             return formatsList;
         }
+
+        public void Dispose()
+        {
+            _wrapped.Dispose();
+        }
     }
 }

src/Windows/Avalonia.Win32/OleDropTarget.cs

@@ -3,7 +3,6 @@ using Avalonia.Input.Raw;
 using Avalonia.MicroCom;
 using Avalonia.Platform;
 using Avalonia.Win32.Interop;
-using IDataObject = Avalonia.Input.IDataObject;
 using DropEffect = Avalonia.Win32.Win32Com.DropEffect;
 
 namespace Avalonia.Win32
@@ -14,7 +13,7 @@ namespace Avalonia.Win32
         private readonly ITopLevelImpl _tl;
         private readonly IDragDropDevice _dragDevice;
         
-        private IDataObject _currentDrag = null;
+        private IDisposableDataObject _currentDrag = null;
 
         public MicroComShadow Shadow { get; set; }
 
@@ -77,7 +76,12 @@ namespace Avalonia.Win32
                 *pdwEffect= (int)DropEffect.None;
             }
 
-            _currentDrag = pDataObj.GetAvaloniaObject();
+            var newDrag = pDataObj.GetAvaloniaObjectFromCOM();
+            if (_currentDrag != newDrag)
+            {
+                _currentDrag?.Dispose();
+                _currentDrag = newDrag;
+            }
 
             var args = new RawDragEvent(
                 _dragDevice,
@@ -85,7 +89,7 @@ namespace Avalonia.Win32
                 _target,
                 GetDragLocation(pt),
                 _currentDrag, 
-                ConvertDropEffect((*pdwEffect)),
+                ConvertDropEffect(*pdwEffect),
                 ConvertKeyState(grfKeyState)
             );
             dispatch(args);
@@ -129,6 +133,7 @@ namespace Avalonia.Win32
             }
             finally
             {
+                _currentDrag?.Dispose();
                 _currentDrag = null;
             }
         }
@@ -143,7 +148,12 @@ namespace Avalonia.Win32
                     *pdwEffect = (int)DropEffect.None;
                 }
 
-                _currentDrag = pDataObj.GetAvaloniaObject();
+                var newDrag = pDataObj.GetAvaloniaObjectFromCOM();
+                if (_currentDrag != newDrag)
+                {
+                    _currentDrag?.Dispose();
+                    _currentDrag = newDrag;
+                }
 
                 var args = new RawDragEvent(
                     _dragDevice, 
@@ -159,6 +169,7 @@ namespace Avalonia.Win32
             }
             finally
             {
+                _currentDrag?.Dispose();
                 _currentDrag = null;
             }
         }
@@ -171,6 +182,7 @@ namespace Avalonia.Win32
 
         public void Dispose()
         {
+            _currentDrag?.Dispose();
         }
 
         public void OnReferencedFromNative()

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -529,6 +529,7 @@ namespace Avalonia.Win32
             if (_dropTarget != null)
             {
                 OleContext.Current?.UnregisterDragDrop(Handle);
+                _dropTarget.Dispose();
                 _dropTarget = null;
             }