Support Windows shell virtual files in drag and drop (#21907)

* Add Win32 virtual file drop support

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* docs(win32): clarify virtual file disposal

* fix(win32): handle empty virtual file reads

* Address virtual file data review feedback

* Address virtual stream review feedback

* Address final virtual stream review nits

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

Author: 3 people

src/Windows/Avalonia.Win32/Avalonia.Win32.csproj

@@ -35,6 +35,7 @@
   <ItemGroup>
     <InternalsVisibleTo Include="Avalonia.Win32.Interoperability, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.WinUI, PublicKey=$(AvaloniaPublicKey)" />
+    <InternalsVisibleTo Include="Avalonia.IntegrationTests.Win32, PublicKey=$(AvaloniaPublicKey)" />
   </ItemGroup>
   <ItemGroup>
     <!-- By default, any projects supports Windows, Linux, MacOS platforms. -->

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1640,6 +1640,14 @@ public static IntPtr GetClassLongPtr(IntPtr hWnd, int nIndex)
         internal static extern int CoCreateInstance(in Guid clsid,
             IntPtr ignore1, int ignore2, in Guid iid, [Out] out IntPtr pUnkOuter);
 
+        [DllImport("ole32.dll", PreserveSig = true)]
+        internal static extern int CoMarshalInterThreadInterfaceInStream(
+            ref Guid riid, IntPtr unknown, out IntPtr stream);
+
+        [DllImport("ole32.dll", PreserveSig = true)]
+        internal static extern int CoGetInterfaceAndReleaseStream(
+            IntPtr stream, ref Guid riid, out IntPtr unknown);
+
         internal static T CreateInstance<T>(in Guid clsid, in Guid iid) where T : IUnknown
         {
             var hresult = CoCreateInstance(in clsid, IntPtr.Zero, 1, in iid, out IntPtr pUnk);
@@ -2646,6 +2654,7 @@ public APPBARDATA()
         public const uint DV_E_FORMATETC = 0x80040064;
         public const uint OLE_E_ADVISENOTSUPPORTED = 0x80040003;
         public const uint COR_E_OBJECTDISPOSED = 0x80131622;
+        public const int STATFLAG_NONAME = 1;
 
         [StructLayout(LayoutKind.Sequential)]
         public struct DROPFILES
@@ -2674,6 +2683,41 @@ public struct FORMATETC
             public TYMED tymed;
         }
 
+        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
+        internal unsafe struct FILEDESCRIPTORW
+        {
+            public const uint FD_FILESIZE = 0x00000040;
+            public const int FileNameLength = 260;
+
+            public uint dwFlags;
+            public Guid clsid;
+            public SIZE sizel;
+            public POINT pointl;
+            public uint dwFileAttributes;
+            public FILETIME ftCreationTime;
+            public FILETIME ftLastAccessTime;
+            public FILETIME ftLastWriteTime;
+            public uint nFileSizeHigh;
+            public uint nFileSizeLow;
+            public fixed char cFileName[FileNameLength];
+        }
+
+        [StructLayout(LayoutKind.Sequential)]
+        internal struct STATSTG
+        {
+            public IntPtr pwcsName;
+            public uint type;
+            public ulong cbSize;
+            public FILETIME mtime;
+            public FILETIME ctime;
+            public FILETIME atime;
+            public uint grfMode;
+            public uint grfLocksSupported;
+            public Guid clsid;
+            public uint grfStateBits;
+            public uint reserved;
+        }
+
         [Flags]
         public enum GlobalAllocFlags : uint
         {

src/Windows/Avalonia.Win32/OleDataObjectHelper.cs

@@ -332,7 +332,7 @@ private static List<string> ReadFileNamesFromHGlobal(IntPtr hGlobal)
         return files;
     }
 
-    private static byte[] ReadBytesFromHGlobal(IntPtr hGlobal)
+    internal static byte[] ReadBytesFromHGlobal(IntPtr hGlobal)
     {
         var source = GlobalLock(hGlobal);
         try

src/Windows/Avalonia.Win32/OleDataObjectToDataTransferWrapper.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using System.Runtime.InteropServices.ComTypes;
@@ -33,20 +33,34 @@ protected override DataFormat[] ProvideFormats()
             formats.Add(format);
 
         bool hasSupportedImageFormat = false;
+        bool hasFile = false;
+        bool hasFileGroupDescriptor = false;
+        bool hasFileContents = false;
 
         foreach (var format in formats)
         {
-            if (ClipboardFormatRegistry.ImageFormats.Contains(format)) {
+            if (ClipboardFormatRegistry.ImageFormats.Contains(format))
                 hasSupportedImageFormat = true;
-                break;
-            }
+
+            if (DataFormat.File.Equals(format))
+                hasFile = true;
+            else if (OleVirtualFileData.FileGroupDescriptorFormat.Equals(format))
+                hasFileGroupDescriptor = true;
+            else if (OleVirtualFileData.FileContentsFormat.Equals(format))
+                hasFileContents = true;
         }
 
         if (hasSupportedImageFormat)
         {
             formats.Add(DataFormat.Bitmap);
         }
 
+        // Shell virtual files have descriptors and indexed contents, but no CF_HDROP path.
+        if (hasFileGroupDescriptor && hasFileContents && !hasFile)
+        {
+            formats.Add(DataFormat.File);
+        }
+
         return formats.ToArray();
 
         static unsafe DataFormat? Next(IEnumFORMATETC enumFormat)
@@ -87,14 +101,21 @@ protected override PlatformDataTransferItem[] ProvideItems()
                     foreach (var storageItem in storageItems)
                         items.Add(PlatformDataTransferItem.Create(DataFormat.File, storageItem));
                 }
+                else if (OleVirtualFileData.TryCreateFiles(_oleDataObject) is { Count: > 0 } virtualFiles)
+                {
+                    hasFiles = true;
+
+                    foreach (var virtualFile in virtualFiles)
+                        items.Add(PlatformDataTransferItem.Create(DataFormat.File, virtualFile));
+                }
             }
             else
                 (nonFileFormats ??= new()).Add(format);
         }
 
         // Single item containing all formats except for DataFormat.File.
         if (nonFileFormats is not null)
-            items.Add(new OleDataObjectToDataTransferItemWrapper(_oleDataObject, Formats));
+            items.Add(new OleDataObjectToDataTransferItemWrapper(_oleDataObject, nonFileFormats.ToArray()));
 
         return items.ToArray();
     }