Showing Image thumbnail with mouse cursor while dragging
See the question and my original answer on StackOverflowOfficially, you're supposed to use the IDragSourceHelper interface to add a preview bitmap to a Drag&Drop operation.
Unfortunately, this interface uses IDataObject::SetData method which is not implemented at COM level by the .NET DataObject class, only at .NET level.
The solution is to re-use an IDataObject provided by the Shell instead for any Shell Item (here a file), using the SHCreateItemFromParsingName function and the IShellItem::BindToHandler method.
Note these functions automatically add clipboard formats like FileDrop, but we still have to use IDragSourceHelper to add the preview image.
This is how you can use it:
...
// get IDataObject from the Shell so it can handle more formats
var dataObject = DataObjectUtilities.GetFileDataObject(imgPath);
// add the thumbnail to the data object
DataObjectUtilities.AddPreviewImage(dataObject, imgPath);
// start d&d
DragDrop.DoDragDrop(this, dataObject, DragDropEffects.All);
...
And here is the code:
public static class DataObjectUtilities
{
public static void AddPreviewImage(System.Runtime.InteropServices.ComTypes.IDataObject dataObject, string imgPath)
{
if (dataObject == null)
throw new ArgumentNullException(nameof(dataObject));
var ddh = (IDragSourceHelper)new DragDropHelper();
var dragImage = new SHDRAGIMAGE();
// note you should use a thumbnail here, not a full-sized image
var thumbnail = new System.Drawing.Bitmap(imgPath);
dragImage.sizeDragImage.cx = thumbnail.Width;
dragImage.sizeDragImage.cy = thumbnail.Height;
dragImage.hbmpDragImage = thumbnail.GetHbitmap();
Marshal.ThrowExceptionForHR(ddh.InitializeFromBitmap(ref dragImage, dataObject));
}
public static System.Runtime.InteropServices.ComTypes.IDataObject GetFileDataObject(string filePath)
{
if (filePath == null)
throw new ArgumentNullException(nameof(filePath));
Marshal.ThrowExceptionForHR(SHCreateItemFromParsingName(filePath, null, typeof(IShellItem).GUID, out var item));
Marshal.ThrowExceptionForHR(item.BindToHandler(null, BHID_DataObject, typeof(System.Runtime.InteropServices.ComTypes.IDataObject).GUID, out var dataObject));
return (System.Runtime.InteropServices.ComTypes.IDataObject)dataObject;
}
private static readonly Guid BHID_DataObject = new Guid("b8c0bd9f-ed24-455c-83e6-d5390c4fe8c4");
[DllImport("shell32", CharSet = CharSet.Unicode)]
private static extern int SHCreateItemFromParsingName(string path, System.Runtime.InteropServices.ComTypes.IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, out IShellItem ppv);
[Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
private interface IShellItem
{
[PreserveSig]
int BindToHandler(System.Runtime.InteropServices.ComTypes.IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid bhid, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, [MarshalAs(UnmanagedType.IUnknown)] out object ppv);
// other methods are not defined, we don't need them
}
[ComImport, Guid("4657278a-411b-11d2-839a-00c04fd918d0")] // CLSID_DragDropHelper
private class DragDropHelper
{
}
[StructLayout(LayoutKind.Sequential)]
private struct POINT
{
public int x;
public int y;
}
[StructLayout(LayoutKind.Sequential)]
private struct SIZE
{
public int cx;
public int cy;
}
// https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/ns-shobjidl_core-shdragimage
[StructLayout(LayoutKind.Sequential)]
private struct SHDRAGIMAGE
{
public SIZE sizeDragImage;
public POINT ptOffset;
public IntPtr hbmpDragImage;
public int crColorKey;
}
// https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nn-shobjidl_core-idragsourcehelper
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("DE5BF786-477A-11D2-839D-00C04FD918D0")]
private interface IDragSourceHelper
{
[PreserveSig]
int InitializeFromBitmap(ref SHDRAGIMAGE pshdi, System.Runtime.InteropServices.ComTypes.IDataObject pDataObject);
[PreserveSig]
int InitializeFromWindow(IntPtr hwnd, ref POINT ppt, System.Runtime.InteropServices.ComTypes.IDataObject pDataObject);
}
}