I can successfully use the provided reproduction program. It works at least on Windows 10, 11, x86 x64 in Release and Debug configuration, when ran without debugging it.

DV_E_FORMATETC (0x80040064 ) is just a standard error code that means the data object doesn't support the clipboard format that's requested. This error is common in copy/paste, drag&drop and clipboard operations.

What doesn't work at all is:

• Putting a breakpoint in the Drag & Drop handling code, for example in void System.Runtime.InteropServices.ComTypes.IDataObject.GetData(ref FORMATETC format, out STGMEDIUM medium) while running in Debug mode.
• Raising an exception in the Drag & Drop handling code when running in Debug mode.

I don't think you can debug a .NET program (with today tools) with breakpoints or exceptions being throw when in a DoDragDrop function call.

DoDragDrop is a big modal loop that eats most messages sent to a window. Unfortunately the .NET debugger facility or exception handler (in clr.dll) seems to also wait on this loop for some reason. So, hitting a breakpoint or handling an exception creates a hang in the program itself (and in the debugger too) that you can only break the dead lock by killing the debugged process the hard way (taskkill /im myprocess.exe /f command line for example).

So my recommendation is to:

• avoid putting breakpoints in these portions of code

• wrap these functions in try/catch block and use some non-intrusive trace mechanism, something like this:

    void System.Runtime.InteropServices.ComTypes.IDataObject.GetData(ref FORMATETC format, out STGMEDIUM medium)
    {
        try
        {
            ....
        }
        catch(Exception ex)
        {
            Trace.WriteLine("GetData Error: " + ex);
            throw; // rethrow as is
        }
    }