.NET 7.0 preview DragEnter bug after update to latest versions.

[c-ohle]

Description

When trying to drag and drop files from folders or other sources into a Windows.Forms.Controls:

NotImplementedException in IDropTargetVtbl.cs
VS2022 preview, .NET 7.0 preview, SDK, all latest versions.

It works compiled for .NET 6.0, it worked for all .NET 7.0 previews before.
Example project is available at GitHub

Reproduction Steps

Should be easy to reproduce:
VS2022 - New Project - Windows Forms App - .NET 7.0 preview
Form1 - AllowDrop = true; - override OnDragEnter
on file drop -> exception

Expected behavior

Should work or message/exception if there is a security problem or what ever.

Actual behavior

NotImplementedException

Regression?

No response

Known Workarounds

no

Configuration

Processor: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz (2112 MHz) × 8
System: Microsoft Windows 10.0.22622 X64 Runtime: .NET 7.0.0-preview.6.22324.4 X64
BigRational: 1.0.0.5 Runtime: v4.0.30319 .NETCoreApp,Version=v7.0

For Test project on: GitHub sample

Other information

No response

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[huoyaoyuan]

WinForms related issue belongs to https://github.com/dotnet/winforms repo.

Can you provide detailed exception stack trace of the exception, for where it's thrown, WinForm-specific code or the runtime COM interop?

[c-ohle]

@huoyaoyuan problem is that it's not possible to debug in ComInterfaceDispatch... The debugger hangs.
The exception message is: Exception thrown: 'System.NotImplementedException' in System.Windows.Forms.Primitives.dll

[c-ohle]

Here is some more, activated unmanaged debug, exception thrown at last line but can't see GUIDS flags.

 protected override object CreateObject(IntPtr externalComObject, CreateObjectFlags flags)
    {
        Debug.Assert(flags == CreateObjectFlags.UniqueInstance || flags == CreateObjectFlags.None || flags == CreateObjectFlags.Unwrap);

        Guid pictureIID = IID.IPicture;
        int hr = Marshal.QueryInterface(externalComObject, ref pictureIID, out IntPtr pictureComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new PictureWrapper(pictureComObject);
        }

        Guid errorInfoIID = IID.IErrorInfo;
        hr = Marshal.QueryInterface(externalComObject, ref errorInfoIID, out IntPtr errorInfoComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new ErrorInfoWrapper(errorInfoComObject);
        }

        Guid enumFormatEtcIID = IID.IEnumFORMATETC;
        hr = Marshal.QueryInterface(externalComObject, ref enumFormatEtcIID, out IntPtr enumFormatEtcComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new EnumFORMATETCWrapper(enumFormatEtcComObject);
        }

        Guid fileOpenDialogIID = IID.IFileOpenDialog;
        hr = Marshal.QueryInterface(externalComObject, ref fileOpenDialogIID, out IntPtr fileOpenDialogComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new FileOpenDialogWrapper(fileOpenDialogComObject);
        }

        Guid fileSaveDialogIID = IID.IFileSaveDialog;
        hr = Marshal.QueryInterface(externalComObject, ref fileSaveDialogIID, out IntPtr fileSaveDialogComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new FileSaveDialogWrapper(fileSaveDialogComObject);
        }

        Guid lockBytesIID = IID.ILockBytes;
        hr = Marshal.QueryInterface(externalComObject, ref lockBytesIID, out IntPtr lockBytesComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new LockBytesWrapper(lockBytesComObject);
        }

        Guid shellItemIID = IID.IShellItem;
        hr = Marshal.QueryInterface(externalComObject, ref shellItemIID, out IntPtr shellItemComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new ShellItemWrapper(shellItemComObject);
        }

        Guid shellItemArrayIID = IID.IShellItemArray;
        hr = Marshal.QueryInterface(externalComObject, ref shellItemArrayIID, out IntPtr shellItemArrayComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new ShellItemArrayWrapper(shellItemArrayComObject);
        }

        Guid autoCompleteIID = IID.IAutoComplete2;
        hr = Marshal.QueryInterface(externalComObject, ref autoCompleteIID, out IntPtr autoCompleteComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new AutoCompleteWrapper(autoCompleteComObject);
        }

        Guid enumVariantIID = IID.IEnumVariant;
        hr = Marshal.QueryInterface(externalComObject, ref enumVariantIID, out IntPtr enumVariantComObject);
        if (hr == S_OK)
        {
            Marshal.Release(externalComObject);
            return new EnumVariantWrapper(enumVariantComObject);
        }

        throw new NotImplementedException();

[c-ohle]

It's in WinFormsComWrappers.cs, for the case: drag a single file from Desktop to WinForm or in my case a UserControl class.

[dreddy-work]

@kant2002, can you take a quick look to see if your latest changes in anyway causing this?

[kant2002]

I think issue was introduced in #6976

[c-ohle]

@kant2002 Yes, this seems to be the same problem. But there is something more.

Since an earlier NET7 version (3 or so) something is wrong with COM refcounts.
For example in my test app: Previously it was possible to drag and drop 3D objects from the 3D view of one app instance to the 3D view of another app instance.
Now the source app instance hangs after a some deep internal COM exceptions.
Compiled with Target NET 6.0 it works.

So it's easy to reproduce with the app:
https://github.com/c-ohle/RationalNumerics

[c-ohle]

@kant2002 Not really important because the exception is handled but maybe as hint:
This exception is also new since this version. (3 or so) (the exception is in AxIWebBrowser - wrong debug display)

[c-ohle]

@kant2002 and one more hint to reproduce:
Drag and drop still works within views of an app instance.
But app exit -1, which has always been a sure sign of a forgotten COM release call.

The program '[14228] Test.exe' has exited with code 4294967295 (0xffffffff).

[merriemcgaw]

@c-ohle would it be possible for you to provide trimmed down sample code so that we can test the fix from @kant2002 (and hopefully even incorporate it into our test bed somehow)?

[c-ohle]

@merriemcgaw hi, the fastest way is really with Test.exe from the Project
It's all up to date, last check in yesterday, based on NET7 dev master-branch
After download change Target framework from NET 6.0 to NET 7.0.
In debug go to TabPage "Polyhedron" and wait for the short animation to finish.
You can move the 3D-objects with mouse. Hold the Control-Key to drag-drop 3D-objects.
This works in the current viewport and also when dragging objects onto the desktop.

• But dragging the objects from the desktop back into the view fails and the exception occurs.
• There is also a WB based toolbox (right click). Dragging objects from there fails - same exception.
• Run a second instance to check drag-drop between views, normally it works.
• App-exit-code should be 0 (zero). Currently after any drag action (also only within a view) it's -1.

[Olina-Zhang]

@c-ohle I tested your Test.exe from the Project, can drag-drop 3D objects from "Polyhedron" TabPage to desktop, but it doesn't allow dragging the objects from the desktop back into the view, is there something wrong with my operation?

[c-ohle]

@Olina-Zhang Yes, but that's exactly the problem - the bug in the latest .NET 7.0 preview.
But thanks for testing, now it's certain it's not just my machine.

[c-ohle]

@Olina-Zhang
VS2022 MainMenu\Debug\Windows\Exception Settings
Common Language Runtime Exceptions - select all
Then you get it: 'System.NotImplementedException' in System.Windows.Forms.Primitives.dll

[Olina-Zhang]

@c-ohle we also created a simple repro project about drag-dropping image on pictureBox in Winforms from file explorer, the DragEnter event cannot be triggered in the latest .Net 7.0, can't repro in .Net 6.0. So it is the same issue in here.
WinFormsApp14.zip

[c-ohle]

@Olina-Zhang good work, now it should be easy for them to reproduce and to add the missing COM interface request.
But please note: There are actually two issues as I mentioned above:

Since an earlier NET7 version (3 or so) something is wrong with COM refcounts.

The other issue is some versions older and deeper in the system and can not reproduce with a "drop target only app".

[c-ohle]

@Olina-Zhang when I switch back to .Net 6.0. it works. Maybe you forgot a rebuild or something?

[Olina-Zhang]

st .Net 7.0, can't repro in .Net 6.0

yes, this issue cannot repro in Net 6.0, you can see my previous comment. When switch app to .Net 6.0, that DragEnter event can be triggered, I set a breakpoint in that app.

[c-ohle]

@Olina-Zhang with my app too? This could also be a SDK version(s) conflict - it would not be the first time.
To make sure - you can compile and test the same project with Target NET 6.0 in a offical VS2022 (not preview).

[c-ohle]

@Olina-Zhang sorry, yes you said, I misunderstood earlier and focussed on this.

[Amy-Li03]

Verified on .NET 7.0 test pass build: .NET 7.0.100-rc.1.22425.12, issue was fixed, now the DragEnter event can be triggered.

[Nora-Zhou01]

Verified on .NET 8.0 latest build from main branch: NET 8.0.100-alpha.1.22451.15, issue was fixed, test result is same as above screenshot.