AllowAutoPlay Never Gets Called

I have a WinForms application. Just before creating the actual form in Program.cs, I instantiate an Autoplay class. Registration is successful, after the obligatory first return value of 65536, but I never get any calls to AllowAutoPlay().

Am I missing something?

Here is the code:

public class RunningObjectTableEntry : IDisposable
{
    private const int ROTFLAGS_REGISTRATIONKEEPSALIVE = 1;

    private HRESULT cookie;
    private IRunningObjectTable rot = null;
    private IMoniker monkey = null;

    private RunningObjectTableEntry() { }

    public RunningObjectTableEntry(object obj)
    {
        this.AddToROT(obj);
    }

    public void AddToROT(object obj)
    {
        int hr = GetRunningObjectTable(0, out rot);
        if (hr != 0)
        {
            throw new COMException("Could not retrieve running object table!", hr);
        }

        Guid clsid = obj.GetType().GUID;

        hr = CreateClassMoniker(ref clsid, out monkey);

        if (hr != 0)
        {
            Marshal.ReleaseComObject(rot);
            throw new COMException("Could not create moniker for CLSID/IID \"" + clsid + "\"!", hr);
        }

        UInt32 iResult = (UInt32)rot.Register(ROTFLAGS_REGISTRATIONKEEPSALIVE, obj, monkey);   // Weak reference, but allow any user

        if (65536 == iResult)
            iResult = (UInt32)rot.Register(ROTFLAGS_REGISTRATIONKEEPSALIVE, obj, monkey);

        cookie = (HRESULT)iResult;
    }

    public void RemoveFromROT()
    {
        if (cookie != 0)
        {
            try
            {
                // Get the running object table and revoke the cookie
                rot.Revoke((int)cookie);
                cookie = 0;
            }
            finally
            {
                if (rot != null) while (Marshal.ReleaseComObject(rot) > 0) ;
            }
        }
    }

    [DllImport("ole32.dll", ExactSpelling = true)]
    private static extern int GetRunningObjectTable([MarshalAs(UnmanagedType.U4)] int reserved, out IRunningObjectTable pprot);

    [DllImport("ole32.dll", CharSet = CharSet.Unicode, ExactSpelling = true)]
    private static extern int CreateClassMoniker([In] ref Guid g, [Out] out IMoniker ppmk);

    #region IDisposable Members

    public void Dispose()
    {
        if (null != monkey)
            Marshal.ReleaseComObject(monkey);
        rot.Revoke((int)cookie);
        Marshal.ReleaseComObject(rot);
    }

    #endregion
}

[ComVisible(true)]
[Guid("331F1768-05A9-4ddd-B86E-DAE34DDC998A")]
[ClassInterface(ClassInterfaceType.None)]
public class Autoplay : IQueryCancelAutoPlay, IDisposable
{
    private RunningObjectTableEntry rotEntry;

    public Autoplay()
    {
        rotEntry = new RunningObjectTableEntry(this);
    }

    public void RemoveFromROT()
    {
        this.rotEntry?.RemoveFromROT();
    }
    #region IQueryCancelAutoPlay Members

    public int AllowAutoPlay(string pszPath, AutorunContent dwContentType, string pszLabel, int dwSerialNumber)
    {
        String msgUser = $"AllowAutoPlay: Path={pszPath}, ContentType={dwContentType.ToString()}, Label={pszLabel}, SerialNumber={dwSerialNumber.ToString()}";
        System.Diagnostics.Debug.WriteLine(msgUser);
        MessageBox.Show(msgUser);
    }

    #endregion

    #region IDisposable Members

    public void Dispose()
    {
        rotEntry.Dispose();
    }

    #endregion
}

The cookie on the second call is fine, consistent, but fine at 131073 or 0x00020001.

I used the following articles: Prevent Autoplay, 65536 error, and CodeProject.

Neither a breakpoint or the message box shows.

I am running on Windows 10 using Visual Studio 2017.

Thoughts?

The Exchange Expert response is the answer, namely

dbtoth Author Commented: 2003-07-30 The above is working fine except for one small glitch... because the code only works when the window has focus,

A key element worth noting is "window". The original that I gave in my question works great with only one form. My main applications has several forms packed together, so if any of them have the focus, then the code will not work.

The code above and the WndProc variant rely on Windows sending the QueryCancelAutoPlay message, which only occurs to the top-layer form, which may NOT be the form that you think.

My applications creates firsts first a FrmMain, but on top of that I have various child forms. Only the top-most form (window) gets the message, which means that to be safe that all child forms need one of the forms of QueryCancelAutoPlay.