SendMessage with TDM_CLICK_BUTTON return value is always zero

Question

In my C# application I'm trying to use the TDM_CLICK_BUTTON message to click a button in a TaskDialog. This basically works just fine. What I expect to happen, happens. I even receive the TDN_BUTTON_CLICKED notification.

But the documentation says, that the return value of SendMessage would be nonzero if the call succeeds. But it always returns zero for me.

This is my code:

public void ClickButton( int buttonId ) {
  bool success = UnsafeNativeMethods.SendMessage(
    WindowHandle,
    (uint)UnsafeNativeMethods.TASKDIALOG_MESSAGES.TDM_CLICK_BUTTON,
    (IntPtr)buttonId,
    IntPtr.Zero ) != IntPtr.Zero;
  if( !success ) {
    int lastWin32Error = Marshal.GetLastWin32Error();
    throw new Win32Exception( lastWin32Error, "SendMessage for TDM_CLICK_BUTTON failed." );
  }
}

lastWin32Error is always zero when the exception is thrown. Which would be another indicator that everything is fine.

SendMessage is declared in my code like this:

[DllImport( "user32.dll", SetLastError = true )]
internal static extern IntPtr SendMessage( IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam );

Is the documentation incorrect or am I using the message incorrectly?

Answer 1

So I took the C++ Windows SDK TaskDialog sample and tried this message out in there as well. The return value of SendMessage remains zero always.

I noticed that a TDN_BUTTON_CLICKED notification will always be sent (even if the button id does not exist). If the button exists or not has no effect on the SendMessage return value.

Then I suspected, that the return value for that notification might have an effect, but no matter what I return from the callback (TRUE/FALSE/S_OK/42), my SendMessage return value remains zero.

So, given that I found no way to make this fail and/or return a nonzero value, I can only assume that the documentation is incorrect.

edit: I got a reply to my documentation feedback I sent a few days ago. Turns out the documentation is indeed incorrect. The return value is to be ignored.