Question

I need to automate a third party program, and the only course of action is to simulate a click on some buttons.

I do this by finding the HWND handle of the button with EnumChildWindows. When I’ve found the “window” (the button), I try to send BM_CLICK to it with SendMessageW. This works, my tests show that the button indeed think it was clicked.

The problem arise in my error handling. There is no feedback given by the BM_CLICK message, so I don’t really know if it has been clicked. I thought I should be diligent and check for any error codes though with Marshal.GetLastWin32Error.

This returns ERROR_PROC_NOT_FOUND, which is not really what I would expect from a successful message handling.

I import SendMessageW as follows:

[DllImport("User32.dll", 
           CharSet = CharSet.Unicode, 
           CallingConvention = CallingConvention.Winapi, 
           SetLastError = true)]
public static extern IntPtr SendMessageW(
     HandleRef hWnd, 
     UInt32 Msg, 
     UIntPtr wParam, 
     IntPtr lParam);

The code doing the call is:

User32.SendMessageW(
    buttonHandle,
    (uint)ButtonControlMessages.BM_CLICK, // value of BM_CLICK = 0x00F5.
    UIntPtr.Zero,
    IntPtr.Zero);

int error = Marshal.GetLastWin32Error();
if (error != ErrorCodes.ERROR_SUCCESS) // love the name of this error code.
    throw new Win32Exception(error);

My tests are just using a simple Windows Forms with a button control attached. Thus, I can procure the handle through button.Handle. It gets clicked; could it be that this error is completely unrelated?

It sure would be nice to get rid of it though, I’d like some way to be sure that the call to SendMessageW at least didn’t fail.

I’m on Windows 7 x86-32 with .NET 4.

Answer 1

The calling convention should be Stdcall, but since that is the default you can just drop it.

I think your SendMessage P/Invoke looks a bit odd but that’s probably not the cause of the issue. In any case I would do it like this:

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

I think what’s happening here is that SendMessage() is working but isn’t assigning the last error. The only thing that the documentation for SendMessage() mentions about errors is:

When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).

The return value for SendMessage() is dependent on the message sent. In the case of BM_CLICK there is apparently no message sent. In other words you simply do not get any feedback.