C# SendMessage click button

Question

Im trying to understand the SendMessage function and here's my actual code:

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

[DllImport("user32.dll")]
private static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, string className, string windowTitle);

static void Main(string[] args)
{
    Process test = Process.GetProcessesByName("calc")[0];
    IntPtr hwndChild = FindWindowEx(test.MainWindowHandle, IntPtr.Zero, "Button", "2");
    SendMessage(hwndChild, 245, IntPtr.Zero, IntPtr.Zero);
    Console.ReadKey();
}

Very simple, I just want to click the calc button 2, but I'm having no success.

Answer 1

Error checking is never optional when you pinvoke winapi functions. It is a C api, it doesn't throw exceptions to keep you out of trouble. You'll have to do that by yourself. Proper code looks like this:

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    private static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, 
                                              string className, string windowTitle);

    ...
    IntPtr hwndChild = FindWindowEx(test.MainWindowHandle, IntPtr.Zero, "Button", "2");
    if (hwndChild == IntPtr.Zero) throw new System.ComponentModel.Win32Exception();

Now you know why your program doesn't work. Next thing you'd do is fire up the Spy++ utility and have a look-see at the calculator window. You'll discover that you have to make more FindWindowEx() calls to drill down to the nested button.

Do consider using a UI Automation library to do this.