SendMessage to simulate right-click crashes the target app

Question

I'm writing a C# automation tool.

Since Microsoft UI Automation doesn't provide any way of simulating right-clicks or raising context menus, I'm using SendMessage to do this instead. I'd rather not use SendInput because I don't want to have to grab focus.

When I call SendMessage, however, it crashes the target app.

Here's my code:

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

    public void RightClick<T>(T element) where T: AutomationElementWrapper
    {
        const int MOUSEEVENTF_RIGHTDOWN = 0x0008; /* right button down */
        const int MOUSEEVENTF_RIGHTUP = 0x0010; /* right button up */

        var point = element.Element.GetClickablePoint();
        var processId = element.Element.GetCurrentPropertyValue(AutomationElement.ProcessIdProperty);
        var window = AutomationElement.RootElement.FindFirst(
            TreeScope.Children,
            new PropertyCondition(AutomationElement.ProcessIdProperty,
                                  processId));
        var handle = window.Current.NativeWindowHandle;

        var x = point.X;
        var y = point.Y;

        var value = ((int)x)<<16 + (int)y;

        SendMessage(new IntPtr(handle), MOUSEEVENTF_RIGHTDOWN, IntPtr.Zero, new IntPtr(value));
        SendMessage(new IntPtr(handle), MOUSEEVENTF_RIGHTUP, IntPtr.Zero, new IntPtr(value));
    }

Any idea what I'm doing wrong?

Can you explain this a bit more: "I'd rather not use SendInput because I don't want to have to grab focus"? - The app that does the SendInput doesn't need to have focus. Or is it that you don't want the right-click itself to have the side-effect of focusing the target app? (BTW, the reason that UIAutomation doesn't have a way to simulate right-click is because it already exists: SendInput!) — BrendanMcK
UIAutomation doesn't grab focus. So far, I've managed to automate everything I want in the background. SendInput does grab focus, so I want to find an alternative (SendMessage doesn't). Also, using user32.dll is in no way as clean or easy to use as UIAutomation! One could argue that SendInput is capable of clicking and typing anywhere, so why would we need UIAutomation at all? — Lunivore
"One could argue that SendInput is capable of clicking and typing anywhere, so why would we need UIAutomation at all?" - Snark answer: because you can't use SendInput to find elements on the screen. UIAutomation does two different classes of things: it lets you discover information about the UI, and perform some types of interactions with it. For the former, it doesn't need to focus the UI, since it's just passively observing. But if you use it to interact with the UI, it will typically focus the target UI - because UI generally expects to have the focus when its being interacted with. — BrendanMcK
SendInput is the most reliable way of simulating input: because the input ends up going through exactly the same pathways that it would go through if an end-user did an actual click. When you use SendMessage/PostMessage, you can end up bypassing a bunch of code, so it's not guaranteed to be an exact equivalent. It might work in some or most cases, but fail silently or mysteriously in others. For example, a mouse hook will still work if the input is sent using SendInput, but won't get called if SendMessage is used. (SendMessage also bypasses any processing in the app's GetMessage loop.) — BrendanMcK
Right, but it grabs focus, which is what I'm explicitly trying to avoid. UI Automation does not typically focus the target UI, and works on logical rather than physical levels (for instance, a button is pressed regardless of whether it was a mouse-click or an enter key). Am now in touch with someone at Microsoft to try and provide a solution for this. — Lunivore

Joe White Joe White · Accepted Answer · 2011-06-28T17:34:35

You're mixing up your types. You're using SendMessage, which takes a window message (which by convention are named WM_...), but you're passing it a MOUSEINPUT.dwFlags value that's meant for SendInput (which are named MOUSEEVENTF_...). You're basically passing gibberish.

What your code is actually doing is sending a window message whose numeric value is 8 (which, in window messages, means WM_KILLFOCUS), followed by a window message of 0x10 == 16 (WM_CLOSE). It's the latter that's likely causing you problems -- you're telling the window to close. I'm not sure why it would crash, but it would certainly exit.

If you're using SendMessage, you need to pass it window messages (WM_, for example WM_RBUTTONDOWN and WM_RBUTTONUP).

SendMessage to simulate right-click crashes the target app

4 Answers