Error: Cross thread operation not valid

Question

I am trying to access a form from a different thread to that on which the form was created, and finally ended up with an error: Cross thread operation not valid

public static void MakeTopMost(Form form)
{
    SetWindowPos(form.Handle, HWND_TOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
}

I am passing a form which is running in another thread. I tried testing InvokeRequired, but it is always false.

I am new to threading.

Answer 1

Make sure you are testing the right object for InvokeRequired:

public static void MakeTopMost(Form form)
{
    if (form.InvokeRequired)
    {
        form.Invoke((Action)delegate { MakeTopMost(form); });
        return;
    }

    SetWindowPos(form.Handle, HWND_TOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
}

I like to wrap all of this up with an extension method like this:

public static class SynchronizeInvokeUtil
{
    public static void SafeInvoke(this ISynchroniseInvoke sync, Action action)
    {
        if (sync.InvokeRequired)
            sync.Invoke(action);
        else
            action();
    }

    public static void SafeBeginInvoke(this ISynchroniseInvoke sync, 
                                       Action action)
    {
        if (sync.InvokeRequired)
            sync.BeginInvoke(action);
        else
            action();
    }
}

You can then just call:

form.SafeInvoke(() => SetWindowPos(form.Handle, HWND_TOPMOST, 
                                   0, 0, 0, 0, TOPMOST_FLAGS));

Which is probably the most readable.

Note that if you are using this within the form class itself, you have to use this.SafeInvoke(...) in order to access the extension method.