Question

I have a custom UserControl that I am drawing using the GDI+. It is a transparent control which draws small shapes on top of the parent control.

All the parent window does is creates the control, gives it a Rectangle in which to draw itself, and then it receives events if the user clicks on the non-transparent areas.

The drawing part works perfectly, but right now what I need to do is to as seamlessly as possible forward all MouseMove, MouseClick, etc. events to the parent control IF those events occurred outside the shapes.

The shapes are drawn using GraphicsPath, and I am already able to detect if the mouse position is over a shape using GraphicsPath.IsVisible().

I want to do this in a way that requires zero or minimal extra code on the parent. The parent should not necessarily know whether the MouseMove event was forwarded from the child control or not, it should treat them all equally.

Do I have to pinvoke/SendMessage() to do this? Or is there an easier way using the .NET framework?

Answer 1

This is possible in the winapi, the WM_NCHITTEST message is sent by the window manager to ask what part of the control the mouse is on top of. What you can do is return HTTRANSPARENT and it will ask the parent window. Here’s a sample UserControl that implements this. Catching the message requires overriding WndProc():

public partial class UserControl1 : UserControl {
    public UserControl1() {
        InitializeComponent();
        paths = new List<GraphicsPath>();
        GraphicsPath example = new GraphicsPath();
        example.AddEllipse(new Rectangle(10, 10, 50, 30));
        paths.Add(example);
    }
    List<GraphicsPath> paths;

    protected override void OnPaint(PaintEventArgs e) {
        foreach (var path in paths) e.Graphics.FillPath(Brushes.Blue, path);
        base.OnPaint(e);
    }

    protected override void WndProc(ref Message m) {
        base.WndProc(ref m);
        // Trap WM_NCHITTEST on the client area
        if (m.Msg == 0x84 && m.Result == (IntPtr)1) {
            Point pos = new Point(m.LParam.ToInt32());
            pos = this.PointToClient(pos);
            bool oncurve = false;
            foreach (var path in paths)
                if (path.IsVisible(pos)) oncurve = true;
            if (!oncurve) m.Result = (IntPtr)(-1);  // HTTRANSPARENT
        }
    }
}

Test code in the form:

    private void userControl11_MouseMove(object sender, MouseEventArgs e) {
        Console.WriteLine("On shape {0}", e.Location);
    }

    private void Form1_MouseMove(object sender, MouseEventArgs e) {
        Console.WriteLine("On form  {0}", e.Location);
    }