C# winform check if control is physicaly visible

Question

Is it possible to determine if at least one pixel of a control can be seen (by a property or maybe using event notification).

NB : I am not looking for the Visible p

Answer 1

If a control is visible the Paint event will be called (repeatedly).

Normally for not visible controls, this event will not be called.

Answer 2

I somewhat finished the answer by Hans Passant. The function below tests for all four corners of the form.

        /// <summary>
    /// determines if a form is on top and really visible.
    /// a problem you ran into is that form.invalidate returns true, even if another form is on top of it. 
    /// this function avoids that situation
    /// code and discussion:
    /// https://stackoverflow.com/questions/4747935/c-sharp-winform-check-if-control-is-physicaly-visible
    /// </summary>
    /// <param name="child"></param>
    /// <returns></returns>
    public bool ChildReallyVisible(Control child)
    {
        bool result = false;

        var pos = this.PointToClient(child.PointToScreen(Point.Empty));

        result = this.GetChildAtPoint(pos) == child;
        //this 'if's cause the condition only to be checked if the result is true, otherwise it will stay false to the end
        if(result)
        {
            result = (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y)) == child);
        }
        if(result)
        {
            result = (this.GetChildAtPoint(new Point(pos.X, pos.Y + child.Height - 1)) == child);
        }
        if(result)
        {
            result = (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y + child.Height - 1)) == child) ;
        }
        return result;
    }

Answer 3

Tried the above but kept getting true even if the winform was covered by another app.

Ended up using the following (inside my winform class):

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace yourNameSpace
{
    public class Myform : Form
    {

        private void someFuncInvokedByTimerOnMainThread()
        {
            bool isVisible = isControlVisible(this);
            // do something.
        }

        [DllImport("user32.dll")]
        static extern IntPtr WindowFromPoint(System.Drawing.Point p);


        ///<summary><para>------------------------------------------------------------------------------------</para>
        ///
        ///<para>           Returns true if the control is visible on screen, false otherwise.                </para>
        ///
        ///<para>------------------------------------------------------------------------------------</para></summary>
        private bool isControlVisible(Control control)
        {
            bool result = false;
            if (control != null)
            {
                var pos = control.PointToScreen(System.Drawing.Point.Empty);
                var handle = WindowFromPoint(new System.Drawing.Point(pos.X + 10, pos.Y + 10)); // +10 to disregard padding   
                result = (control.Handle == handle); // should be equal if control is visible
            }
            return result;
        }
    }
}