C# winform check if control is physicaly visible

Question

A pragmatic solution is to use the form's GetChildAtPoint() method, passing the 4 corners of the control. If one of them returns true then the control is definitely visible. It is not 100% reliable, all 4 corners could be overlapped by another control but still leave part of interior visible. I would not worry about that, too bizarre.

public bool ChildReallyVisible(Control child) {
    var pos = this.PointToClient(child.PointToScreen(Point.Empty));

    //Test the top left
    if (this.GetChildAtPoint(pos) == child) return true;

    //Test the top right
    if (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y)) == child) return true;

    //Test the bottom left
    if (this.GetChildAtPoint(new Point(pos.X, pos.Y + child.Height -1)) == child) return true;

    //Test the bottom right
    if (this.GetChildAtPoint(new Point(pos.X + child.Width - 1, pos.Y + child.Height -1)) == child) return true;

    return false;
}

You can invalidate the control and then call GetUpdateRect (Win32 api function) to find this out. It does have the side effect of causing a repaint, though.

Inspired by Hans's answer I've implemented this behavior in this way;

    [DllImport("user32.dll")]
    static extern IntPtr WindowFromPoint(POINT Point);

    [StructLayout(LayoutKind.Sequential)]
    public struct POINT
    {
        public int X;
        public int Y;

        public POINT(int x, int y)
        {
            this.X = x;
            this.Y = y;
        }

        public static implicit operator System.Drawing.Point(POINT p)
        {
            return new System.Drawing.Point(p.X, p.Y);
        }

        public static implicit operator POINT(System.Drawing.Point p)
        {
            return new POINT(p.X, p.Y);
        }
    }

    public static bool IsControlVisibleToUser(this Control control)
    {
        var pos = control.PointToScreen(control.Location);
        var pointsToCheck = new POINT[]
                                {
                                    pos,
                                    new Point(pos.X + control.Width - 1, pos.Y),
                                    new Point(pos.X, pos.Y + control.Height - 1),
                                    new Point(pos.X + control.Width - 1, pos.Y + control.Height - 1),
                                    new Point(pos.X + control.Width/2, pos.Y + control.Height/2)
                                };

        foreach (var p in pointsToCheck)
        {
            var hwnd = WindowFromPoint(p);
            var other = Control.FromChildHandle(hwnd);
            if (other == null)
                continue;

            if (control == other || control.Contains(other))
                return true;
        }

        return false;
    }

In order to facilitate a previous answer to your question.

Here is the source code that you will need to work with the GetUpdateRect function as jdv-Jan de Vaan answered.

[System.Runtime.InteropServices.StructLayout(System.Runtime.InteropServices.LayoutKind.Sequential)]
internal struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;
    public int Width { get { return this.Right - this.Left; } }
    public int Height { get { return this.Bottom - this.Top; } }
}
[System.Runtime.InteropServices.DllImport("user32.dll")]
internal static extern bool GetUpdateRect(IntPtr hWnd, ref RECT rect, bool bErase);
public static bool IsControlVisibleToUser(Control control)
{
    control.Invalidate();
    Rectangle bounds = control.Bounds;
    RECT rect = new RECT { Left = bounds.Left, Right = bounds.Right, Top = bounds.Top, Bottom = bounds.Bottom };
    return GetUpdateRect(control.Handle, ref rect, false);
}

When you need to check if a specified is visible just do something like the following:

if (IsControlVisibleToUser(controlName) == true)
{
    // The Specified Control is visible.
    // ... do something 
}
else
{
    // Control is not visible.
    // ... do something else
}

Good luck.

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

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