[GetColorAtPoint]System.ArgumentException: Parameter is not valid.
   at System.Drawing.Graphics.GetHdc()
   at SGetColorAtPoint(Point p, IntPtr hWnd)

[DllImport("gdi32.dll")]
    private static extern int BitBlt(IntPtr srchDC, int srcX, int srcY, int srcW, int srcH, IntPtr desthDC, int destX, int destY, int op);
    [DllImport("user32.dll")]
    static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDC);
    public static Color GetColorAtPoint(Point p, IntPtr hWnd)
    {
        try
        {
            Bitmap screenPixel = new Bitmap(1, 1);
            Color c = System.Drawing.Color.Black;

            //method to test
            if (IsWindow(hWnd))
            using (Graphics gdest = Graphics.FromImage(screenPixel))
            {
                if (IsWindow(hWnd))
                {
                    using (Graphics gsrc = Graphics.FromHwnd(hWnd))
                    {
                        IntPtr hSrcDC = gsrc.GetHdc();
                        IntPtr hDestDC = gdest.GetHdc();
                        int retval = BitBlt(hDestDC, 0, 0, 1, 1, hSrcDC, p.X, p.Y, (int)CopyPixelOperation.SourceCopy);
                        gdest.ReleaseHdc();
                        gsrc.ReleaseHdc();
                        DeleteDC(hSrcDC);
                        DeleteDC(hDestDC);

                    }
                }
            }
            c = screenPixel.GetPixel(0, 0);
            return c;

        }
        catch (Exception ex)
        {
            Console.WriteLine("[GetColorAtPoint]"+ex.ToString());
            return System.Drawing.Color.Black;
        }
    }

public IntPtr GetHdc() {
        IntPtr hdc = IntPtr.Zero;

        int status = SafeNativeMethods.Gdip.GdipGetDC(new HandleRef(this, this.NativeGraphics), out hdc);

        if (status != SafeNativeMethods.Gdip.Ok) {
            throw SafeNativeMethods.Gdip.StatusException(status);
        }

        this.nativeHdc = hdc; // need to cache the hdc to be able to release with a call to IDeviceContext.ReleaseHdc().

        return this.nativeHdc;
    }