public partial class Form1 : Form
    {
        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;
            
        }

        [DllImport("user32.dll")]
        public static extern bool GetUpdateRect(IntPtr hWnd, ref RECT lpRect, bool bErase);
        
        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        public static extern IntPtr GetDesktopWindow();
      
        [DllImport("user32.dll")]
        public static extern bool GetWindowRect(IntPtr handle, ref RECT rect);

        RECT rc = new RECT();

        public Form1()
        {
            InitializeComponent();
        }

        public void GetUpdRectWindow()
        {
            if (GetWindowRect(GetDesktopWindow(), ref rc))
            {
                if (GetUpdateRect(GetDesktopWindow(), ref rc, true))
                {
                    MessageBox.Show("Область получена!");
                }
                else
                {
                    MessageBox.Show("Область не получена!");
                }
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            GetUpdRectWindow();
        }

    }

public partial class Form1 : Form
    {
        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;
            
        }

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        public static extern bool GetUpdateRect(IntPtr hWnd, ref RECT lpRect, bool bErase);
        
        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        public static extern IntPtr GetDesktopWindow();

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        public static extern bool GetWindowRect(IntPtr handle, ref RECT rect);

        [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
        static extern IntPtr GetDC(IntPtr hWnd);

        RECT rc = new RECT();

        public Form1()
        {
            InitializeComponent();
        }

        public void GetUpdRectWindow()
        {
            if (GetWindowRect(GetDC(this.Handle), ref rc))
            {
                if (GetUpdateRect(GetDC(this.Handle), ref rc, true))
                {
                    MessageBox.Show("Область получена!");
                }
                else
                {
                    MessageBox.Show("Область не получена!");
                }
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            GetUpdRectWindow();
        }

public partial class Form1 : Form
    {

        Thread th = null;
        HandleRef hndlref;
        
        public Form1()
        {
            InitializeComponent();
            th = new Thread(new ThreadStart(thread));
            hndlref = new HandleRef(this, this.Handle);
        }

        [DllImport("user32.dll")]
        static extern IntPtr GetDC(HandleRef hWnd);

        [DllImport("user32.dll")]
        static extern int GetUpdateRgn(IntPtr hWnd, IntPtr hRgn, bool bErase);

        [DllImport("user32.dll")]
        static extern bool GetUpdateRect(IntPtr hWnd, IntPtr lpRect, bool bErase);

        [DllImport("user32.dll")]
        static extern bool GetUpdateRect(HandleRef hWnd, out RECT rect, bool bErase);
        
        [DllImport("user32.dll")]
        private static extern IntPtr GetDesktopWindow();

        [DllImport("user32.dll")]
        private static extern IntPtr GetWindowDC(IntPtr handle);

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;
        }
 
        RECT rct;

        private void thread()
        {
            while (true)
            {
                if (GetUpdateRect(hndlref, out rct, false)) { 
                    MessageBox.Show(rct.Left.ToString()); 
                } 
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            th.Start();
        }

    }

        [DllImport("user32.dll")]
        static extern bool GetUpdateRect(IntPtr hWnd, ref RECT lpRect, bool bErase);
       
        <....................>
        
        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_PAINT)
            {
                if (!GetUpdateRect(desktopDC, ref rct, false)){return;}
                base.WndProc(ref m);
                MessageBox.Show(rct.Bottom.ToString()+":"+rct.Left.ToString()+":"+rct.Right.ToString());
            }
            else
            {

                base.WndProc(ref m); 

            }
            
        }