RichTextBox with a custom border

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public class RichTextBoxEx : RichTextBox
    {

        private int _borderWidth = 1;
        private Color _borderColor = Color.Black;

        public RichTextBoxEx() {  }

        [Category("Border")]
        public int BorderWidth
        {
            get {
                return _borderWidth;
            }
            set {
                if (_borderWidth < 0)
                    throw new ArgumentException("Border width must be non-negative", "BorderWidth");
                _borderWidth = value;
                Invalidate();
                InvalidateNC();
            }

        }

        [Category("Border")]
        public Color BorderColor
        {
            get
            {
                return _borderColor;
            }
            set
            {
                _borderColor = value;
                Invalidate();
                InvalidateNC();
            }

        }

        protected override CreateParams CreateParams
        {
            get
            {
                var cp = base.CreateParams;
                cp.ExStyle &= ~0x200; // WS_EX_CLIENTEDGE
                cp.Style &= ~0x00800000; // WS_BORDER
                return cp;
            }
        }

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case (int)NativeMethods.WM_NCCALCSIZE:
                    if (m.WParam == IntPtr.Zero || m.WParam == new IntPtr(1))
                        RecalcNonClientArea(m.LParam);
                    break;
                case (int)NativeMethods.WM_NCPAINT:
                    OnWMNCPaint(ref m);
                    break;
                case (int)NativeMethods.WM_NCHITTEST:
                    m.Result = PerformHitTest(new Point(m.LParam.ToInt32()));
                    return;
            }

            base.WndProc(ref m);
        }

        protected override void OnResize(EventArgs eventargs)
        {
            InvalidateNC();
            base.OnResize(eventargs);
        }

        private void OnWMNCPaint(ref Message m)
        {
            var hDC = NativeMethods.GetWindowDC(m.HWnd);
            if (hDC == IntPtr.Zero) return;

            var hrgnWindow = IntPtr.Zero;
            var hrgnClient = IntPtr.Zero;

            try
            {
                var rect = new Rectangle(_borderWidth, _borderWidth, Width - (_borderWidth * 2), Height - (_borderWidth * 2));
                hrgnWindow = NativeMethods.CreateRectRgn(0, 0, Width, Height);
                hrgnClient = NativeMethods.CreateRectRgn(rect.Left, rect.Top, rect.Right, rect.Bottom);

                NativeMethods.CombineRgn(hrgnWindow, hrgnWindow, hrgnClient, 4); // RGN_DIFF
                NativeMethods.SelectClipRgn(hDC, hrgnWindow);

                using (Graphics g = Graphics.FromHdc(hDC))
                    g.Clear(BorderColor);

                m.Result = IntPtr.Zero;
            }
            finally
            {
                NativeMethods.ReleaseDC(m.HWnd, hDC);

                if (hrgnWindow != IntPtr.Zero)
                    NativeMethods.DeleteObject(hrgnWindow);

                if (hrgnClient != IntPtr.Zero)
                    NativeMethods.DeleteObject(hrgnClient);
            }

        }

        private void InvalidateNC()
        {
            NativeMethods.SetWindowPos(this.Handle, IntPtr.Zero, 0, 0, 0, 0,
                                 NativeMethods.SWP_NOMOVE |
                                 NativeMethods.SWP_NOSIZE |
                                 NativeMethods.SWP_NOZORDER |
                                 NativeMethods.SWP_NOACTIVATE |
                                 NativeMethods.SWP_DRAWFRAME);
        }

        private void RecalcNonClientArea(IntPtr lParam)
        {
            NativeMethods.NCCALCSIZE_PARAMS csp;

            csp = (NativeMethods.NCCALCSIZE_PARAMS)Marshal.PtrToStructure(
                lParam,
                typeof(NativeMethods.NCCALCSIZE_PARAMS));

            csp.rcNewWindow.Top += _borderWidth;
            csp.rcNewWindow.Bottom -= _borderWidth;
            csp.rcNewWindow.Left += _borderWidth;
            csp.rcNewWindow.Right -= _borderWidth;

            Marshal.StructureToPtr(csp, lParam, false);
        }

        private IntPtr PerformHitTest(Point screenPos)
        {
            Point windowPos = this.Parent.PointToClient(screenPos);
            windowPos.Offset(-this.Location.X, -this.Location.Y);

            var clientWindowRect = new Rectangle(_borderWidth, _borderWidth, Width, Height);

            return clientWindowRect.Contains(windowPos) ? (IntPtr)NativeMethods.HTCLIENT : (IntPtr)NativeMethods.HTBORDER;
        }


    }

    public class NativeMethods
    {

        public const int WM_NCCALCSIZE = 0x83;
        public const int WM_NCHITTEST = 0x84;
        public const int WM_NCPAINT = 0x85;

        public const int SWP_NOSIZE = 0x0001;
        public const int SWP_NOMOVE = 0x0002;
        public const int SWP_NOZORDER = 0x0004;
        public const int SWP_NOREDRAW = 0x0008;
        public const int SWP_NOACTIVATE = 0x0010;
        public const int SWP_FRAMECHANGED = 0x0020;
        public const int SWP_DRAWFRAME = SWP_FRAMECHANGED;

        public const int HTCLIENT = 1;
        public const int HTBORDER = 18;

        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowDC(IntPtr hWnd);

        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);

        [DllImport("user32.dll")]
        public static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDC);

        [DllImport("gdi32.dll")]
        public static extern bool DeleteObject(IntPtr hObject);

        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateRectRgn(int nLeftRect, int nTopRect, int nRightRect, int nBottomRect);

        [DllImport("gdi32.dll")]
        public static extern int CombineRgn(IntPtr hrgnDest, IntPtr hrgnSrc1,
           IntPtr hrgnSrc2, int fnCombineMode);

        [DllImport("gdi32.dll")]
        public static extern int SelectClipRgn(IntPtr hdc, IntPtr hrgn);

        public struct RECT { public int Left, Top, Right, Bottom; }
        public struct NCCALCSIZE_PARAMS
        {
            public RECT rcNewWindow;
            public RECT rcOldWindow;
            public RECT rcClient;
            IntPtr lppos;
        }

    }
}