Themed RichTextBox - A RichTextBox with XP-styled borders

using System;
using System.Collections;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace CustomControls
{
    public class CustomButton : System.Windows.Forms.Button
    {
        private System.ComponentModel.Container components = null;

        public CustomButton()
        {
            InitializeComponent();

        }

        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if(components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        private void InitializeComponent()
        {
            components = new System.ComponentModel.Container();
        }

        private const int WM_PAINT =0x000F;
        public const int WM_NCPAINT = 0x85;
        public const int WM_NCCALCSIZE = 0x83;
        public const int WM_THEMECHANGED = 0x031A;

        private enum BUTTONPARTS:int
        {
            BP_UNKNOWN = 0,
            BP_PUSHBUTTON = 1,
            BP_RADIOBUTTON = 2,
            BP_CHECKBOX = 3,
            BP_GROUPBOX = 4,
            BP_USERBUTTON = 5,
            BP_COMMANDLINK = 6,
            BP_COMMANDLINKGLYPH = 7
        };

        private enum PUSHBUTTONSTATES:int
        {
            PBS_NORMAL = 1,
            PBS_HOT = 2,
            PBS_PRESSED = 3,
            PBS_DISABLED = 4,
            PBS_DEFAULTED = 5,
            PBS_DEFAULTED_ANIMATING = 6
        };

        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;

            public RECT(int left_, int top_, int right_, int bottom_)
            {
                Left = left_;
                Top = top_;
                Right = right_;
                Bottom = bottom_;
            }
        }

        [DllImport("uxtheme.dll", ExactSpelling=true)]
        public extern static bool IsThemeActive();

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

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

        [DllImport("user32.dll")]
        public static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);

        [DllImport("uxtheme.dll", ExactSpelling=true, CharSet=CharSet.Unicode)]
        public static extern IntPtr OpenThemeData(IntPtr hWnd, String classList);

        [DllImport("uxtheme.dll", ExactSpelling=true)]
        public extern static Int32 DrawThemeBackground(IntPtr hTheme, IntPtr hdc, int iPartId,
            int iStateId, ref RECT pRect, IntPtr pClipRect);

        [DllImport("uxtheme.dll", ExactSpelling=true)]
        public extern static Int32 CloseThemeData(IntPtr hTheme);

        protected override void WndProc(ref Message m)

        {
            switch(m.Msg)
            {
                case WM_NCPAINT:
                    WmPaint(ref m);
                    break;
                default:
                    base.WndProc (ref m);
                    break;
            }
        }

        public void WmPaint(ref Message m) {
            try
            {
                base.WndProc(ref m);

                if (!IsThemeActive()) {
                    return;
                }

                /////////////////////////////////////////////////////////////////////////////
                // Get the DC of the window frame and paint the border using uxTheme API
                /////////////////////////////////////////////////////////////////////////////

                // set the part id to TextBox
                int partId = (int)BUTTONPARTS.BP_PUSHBUTTON;
                int stateId = (int)PUSHBUTTONSTATES.PBS_NORMAL;

                // define the windows frame rectangle of the TextBox
                RECT windowRect;
                GetWindowRect(this.Handle, out windowRect);

                // get the device context of the window frame
                IntPtr hDC = GetWindowDC(this.Handle);

                // open theme data
                IntPtr hTheme = OpenThemeData(this.Handle, "Button");

                // draw background
                DrawThemeBackground(hTheme, hDC, partId, stateId, ref windowRect, IntPtr.Zero);

                // close theme data
                CloseThemeData(hTheme);

                // release dc
                ReleaseDC(this.Handle, hDC);

                // we have processed the message so set the result to zero
                m.Result = IntPtr.Zero;

            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}

Imports System.Runtime.InteropServices
Imports System.Windows.Forms.VisualStyles

Public Class RichTextBoxEx
	Inherits RichTextBox

	Public Event NonClientPaint As EventHandler(Of PaintEventArgs)

	Private m_Render As VisualStyles.VisualStyleRenderer

	Protected Overrides Sub WndProc(ByRef m As Message)
		Select Case m.Msg
			Case WM_NCPAINT
				WmNcPaint(m)
			Case Else
				MyBase.WndProc(m)
		End Select
	End Sub

	Private Sub WmNcPaint(ByRef m As Message)
		MyBase.WndProc(m)
		Dim hDC As IntPtr = GetWindowDC(m.HWnd)
		Dim b As New Rectangle(0, 0, Me.Width, Me.Height)
		Using a As New PaintEventArgs(Graphics.FromHdc(hDC), b)
			Me.OnNonClientPaint(a)
		End Using
	End Sub

	Protected Overridable Sub OnNonClientPaint(ByVal e As PaintEventArgs)
		If Application.RenderWithVisualStyles Then
			If m_Render Is Nothing Then
				m_Render = New VisualStyleRenderer(VisualStyleElement.TextBox.TextEdit.Normal)
			End If
			m_Render.DrawBackground(e.Graphics, e.ClipRectangle)
		End If
		RaiseEvent NonClientPaint(Me, e)
	End Sub

	Private Const WM_NCPAINT As Integer = &H85

	<DllImport("user32.dll")> _
	Private Shared Function GetWindowDC(ByVal hWnd As IntPtr) As IntPtr
	End Function
End Class

if (this.ReadOnly)<br />
    stateId = NativeMethods.ETS_READONLY;<br />
else