Winform重畫ComboBox背景色

//傳回hWnd參數所指定的視窗的裝置環境。
 [System.Runtime.InteropServices.DllImport("user32.dll")]
        static extern IntPtr GetWindowDC(IntPtr hWnd);     

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        //函數釋放裝置上下文環境（DC）  
        static extern int ReleaseDC(IntPtr hWnd, IntPtr hDC);    
        int WM_PAINT = 0xf; //要求一個視窗重畫自己,即Paint事件時         


        /// <summary>
        /// 
        /// </summary>
        /// <param name="m"></param>
        protected override void WndProc(ref Message m)
        { 

            base.WndProc(ref m);
            if (m.Msg == WM_PAINT)
            {
                IntPtr hDC = GetWindowDC(m.HWnd);
                if (hDC.ToInt32() == 0) //如果取裝置上下文失敗則傳回     
                {
                    return;
                }

                PaintComboBox(Graphics.FromHdc(hDC));
                ReleaseDC(m.HWnd, hDC);
            }
        }      

private void PaintComboBox(Graphics g)
        {
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.TextRenderingHint = TextRenderingHint.AntiAliasGridFit;


            int iDropDownButtonWidth = 17;
            int iDropDownButtonHeight = this.Height;
            int iDropDownButtonLocatinX = 17;
            int iDropDownButtonLocatinY = 0;
            
            if (!Util.PublicFunction.IsHigherWinXP())
            {
                iDropDownButtonWidth = 17;
                iDropDownButtonHeight = this.Height-2;
                iDropDownButtonLocatinX = 19;
                iDropDownButtonLocatinY = 1;
            }
            //下拉按鈕
            Rectangle dropDownRectangle = new Rectangle(ClientRectangle.Width - iDropDownButtonLocatinX, iDropDownButtonLocatinY, iDropDownButtonWidth, iDropDownButtonHeight);


            //背景色刷
            Brush bkgBrush;

            //字型色刷
            Brush fcBrush;

            //設定背景色和字型色
            bkgBrush = new SolidBrush(this._backColor);
            fcBrush = new SolidBrush(this._foreColor);

            //畫3D邊框
            //ControlPaint.DrawBorder3D(g, new Rectangle(0, 0, this.Width, this.Height), Border3DStyle.SunkenInner, Border3DSide.All);


            int iBackColorX = 0;
            //為了字型正常，Enable時隻是重畫按鈕區域
            if (this.Enabled)
            {
                iBackColorX = this.Width - 20;
            }
            //畫背景
            g.FillRectangle(bkgBrush, iBackColorX, 0, ClientRectangle.Width, ClientRectangle.Height);

            //為了字型正常，Disable時才重畫文本
            if (!this.Enabled)
            {
                //畫文本
                g.DrawString(base.Text, this.Font, fcBrush, 2, this.ClientSize.Height / 2, new StringFormat() { LineAlignment = StringAlignment.Center });
            }

            //畫邊框
            //g.DrawRectangle(_BorderPen, new Rectangle(0, 0, this.Width, this.Height));
            ControlPaint.DrawBorder(g, new Rectangle(0, 0, this.Width, this.Height), borderColor, ButtonBorderStyle.Solid);

            //畫下拉按鈕
            if (Util.PublicFunction.IsHigherWinXP())
            {
                ControlPaint.DrawComboButton(g, dropDownRectangle, this.Enabled ? System.Windows.Forms.ButtonState.Flat : System.Windows.Forms.ButtonState.All);
            }
            else
            {
                ComboBoxRenderer.DrawDropDownButton(g, dropDownRectangle, this.Enabled ? ComboBoxState.Normal : ComboBoxState.Disabled);
            }

            g.Dispose();
            bkgBrush.Dispose();
            fcBrush.Dispose();

        }