C# winform TextBox 边框只有最下面一条边框

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Drawing;

namespace ChineseCalender
{
   public  class myTextBox:TextBox
    {

       public myTextBox()
         {
            //去掉边框
             this.BorderStyle = BorderStyle.None;
             // BackColor也可以自己设置
             this.BackColor = SystemColors.Control;
         }
         //截获消息，在原来的下边框位置在画一条线
         private int WM_PAINT = 0x000F;
         protected override void WndProc(ref Message m)
         {
             base.WndProc(ref m);

             if (m.Msg == WM_PAINT)
             {
                 Pen pen = new Pen(Brushes.Black, 1.5f);
                 using (Graphics g = this.CreateGraphics())
                 {
                     g.DrawLine(pen, new Point(0, this.Size.Height - 1), new Point(this.Size.Width, this.Size.Height - 1));
                 }
             }
         }
    }
}