自定义控件-TextBoxEx，让其无边框只有下划线

自定义控件，继承TextBox 

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Drawing;
namespace Do
{
     class TextBoxEx : TextBox
     {
         public TextBoxEx()
         {
             this.BorderStyle = BorderStyle.None;
             // BackColor也可以自己设置
             this.BackColor = SystemColors.Control;
         }
         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));
                 }
             }
         }
     }
}