重写Textbox

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

namespace  MyTextBox
{
    public partial class MyTextBox : TextBox
    {
        bool isMouseClick = false;
        public MyTextBox()
        {
            InitializeComponent();
        }
        protected override void OnMouseClick(MouseEventArgs e)
        {
            base.OnMouseClick(e);
            //
            isMouseClick = true;
            isMouseLeave = false;
            Refresh();//产生点击事件以后，重画控件,NET自己会调用OnPaint()方法
        }
        protected override void OnMouseLeave(EventArgs e)
        {
            base.OnMouseLeave(e);
            //
            isMouseClick = false;
            Refresh();//鼠标离开以后，重画控件，NET自己会调用OnPaint()方法
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);//这里是 TextBox 原有的绘制代码
            //下面可以绘制放大镜

            if (isMouseClick)
            {
                //写绘制放大镜代码
            }
        }
    }
}