c#多选下拉框（ComboBox）

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代码如下
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 <img alt="" class="code_img_opened" id="code_img_opened_d569f1df-fa0b-4534-a2fb-3ce1019c87fc" src="https://beijingoptbbs.oss-cn-beijing.aliyuncs.com/cs/5606289-961ddebeb323a10fe0623af514929fc1.gif">
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<pre class="blockcode"> 1 using System;
 2 using System.Collections.Generic;
 3 using System.ComponentModel;
 4 using System.Drawing;
 5 using System.Data;
 6 using System.Linq;
 7 using System.Text;
 8 using System.Windows.Forms;
 9 using System.Collections;
 10 using System.Reflection;
 11 using Newtonsoft.Json.Linq;
 12
 13 namespace ControlsAA
 14 {
 15 public class ComboBoxEx : ComboBox
 16 {
 17 TreeView lst = new TreeView();
 18
 19 public ComboBoxEx()
 20 {
 21 this.DrawMode = DrawMode.OwnerDrawFixed;//只有设置这个属性为OwnerDrawFixed才可能让重画起作用
 22 lst.KeyUp += new KeyEventHandler(lst_KeyUp);
 23 lst.MouseUp += new MouseEventHandler(lst_MouseUp);
 24 // lst.KeyDown += new KeyEventHandler(lst_KeyDown);
 25 lst.Leave += new EventHandler(lst_Leave);
 26 lst.CheckBoxes = true;
 27 lst.ShowLines = false;
 28 lst.ShowPlusMinus = false;
 29 lst.ShowRootLines = false;
 30 this.DropDownHeight = 1;
 31 }
 32
 33 void lst_Leave(object sender, EventArgs e)
 34 {
 35 lst.Hide();
 36 }
 37 <span style="color:#