多线程编辑问题

这是后台代码：

 

using System;
 2  using System.Collections.Generic;
 3  using System.ComponentModel;
 4  using System.Data;
 5  using System.Drawing;
 6  using System.Text;
 7 using System.Windows.Forms;
 8 using System.Threading;//添加名称空间引用
 9 
10 namespace ThreadMutex1
11 {
12 publicpartialclass Form1 : Form
13     {
14 public Form1()
15         {
16             InitializeComponent();
17             CheckForIllegalCrossThreadCalls =false;//禁用此异常
18         }
19 
20 //创建显示字符的线程对象
21 private Thread thread1 =null;
22 private Thread thread2 =null;
23 
24 //显示字符
25 privatevoid ShowChar(char ch)
26         {
27 lock (this)
28             {
29                 richTextBox1.Text += ch;
30             }
31         }
32 
33 //线程thread1调用的方法（显示字符a）
34 privatevoid thread1Show()
35         {
36 while (true)
37             {
38                 ShowChar('a');
39                 Thread.Sleep(60);
40             }
41         }
42 
43 //线程thread2调用的方法（显示字符A）
44 privatevoid thread2Show()
45         {
46 while (true)
47             {
48                 ShowChar('A');
49                 Thread.Sleep(30);
50             }
51         }
52 
53 //线程初始化，并启动线程
54 privatevoid button1_Click(object sender, EventArgs e)
55         {
56             
57             thread1 =new Thread(new ThreadStart(thread1Show));
58             thread2 =new Thread(new ThreadStart(thread2Show));
59             thread1.Start();
60             thread2.Start();
61             button1.Enabled =false;
62             button2.Enabled =true;
63         }
64 
65 //终止线程
66 privatevoid button2_Click(object sender, EventArgs e)
67         {
68             thread1.Abort();
69             thread2.Abort();
70             button1.Enabled =true;
71             button2.Enabled =false;
72         }
73 
74 //关闭窗体时终止线程（否则，VS调试程序将仍处于运行状态）
75 privatevoid Form1_FormClosing(object sender, FormClosingEventArgs e)
76         {
77 if (thread1 !=null) thread1.Abort();
78 if (thread2 !=null) thread2.Abort();
79         }
80     }
81 }

 

 

前台显示：

 

正常运行：

 

但，当我把排他锁去掉时，在启动线程，文本框的A和a显示就会自动清空。。。不解！