源码如下：

using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;using System.Threading;namespace PracticeThreadAppendString{    public partial class threadAppendStrForm : Form    {        StringBuilder sb = new StringBuilder();        Thread threadA, threadB;        public bool isStart = false;        private void appendString(string s)        {            lock (sb)            {                sb.Append(s);            }        }        private void methodA()        {            while (true)            {                if( !this.isStart){                    break;                }                Thread.Sleep(100);                appendString("A");            }        }        private void methodB()        {            while (true)            {                if( !this.isStart){                    break;                }                Thread.Sleep(100);                appendString("B");            }        }        public threadAppendStrForm()        {            InitializeComponent();        }        private void startThreadButton_Click(object sender, EventArgs e)        {            if (this.isStart)            {                return;            }            this.isStart = true;            //清空缓冲区            sb.Remove(0, sb.Length);            appendStrTimer.Enabled = true;            threadA = new Thread(new ThreadStart(methodA));            threadB = new Thread(new ThreadStart(methodB));            threadA.Start();            threadB.Start();        }        private void exitThreadButton_Click(object sender, EventArgs e)        {            this.isStart = false;                    }        private void appendStrTimer_Tick(object sender, EventArgs e)        {            if(threadA.IsAlive == true || threadB.IsAlive == true){                showRichTextBox.Text = sb.ToString();            }else{                appendStrTimer.Enabled = false;            }        }    }}

效果如下：