private void textBox1_TextChanged(object sender, EventArgs e)
        {
            string s1 = textBox1.Text;

            textBox1.Text = "";

            MatchCollection m1 = Regex.Matches(s1, @"^[\u4e00-\u9fa5]{0,}$");


            foreach (Match m in m1)
            {

                textBox1.Text += m.ToString();


            }
            k += textBox1.Text;
            textBox1.Text = k;
            textBox1.SelectionStart = textBox1.Text.Length;

        }

代码貌似有点乱，
textBox1.Text = "";
textBox1.Text += m.ToString();
textBox1.Text = k;
多次回调textBox1_TextChanged(）函数
k又是干什么的
 改下试试看

   private void textBox1_TextChanged(object sender, EventArgs e)
        {
            string s = textBox1.Text;
            MatchCollection m1 = Regex.Matches(s, @"^[\u4e00-\u9fa5]{0,}");
            string str = "";
            foreach (Match m in m1)
            {
                str += m.Value;
            }
            textBox1.Text = str;
            textBox1.SelectionStart = textBox1.Text.Length;
        }

 

很感谢你，我知道了

textBox1.Text += m.ToString();
又引发textBox1_TextChanged事件
所以是一无限循环；

命名规范。。其他的我不会。