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;

        }

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；