程序代码：

public partial class Form1 : Form
    {
        private string initString = null;
        private bool hasEnterNumber = false;

        public Form1()
        {
            InitializeComponent();
            this.textBox1.KeyPress += new KeyPressEventHandler(textBox1_KeyPress);
            this.textBox1.TextChanged += new EventHandler(textBox1_TextChanged);
        }

        void textBox1_TextChanged(object sender, EventArgs e)
        {
            if (this.hasEnterNumber)
            {
                this.textBox1.Text = this.initString;
                this.hasEnterNumber = false;
                this.textBox1.SelectionStart = this.textBox1.Text.Length;
            }
        }

        void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            if ( (int)e.KeyChar >= (int)Keys.D0 && (int)e.KeyChar <= (int)Keys.D9)
            {
                this.hasEnterNumber = true;
                initString = this.textBox1.Text;
            }
        }
    }