ASP.NET的一个问题！！

Question

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        switch (this.DropDownList1.SelectedValue)
        {
            case 1:
                {
                    this.TextBox1.Visible = true;
                    this.TextBox2.Visible = false;
                    this.TextBox3.Visible = false;
                    this.TextBox4.Visible = false;
                }
            case 2:
                {
                    this.TextBox1.Visible = true;
                    this.TextBox2.Visible = true;
                    this.TextBox3.Visible = false;
                    this.TextBox4.Visible = false;
                }
            case 3:
                {
                    this.TextBox1.Visible = true;
                    this.TextBox2.Visible = true;
                    this.TextBox3.Visible = true;
                    this.TextBox4.Visible = false;
                }
            case 4:
                {
                    this.TextBox1.Visible = true;
                    this.TextBox2.Visible = true;
                    this.TextBox3.Visible = true;
                    this.TextBox4.Visible = true;
                }
        }        this.TextBox5.Text = 4 - Int32.Parse(this.DropDownList1.SelectedValue.ToString());
    }
我也是菜鸟，要是不对请不要介意``