文本框使用textmode密码不显示文本asp.net c#

这是我试过的：

protected void btn_punch_7_Click(object sender,EventArgs e)
    {

        const string string_punch_Number_7 = "7";
        var text = txt_punch.Text;
        text += string_punch_Number_7;

        txt_punch.Text = text;


    }

    protected void btn_punch_8_Click(object sender,EventArgs e)
    {
        const string string_punch_Number_8 = "8";
        var text = txt_punch.Text;
        text += string_punch_Number_8;

        txt_punch.Text = text;

    }

我也累了这个：

public partial class WebForm3 : System.Web.UI.Page
{
    public string string_punch;
    protected void Page_Load(object sender,EventArgs e)
    {
        MultiView1.SetActiveView(View1);

        txt_punch.Width = 300;
        txt_punch.Height = 50;
        txt_punch.MaxLength = 4;
        txt_punch.Attributes.Add("OnChange",string_punch);

    }

    protected void btn_punch_7_Click(object sender,EventArgs e)
    {

        const string string_punch_Number_7 = "7";
        string_punch = txt_punch.Text;
        string_punch += string_punch_Number_7;

        txt_punch.Text = string_punch;


    }

    protected void btn_punch_8_Click(object sender,EventArgs e)
    {
        const string string_punch_Number_8 = "8";
        string_punch = txt_punch.Text;
        string_punch += string_punch_Number_8;

        txt_punch.Text = string_punch;

    }