how to change the textboxmode property on checkbox click ?

Question

 protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            txtCl.Text = TextBoxMode.SingleLine;


            }
        else
        {
            txtCl.TextMode = TextBoxMode.Password;
        }
    }
but it doesnt work..