Limiting what a textbox can contain

Try this:

private void txtZip_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (char.IsDigit(e.KeyChar) || char.IsControl(e.KeyChar))
            {
                e.Handled = false;
            }
            else
            {
                e.Handled = true;
            }
        }

Use the following java script function and call it on keypress event of textbox.

function isNumberKey(evt){
                 var charCode = (evt.which) ? evt.which : event.keyCode
                 if (charCode > 31 && (charCode < 48 || charCode > 57)){
                 alert("Please enter digits");
                      return false;
                  }
                  else{   
                      return true;
                  }   
            }