title: How to get button code error if textbox empty ? VS 2017
body: <p>Hello I'm a newbie at this, I make a form in VS 2017, but there is something bugging me. What code to make button error if textbox is empty ?</p>
  
  <p>its like
  ID          (TextBox)
  Password    (Textbox)
  Button.. I want if someone didnt fill Id and password when they click button, it will pop up msgbox error that say id and password still empty. </p>