I want to show a modal dialog

Question

I want to show a modal dialog after execute btn_click event to inform user ("saving complete").

 

what should i write in the behind code. do i need to use javascript?  

 

if i click the button and i need to validate something some alert will pop on and im done with that. 

 

now my problem is i wanna show the modal if the validation is done or i dont have any validation