Detect if Browser is closed or Machine is switched off in ASP.Net

Hi Everyone;

My program need to detect the user visibility, if the user is login or not. I Put the information on the database 

here the pseudocode:

if the user click "LOGIN" I Update the user information to ONLINE = TRUE;

else if the user click "LOGOUT"  I Update the user information to ONLINE = FALSE;

My Question is: How can I detect if the user leave the website or turn off the computer without cllicking "logout"

thanks best wishes