how to you show if a user is online or not

hello,

 i want to show if a user is logged in or not by displaying a msg in a label.

I have to code this in c# and i'm not as familiar with the syntax's. 

 the page that it will be displayed on is a persons profile page, when they create the page, their username is appened to the end of the URL of that page.  I have that sent to a variable i can then use, that contains that users login name for that profile page.

 i figure i can do something like

if membership. something something (username) = true

blah blah blah

just not sure what syntax to check the specified username to see if online or not.

 

thanks for any help.
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