How store user id after login

1. In a ASP.NET application the session for the current user could expire after few or more minutes, based on the associated setting from web.config.

In order to fix this you should use the next setting from your web.config:

<sessionstate mode="InProc" cookieless="false" timeout="1440" />

2. If you want to preserve the user ID between postbacks, after the user login you should cache its value in the current Session like in the next example:

Session["UserID"]= userID;

Then during the postabcks, when you need the user ID, you could access it form the Session cache like in the code below:

int userID = (int)Session["UserID"];

See links..
All about the Session..
http://msdn.microsoft.com/en-us/library/ms178581(v=vs.100).aspx[^]
http://asp.net-tutorials.com/state/sessions/[^]
http://runnable.com/Uhxav7qocnlxAAA8/asp-net-how-to-use-session[^]