How to restrict the user that is already connected

I am working in Asp.net and I want to restrict the user while login, if the same user is already logged in or already connected.. I am creating a table in sql server USERS_CONNECTED and placed a single field USER_ID in it. When ever a user is logged in it's id is searched in 开发者_C百科USERS_CONNECTED table. If id is not found then the user is allowed to connect and the user id is added in the said table. But the problem is when the X button(present on right top corner of the browser) is clicked to exit then the user id should be deleted from the USERS_CONNECTED table. WHERE SHOULD I WRITE THIS CODE ?? I MEAN ON WHAT EVENT .. can anyone help... Dev..

You can handle end of a session. Add something like this to global.asax.cs file:

protected void Session_End(object sender, EventArgs e)
{
    // Remove user from the USERS_CONNECTED table
}

Just one thing to remember: it will not be fired immediately when the user closed his browser. This event will be fired when the session expires.

To me it sounds like you are trying to implement a form of authentication. What you can do is use Forms authentication that uses a cookie that will keep track of the user's activity. Have a look at this tutorial to see how it is implemented: Forms Auth Tutorial