How to access User Id on all pages in ASP.Net

After the successful login store the UserName in Session. Using the Session you can access the UserName in all webpages of your Website.

 In Login Page.

// After Successful Authentication write this line
Session["UserName"] = this.txtUserName.Text.Trim();

And when User logout from the website you can clear the Session like this

Session.Abandon();
Session.Clear();