How to identify the username (from aspnet_Users) when logged in?

 When someone logs into my website (using forms authentication), I can show what their username is, in the .aspx page, using: <asp:LoginName ID="LoginName1" runat="server" />
What I would like to do is to get this same username using the code-behind page, throw it into a string, and pass it around and do stuff knowing who the user is.
Obviously very easy to do, when you know how... all the other forum posts seem to be concerned with getting the Windows username, which, in my case here, is of no interest.
Thanks,
David
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