store logged in user id in database from class file

Hello all,
I am storing the user id for a record who creates it. It is working fine in aspx.cs file.
myReview.Owner = (Guid)Membership.GetUser().ProviderUserKey;
but when I use the same code in a class file in App_Code folder then it does not work and code breaks. 
Is there any other way to get user id in case of class file?
Please help me I am stuck on it for a long time.
Many thanks...
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