How can I get MembershipUser from the PrincipalInfo.CurrentPrincipal.GetUserGuid

Hi,

I need to get the membership user and I have the EPiServer.Security.PrincipalInfo.CurrentPrincipal.GetUserGuid() in a variable. 

I tried Membership

Membership.GetUser(guidVariable);

but it returns null, since the GetUserGuid property is not the membership GUID.

After a lot of searches, I came up with this code:
 Mediachase.Commerce.Customers.CustomerContext customerContext = new Mediachase.Commerce.Customers.CustomerContext();
 Mediachase.Commerce.Customers.CustomerContact userContact = customerContext.GetContactById(modifierId);
 return Membership.GetUser(userContact.ProviderUserKey.ToString());

which simes to work but ProviderUserKey is Obsolete.

What shall I do?

Thanks in advance,

Ashkan