Como definir o usuário atual httpcontext.current.user asp.net -mvc

String[] newRoles = { "Administrator", "Customer" };
GenericIdentity newIdentity = new GenericIdentity(model.AccountName);
GenericPrincipal newPrincipal = new GenericPrincipal(newIdentity, newRoles);                       
System.Web.HttpContext.Current.User = newPrincipal;  
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