Unfortunately, the membership provider and the other providers use IQueryable when getting collection of items, in order to be able to filter it later on. The provider has to be implemented using queryable collections since the derived classes and interfaces need them. The MembershipDataProvider we have provided is made the way it is, so we ensure if you override the methods we can provide all the functionality working as expected and as the default membership provider.
You can use the AsQueryable extension method on the queried collection, however, since all items are loaded this will slow down performance. You can see from our full code sample
that we have also done this in some of the methods where this is inevitable.
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