What is the difference between an IQueryable's Contains() and an IEnumerable's Contains()?

Question

I have an IQueryable custs, a Customer cust, a CustomerComparer custCp which implements IEqualityComparer.

When I call custs.Contains(cust, custCp) I get an exception:

System.NotSupportedException: Unsupported overload used for query operator 'Contains'

But when I call custs.AsEnumerable().Contains(cust,custCp) it works. Can anyone explain why?

Answer 1

An operation on an IQueryable (in the case of Linq to Entities) is translated to SQL and executed in the database. Since you have an IEqualityComparer written in C#, the comparer can't be translated to SQL and the overload can't be supported.

When you translate it to an IEnumerable using AsEnumerable(), all the data is transferred from the database to memory, where the overload can be easily supported. The downside of course being that you're transferring more data than necessary (potentially the whole table) to have it filtered in memory.

Answer 2

IQueryable is implemented by query providers and under the hood translated into something the target system can understand. For example, IQueryable.Contains might be translated to a x IN y SQL expression.

On the other hand, IEnumerable is not translated like that. Its Contains works on anything that implements IEnumerable properly.

Using the IQueryable operators are executed server-side and therefore will gain you some performance as the query is executed remotely, whereas IEnumerable is executed locally (client-side). This means that if you were to get a database table, convert it to enumerable and then apply Contains on it, the whole table needs to be downloaded to your computer to be enumerated.