'Contains()' workaround using Linq to Entities?

Question

I\'m trying to create a query which uses a list of ids in the where clause, using the Silverlight ADO.Net Data Services client api (and therefore Linq To Entities). Does any

Answer 1

Update: EF ≥ 4 supports Contains directly (Checkout Any), so you don't need any workaround.

public static IQueryable WhereIn
  (
    this ObjectQuery query,
    Expression> selector,
    IEnumerable collection
  )
{
  if (selector == null) throw new ArgumentNullException("selector");
  if (collection == null) throw new ArgumentNullException("collection");
  if (!collection.Any()) 
    return query.Where(t => false);

  ParameterExpression p = selector.Parameters.Single();

  IEnumerable equals = collection.Select(value =>
     (Expression)Expression.Equal(selector.Body,
          Expression.Constant(value, typeof(TValue))));

  Expression body = equals.Aggregate((accumulate, equal) =>
      Expression.Or(accumulate, equal));

  return query.Where(Expression.Lambda>(body, p));
}

//Optional - to allow static collection:
public static IQueryable WhereIn
  (
    this ObjectQuery query,
    Expression> selector,
    params TValue[] collection
  )
{
  return WhereIn(query, selector, (IEnumerable)collection);
}

USAGE:

public static void Main()
{
  using (MyObjectContext context = new MyObjectContext())
  {
    //Using method 1 - collection provided as collection
    var contacts1 =
      context.Contacts.WhereIn(c => c.Name, GetContactNames());

    //Using method 2 - collection provided statically
    var contacts2 = context.Contacts.WhereIn(c => c.Name,
      "Contact1",
      "Contact2",
      "Contact3",
      "Contact4"
      );
  }
}