In SQL, you might write a query that looks something like this: SELECT * FROM Foo WHERE blah IN (1, 3, 5, 7). With LINQ to Entities you might have a similar scenario except that you are selecting from an entityset and the list of values you want to compare against is stored in a LIST<T>. Unfortunately, the Entity Framework does not currently support collection-valued parameters. To work around this restriction, you can manually construct an expression given a sequence of values using the following utility method:
static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(
Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
{
if (null == valueSelector) { throw new ArgumentNullException("valueSelector"); }
if (null == values) { throw new ArgumentNullException("values"); }
ParameterExpression p = valueSelector.Parameters.Single();
// p => valueSelector(p) == values[0] || valueSelector(p) == ...
if (!values.Any())
{
return e => false;
}
var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));
var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));
return Expression.Lambda<Func<TElement, bool>>(body, p);
}
Using this utility method, you can rewrite:
var query1 = from e in context.Entities
where ids.Contains(e.ID)
select e;
as
var query2 = context.Entities.Where(
BuildContainsExpression<Entity, int>(e => e.ID, ids));