How do I write a LINQ to Entities query which has the equivalent of the SQL “in” keyword?

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));