Question

I want to generate this SQL statement in LINQ:

select * from Foo where Value in ( 1, 2, 3 )

The tricky bit seems to be that Value is a column that allows nulls.

The equivalent LINQ code would seem to be:

IEnumerable<Foo> foos = MyDataContext.Foos;
IEnumerable<int> values = GetMyValues();
var myFoos = from foo in foos
             where values.Contains(foo.Value)
             select foo;

This, of course, doesn’t compile, since foo.Value is an int? and values is typed to int.

I’ve tried this:

IEnumerable<Foo> foos = MyDataContext.Foos;
IEnumerable<int> values = GetMyValues();
IEnumerable<int?> nullables = values.Select( value => new Nullable<int>(value));
var myFoos = from foo in foos
             where nullables.Contains(foo.Value)
             select foo;

…and this:

IEnumerable<Foo> foos = MyDataContext.Foos;
IEnumerable<int> values = GetMyValues();
var myFoos = from foo in foos
             where values.Contains(foo.Value.Value)
             select foo;

Both of these versions give me the results I expect, but they do not generate the SQL I want. It appears that they’re generating full-table results and then doing the Contains() filtering in-memory (ie: in plain LINQ, without -to-SQL); there’s no IN clause in the DataContext log.

Is there a way to generate a SQL IN for Nullable types?

NOTE

As it turns out, the problem I was having didn’t have anything to do Contains or Nullable, and so the phrasing of my question is largely irrelevant. See @Nick Craver’s accepted answer for details.

Answer 1

This should work for your example:

IEnumerable<int> values = GetMyValues();
var myFoos = from foo in MyDataContext.Foos;
             where values.Contains(foo.Value.Value)
             select foo;

Casting as an IEnumerable<T> from the start means execution will be outside of SQL, instead call the IQueryable<Foo> directly. If you cast as IEnumerable and use it in a query, it gets all MyDataContext.Foos then uses that iterator and executes the rest of the query in C# instead of in SQL.

If you want to run in SQL, don’t cast as IEnumerable anywhere along the way. The effect is the same as using MyDataContext.Foos.AsEnumerable() in the query.