Entity Framework - "Unable to create a constant value of type 'Closure type'..." error

It looks like you're trying to do the equivalent of a "WHERE...IN" condition. Check out How to write 'WHERE IN' style queries using LINQ to Entities for an example of how to do that type of query with LINQ to Entities.

Also, I think the error message is particularly unhelpful in this case because .Contains is not followed by parentheses, which causes the compiler to recognize the whole predicate as a lambda expression.

I've spent the last 6 months battling this limitation with EF 3.5 and while I'm not the smartest person in the world, I'm pretty sure I have something useful to offer on this topic.

The SQL generated by growing a 50 mile high tree of "OR style" expressions will result in a poor query execution plan. I'm dealing with a few million rows and the impact is substantial.

There is a little hack I found to do a SQL 'in' that helps if you are just looking for a bunch of entities by id:

private IEnumerable<Entity1> getByIds(IEnumerable<int> ids)
{
    string idList = string.Join(",", ids.ToList().ConvertAll<string>(id => id.ToString()).ToArray());
    return dbContext.Entity1.Where("it.pkIDColumn IN {" + idList + "}");
}

where pkIDColumn is your primary key id column name of your Entity1 table.

BUT KEEP READING!

This is fine, but it requires that I already have the ids of what I need to find. Sometimes I just want my expressions to reach into other relations and what I do have is criteria for those connected relations.

If I had more time I would try to represent this visually, but I don't so just study this sentence a moment: Consider a schema with a Person, GovernmentId, and GovernmentIdType tables. Andrew Tappert (Person) has two id cards (GovernmentId), one from Oregon (GovernmentIdType) and one from Washington (GovernmentIdType).

Now generate an edmx from it.

Now imagine you want to find all the people having a certain ID value, say 1234567.

This can be accomplished with a single database hit with this:

dbContext context = new dbContext();
string idValue = "1234567";
Expression<Func<Person,bool>> expr =
    person => person.GovernmentID.Any(gid => gid.gi_value.Contains(idValue));

IEnumerable<Person> people = context.Person.AsQueryable().Where(expr);

Do you see the subquery here? The generated sql will use 'joins' instead of sub-queries, but the effect is the same. These days SQL server optimizes subqueries into joins under the covers anyway, but anyway...

The key to this working is the .Any inside the expression.