IQueryable member methods not behaving?

Question

This is my code:

    IQueryable<Car> list = new List<Car>().AsQueryable<Car>();

    foreach (TreeNode node in DataHierarchyTree.CheckedNodes)
    {
        var a = from c in ContextDB.Cars
                where c.CarID == Int32.Parse(node.Value)
                select c;

        list.Union(a.ToList());
    }

    CarGridView.DataSource = list;
    CarGridView.DataBind();

This renders nothing. I have run it through with stop points and it iterates 5 times. If I inspect the a value there is a SELECT statement generated which produces plenty of rows in the result set when used with the values in the checked nodes list.

The problem is that no matter how many results are produced when I step the through code, the list is always empty.

Why is this and what should I do to get the list of cars I'm looking for?

Answer 1

Like all LINQ methods, the Union method returns a new IQueryable<T> containing the results of the union.
It does not modify the original instance.

Therefore, when you write list.Union(a.ToList()), you are creating a new IQueryable<Car> containing all of the cars, but not doing anything with it.
The list variable doesn't change.

You need to assign the new sequence to the list variable, like this:

list = list.Union(a.ToList());