IQueryable member methods not behaving?

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 man开发者_如何学Pythony 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?

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());