Question

I have a piece of code which combines an in-memory list with some data held in a database. This works just fine in my unit tests (using a mocked Linq2SqlRepository which uses List).

    public IRepository<OrderItem> orderItems { get; set; }

    private List<OrderHeld> _releasedOrders = null;
    private List<OrderHeld> releasedOrders
    {
        get
        {
            if (_releasedOrders == null)
            {
                _releasedOrders = new List<nOrderHeld>();
            }
            return _releasedOrders;
        }
    }

    .....

    public int GetReleasedCount(OrderItem orderItem)
    {
        int? total =
            (
                from item in orderItems.All
                join releasedOrder in releasedOrders
                    on item.OrderID equals releasedOrder.OrderID
                where item.ProductID == orderItem.ProductID
                select new
                {
                    item.Quantity,
                }

            ).Sum(x => (int?)x.Quantity);

        return total.HasValue ? total.Value : 0;
    }

I am getting an error I don’t really understand when I run it against a database.

Exception information:
    Exception type: System.NotSupportedException
    Exception message: Local sequence cannot be used in LINQ to SQL
implementation of query operators
except the Contains() operator.

What am I doing wrong?

I’m guessing it’s to do with the fact that orderItems is on the database and releasedItems is in memory.

---

EDIT

I have changed my code based on the answers given (thanks all)

    public int GetReleasedCount(OrderItem orderItem)
    {
        var releasedOrderIDs = releasedOrders.Select(x => x.OrderID);

        int? total =
            (
                from item in orderItems.All
                where releasedOrderIDs.Contains(item.OrderID)
                   && item.ProductID == orderItem.ProductID
                select new
                {
                    item.Quantity,
                }

            ).Sum(x => (int?)x.Quantity);

        return total.HasValue ? total.Value : 0;
    }

Answer 1

I’m guessing it’s to do with the fact
that orderItems is on the database
and releasedItems is in memory.

You are correct, you can’t join a table to a List using LINQ.

Take a look at this link:

http://flatlinerdoa.spaces.live.com/Blog/cns!17124D03A9A052B0!455.entry

He suggests using the Contains() method but you’ll have to play around with it to see if it will work for your needs.