Question

I need to build a configurable query builder that allows the user to compound any number of possible conditions. For example, lets say we have the classic case of Customer, Order, OrderItem and Product. The user wants to be able to draw a report on all orders that have more than 3 items. Or maybe orders that have exactly one item. Should be basically the same query, except that the comparator will be different.

The user might also want to tack on a condition that the customer lives in area code 10024. Or that the order includes at least/most/exactly 2 items of Product ID 14. Each condition will obviously need to be hard coded in its own method, but the basic signature for each condition will be:

private static Expression<Func<Order, bool>> SomePredicate(object someParameters)

So far, here’s what I’ve got for the last condition I described:

private static Expression<Func<Order, bool>> CountProductItems(int productID, int count, Comparator comparator) {
  Expression<Func<Order, int>> productOrderItems = 
    order => order.Items.Where(i => i.ProductID == productID)
                  .Sum(i => i.Quantity);
  switch (comparator) {
    case Comparator.Equals:
      // return... uh... now what?
    case Comparator.GreaterThan:
      // you get the picture....
  }
}

I’m just missing that last syntactical step: how to use that expression productOrderItems and return orders where that expression equals/is less than/greater than/etc. the supplied value count.

How do you do it?

Answer 1

Instead of working with Expression objects inside the method, use Func:

Func<Order, int> productOrderItems =
    (Order o) => o.Items.Where(i => i.ProductID == productID)
                        .Sum(i => i.Quantity);

Then inside your switch, you can compose another Lambda expression on top of productOrderItems as your return value. It will automatically be converted to an Expression, as Expression and Func are technically interchangeable:

switch (comparator) {
    case Comparator.Equals:
        return (Order o) => productOrderItems(o) == count;
        break;
    // etc.
}