Question

I have 2 Lists containing entries of a custom class I have created. Basically, each lists contains a customers orders – but they come from 2 different sources, hence the 2 List’s. Both list’s are the same type however (my custom class), and there is a field to identify which customer the order belongs to.

What I want to do is make a new list, containing each customer that has an order in either of the lists, and the count of the orders from both lists. So, they’ll be 3 fields: The customer’s ID, the number of orders from list 1, and the number of orders from list 2.

I know there’s many ways to do this, but all the ways I’ve thought of seem needlessly complex. Is there a way I can do this?

Answer 1

Something like this would be a good starting point for further tweaks:

var ordersInList1 = list1.GroupBy(order => order.CustomerId)
                         .ToDictionary(g => g.Key, g => g.Count());
var ordersInList2 = list2.GroupBy(order => order.CustomerId)
                         .ToDictionary(g => g.Key, g => g.Count());

var results = ordersInList1.Keys.Union(ordersInList2.Keys)
                           .Select(id =>
                           new { 
                                CustomerId = id, 
                                CountOrders1 = ordersInList1[id], 
                                CountOrders2 = ordersInList2[id]
                           })
                           .ToArray();