Question

Having IEnumerable<Order> orders, how to get a Dictionary<string, IEnumerable<Order>> using Linq, where the key is Order.CustomerName mapped to a IEnumerable of customer’s orders.

orders.ToDictionary(order => order.CustomerName) is not going to work right away, since there could be multiple orders that could have the same CustomerName.

Solution: orders.ToLookup(order => order.CustomerName);

Answer 1

The ILookup interface is designed for this purpose, and represents a dictionary-like structure that contains many values per key. It has similar performance characteristics to those of a dictionary (i.e. it’s a hashtable type structure)

You can create an ILookup using the .ToLookup extension method as follows:

ILookup<string, Order> ordersLookup = orders.ToLookup(o => o.CustomerName)

then:

IEnumerable<Order> someCustomersOrders = ordersLookup[someCustomerName];