title: C# List<> GroupBy 2 Values
body: <p>I'm using C# on Framework 3.5. I'm looking to quickly group a Generic List&lt;> by two properties. For the sake of this example lets say I have a List of an Order type with properties of CustomerId, ProductId, and ProductCount. How would I get the sum of ProductCounts grouped by CustomerId and ProductId using a lambda expression?</p>