The expression editor uses the standard .NET Framework extension methods of the Enumerable class which you can find here. We simply parse the string that the user has entered and then create a lambda expression that calls one of those standard .NET methods of the Enumerable class.
So let's take the Average method for example. Here is its signature:
public static float Average<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector)
If you were writing C# code yourself you could write something like this:
var result = customers.Average(customer => customer.Age);
The same applies for the Sum method.
the Telerik team