The Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) method makes it simple to perform a calculation over a sequence of values. This method works by calling func one time for each element in source except the first one. Each time func is called, Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) passes both the element from the sequence and an aggregated value (as the first argument to func). The first element of source is used as the initial aggregate value. The result of func replaces the previous aggregated value. Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) returns the final result of func.
This overload of the Aggregate method isn't suitable for all cases because it uses the first element of source as the initial aggregate value. You should choose another overload if the return value should include only the elements of source that meet a certain condition. For example, this overload isn't reliable if you want to calculate the sum of the even numbers in source. The result will be incorrect if the first element is odd instead of even.
To simplify common aggregation operations, the standard query operators also include a general purpose count method, Count, and four numeric aggregation methods, namely Min, Max, Sum, and Average.