The LINQ Aggregate() extension method uses a Func<int, int, int> to operate on items in a series. If you want to use it, for example, to return the product of each value with its successor, you can do something like this:

Func&lt;<span style="color: #0000ff">int</span>, <span style="color: #0000ff">int</span>, <span style="color: #0000ff">int</span>&gt; producter = (one, two) =&gt; one * two;
var result = subString.ToCharArray().ToDigits().Aggregate(producter);

Of course, you don’t need the intermediate value. You can simply use a lambda directly for the Aggregate()’s parameter:

<span style="color: #008000">//Func&lt;int, int, int&gt; producter = (one, two) =&gt; one * two;</span>
var result = subString.ToCharArray().ToDigits().Aggregate((p1,p2) =&gt; p1 * p2);

With a loop to keep track of the largest result returned for a substring of length 5, you can easily use this technique to solve Euler 8.