Demonstration of “inner functions” in C#

InnerFunctions.cs

public static int Factorial(int number)
{
    if (number < 0)
        throw new Exception("Sorry, number must be zero or positive");

    Func<int, int> innerFactorial = null;
    innerFactorial = (_number) =>
    {
        if (_number <= 1)
            return 1;
        return _number * innerFactorial(_number - 1);
    }
    return innerFactorial(number);
}