When C# 6 came out to the market, I was thinking, why should I bother. What is so nice about C#6 so that I need to upgrade my VS 2013 to VS 2015. I found one, and that was that was compelling enough for me to do the switch. It was nameof(..) operator. Yes, I was highly excited to replace all of my magic strings with nameof(..). All places where I had to use hard-coded property names (WPF ViewModels, dependency properties...etc) was looking nice and strongly typed with nameof(..).

This time, with c#7, I was looking for at least one feature that can convince myself to do the upgrade again. I found that one feature. It is the 'Local Function'.

Local functions

Here is an example of a local function.

private static void Main(string[] args)
{
    int Square(int number)
    {
        return number * number;
    }
 
    Console.WriteLine(Square(100));
}

Well, initially I was thinking, what is the extra benefit in a local function? I can achieve the same goal by using Func<T,T> delegates. Can't I ? Moreover, I was thinking, what an ugly way of writing code. A method should not be complicated; a method should do only one thing. An obese method with many local functions simply stinks. Whenever we see a method is doing lots of things, we should think about encapsulating the logic to a separate class. Why it is beneficial over a Func<T,T> delegate! Then I found the following article where it is explained nicely.

https://github.com/dotnet/roslyn/issues/3911

The bottom line is, the local function is more efficient (performance wise) than a local lambda delegate. When I heard the word ‘performance,' I was sold.

Here is the answer from that page: You want a helper function. You are only using it from within a single function, and it likely uses variables and type parameters that are in scope in that containing function. On the other hand, unlike a lambda, you don't need it as a first class object, so you don't care to give it a delegate type and allocate an actual delegate object. Also, you may want it to be recursive or generic, or to implement it as an iterator.

The answer given by a stack overflow member named ‘svick’ was splendid:
http://stackoverflow.com/a/40949214/647459