Expect everything to change...

Delegate Types

A delegate is simply a reference to a method. Delegates can be stored and passed around in a variable, and hence they must have a type definition:

private delegate int FuncTwoInts(int one, int two);

The line above defines the type FuncTwoInts. The FuncTwoInts type is a reference to a method that takes two int parameters and returns a single int result.

Delegate expressions

The FuncTwoInts type can be used to declare variables like this:

private static int Add(int one, int two)
{
    return one + two;
}

private FuncTwoInts theFunc = Add;

Or like this:

FuncTwoInts theFunc = delegate (int one, int two)
{
    return one + two;
};

Or this:

FuncTwoInts theFunc = (one, two) =>
{
    return one + two;
};

Or even like this:

FuncTwoInts theFunc = (one, two) => one + two;

Lambda expressions

The last two delegate examples above (the ones utilizing the => operator) are called lambda expressions. Lambda expressions are just a more efficient way of defining a delegate. If the function defined by the lambda expression is more than a single line, then the { } are required, as is the return keyword.

Built-in delegate types

There is a family of built-in delegate types that can be used to declare delegate variables without the need to define a custom type.

Action

The types that represent a delegate without a return value (void return) are the Action types in the System namespace.

private Action supFunction = () => Console.WriteLine("Sup?!");

Here's an example of a delegate that takes three parameters of different types:

private Action<string, int, bool> printThreeValues =
    (s, i, b) => Console.WriteLine($"string: {s}, int: {i}, bool: {b}");

Func

The Func family of delegate types can return a value of any type you wish. For example: Func<TResult> for no params and TResult return type.

private Func twoPlusThree = () => 2 + 3;

Func<T1, T2, TResult> for two parameters and TResult return type. Here is an example of a delegate declaration that takes two string parameters and returns an int:

private Func<string, string, int> sumCharacters = 
    (s1, s2) => s1.Length + s2.Length;

my thanks to this course:
https://www.codingame.com/playgrounds/345/c-linq-background-topics/review