Functions

There are two ways to define anonymous functions in C#:

• Delegates, which take a list of arguments greater than or equal to 0 and have a return type
• Actions, which take a list of arguments greater than or equal to 0 and have no return type

Delegate

You can define an anonymous delegate function with a return type by using the Func object. Func takes some number of arguments and returns something. When declaring a Func you should follow it by < > angle brackets with the input argument type and return type. You can use standard lambda calculus syntax to define anonymous functions.

Func<int, int> Double = x => x * 2
Func<int> Three = () => 3 // delegate that takes no arguments

You can use lexical scopes with delegates.

var y = 20;

Func<int, int> Product = x =>
{
  // y contained in the lexical scope of Product
  return x * y;
};

Product(10); // => 200

You can define a function that takes another function with a return type as an argument by using the Func delegate object. Func delegates define inputs in the <> angle brackets at definition with the last type definition in the angle brackets being the output type. In the example below MyFunc is a delegate that takes an int and returns an int.

public void RunThis(int[] arr, Func<int, int> MyFunc)
{
  foreach(int i in arr)
  {
    var result = MyFunc(i);
    Console.WriteLine(result);
  }
}

int[] arr = new int[] {1, 2, 3, 4, 5};
RunThis(arr, x => x * 2); // => 2, 4, 6, 8, 10

Action

Many of the rules that apply to delegates also apply to actions, so unless functionality is specifically stated assume that they overlap.

You can define an anonymous function without a return type by using the Action. Action takes some number of arguments are returns void. You can define an Action by using the Action type followed by < > angle brackets with the argument types in them.

Action<int> EchoInt = i => Console.WriteLine(i);

Splat Arguments

You can pass multiple arguments to a function using the param keyword and then accessing the arguments as an array.

void EchoNumbers(params int[] foo)
{
  foreach(int i in foo)
  {
    Console.WriteLine(i);
  }
}

EchoNumbers(1, 2, 3);
// => 1
// => 2
// -> 3