How do you use Func<> and Action<> when designing applications?

Question

All the examples I can find about Func<> and Action<> are simple as in the one below where you see how they technically work but I would like to see them used in examples where they solve problems that previously could not be solved or could be solved only in a more complex way, i.e. I know how they work and I can see they are terse and powerful, so I want to understand them in a larger sense of what kinds of problems they solve and how I could use them in the design of applications.

In what ways (patterns) do you use Func<> and Action<> to solve real problems?

using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace TestFunc8282 {     class Program     {         static void Main(string[] args)         {             //func with delegate             Func<string, string> convert = delegate(string s)             {                 return s.ToUpper();             };              //func with lambda             Func<string, string> convert2 = s => s.Substring(3, 10);              //action             Action<int,string> recordIt = (i,title) =>                 {                     Console.WriteLine("--- {0}:",title);                     Console.WriteLine("Adding five to {0}:", i);                     Console.WriteLine(i + 5);                 };              Console.WriteLine(convert("This is the first test."));             Console.WriteLine(convert2("This is the second test."));             recordIt(5, "First one");             recordIt(3, "Second one");              Console.ReadLine();          }     } } 

Answer 1

They're also handy for refactoring switch statements.

Take the following (albeit simple) example:

public void Move(int distance, Direction direction) {     switch (direction)     {         case Direction.Up :             Position.Y += distance;             break;         case Direction.Down:             Position.Y -= distance;             break;         case Direction.Left:             Position.X -= distance;             break;         case Direction.Right:             Position.X += distance;             break;     } } 

With an Action delegate, you can refactor it as follows:

static Something() {     _directionMap = new Dictionary<Direction, Action<Position, int>>     {         { Direction.Up,    (position, distance) => position.Y +=  distance },         { Direction.Down,  (position, distance) => position.Y -=  distance },         { Direction.Left,  (position, distance) => position.X -=  distance },         { Direction.Right, (position, distance) => position.X +=  distance },     }; }  public void Move(int distance, Direction direction) {     _directionMap[direction](this.Position, distance); }