delegate, Action/Func, closures

using System;

// Custom delegate type: a method taking two ints and returning int
public delegate int Op(int x, int y);

public class Program
{
  static int Add(int a, int b) { return a + b; }
  static int Mul(int a, int b) { return a * b; }

  public static void Main(string[] args)
  {
    Op f = Add;                // method group conversion
    Console.WriteLine(f(2, 3)); // 5
    f = Mul;
    Console.WriteLine(f(2, 3)); // 6
  }
}

using System;

public class Program
{
  static void Log(string s) { Console.WriteLine("LOG: " + s); }

  public static void Main(string[] args)
  {
    Action<string> a = Log;                        // no return
    a("hello");

    Func<int, int, int> sum = delegate(int x, int y) { return x + y; }; // lambda/anon method
    Console.WriteLine(sum(3, 4)); // 7

    // Method group to Func
    Func<string, int> len = GetLen;
    Console.WriteLine(len("abc")); // 3
  }

  static int GetLen(string s) { return s == null ? 0 : s.Length; }
}

using System;

public class Program
{
  public static void Main(string[] args)
  {
    int counter = 0; // captured variable (lives as long as delegate)
    Action inc = delegate() { counter = counter + 1; Console.WriteLine(counter); };

    inc(); // 1
    inc(); // 2
    Console.WriteLine("Final=" + counter); // 2
  }
}

using System;

public class Program
{
  // Apply a transform to each number and sum the results
  static int SumBy(int[] xs, Func<int, int> transform)
  {
    int total = 0;
    for (int i = 0; i < xs.Length; i++)
    {
      total += transform(xs[i]);
    }
    return total;
  }

  public static void Main(string[] args)
  {
    int[] data = new int[] { 1, 2, 3 };
    int sumSquares = SumBy(data, delegate(int x) { return x * x; });
    Console.WriteLine(sumSquares); // 14
  }
}