using System;

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// Demo: Delegation

public delegate void Log(string message);

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public class Calculator

{

    private readonly Log _logger;

    public Calculator(Log logger)

    {

        _logger = logger;

    }

    public int Add(int a, int b)

    {

        _logger(String.Format("Adding {1} to {0}", a, b));

        return a + b;

    }

    public int Subtract(int a, int b)

    {

        _logger(String.Format("Subtracting {1} from {0}", a, b));

        return a - b;

    }

}

​

​

var calc = new Calculator(Console.WriteLine);

​

Console.WriteLine("Sum: {0}", calc.Add(10, 20));

Console.WriteLine("Diff: {0}", calc.Subtract(20, 10));

​

​

// Built-in Delegates

// Action<T> - Statements, side-effects (numerous generic overloads)

// Func<T> - Functions, return values (numerous generic overloads)

// Predicate<T> - Seldom used, same as Func<T, bool>

​

​

public class Calculator2

{

    private readonly Action<string> _logger;

    public Calculator2(Action<string> logger)

    {

        _logger = logger;

    }

    public int Add(int a, int b)

    {

        _logger(String.Format("Adding {1} to {0}", a, b));

        return a + b;

    }

    public int Subtract(int a, int b)

    {

        _logger(String.Format("Subtracting {1} from {0}", a, b));

        return a - b;

    }

}

​

var calc2 = new Calculator(Console.WriteLine);

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Console.WriteLine("Sum: {0}", calc2.Add(10, 20));

Console.WriteLine("Diff: {0}", calc2.Subtract(20, 10));

​

Console.WriteLine();

​

​

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// Delegates vs simple interfaces

// Delegates are functional programming's version of an interface

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// Which would you rather write and maintain?

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interface IDateTimeService

{

    DateTime GetCurrent();

}

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public class DateTimeService

    : IDateTimeService

{

    public DateTime GetCurrent() { return DateTime.Now; }

}

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var dtSvc = new DateTimeService();

Console.WriteLine(dtSvc.GetCurrent());

​

// - or -

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Func<DateTime> dtSvc2 = () => DateTime.Now;

Console.WriteLine(dtSvc2());

​

​