When & why to use delegates?

A delegate is a reference to a method. Whereas objects can easily be sent as parameters into methods, constructor or whatever, methods are a bit more tricky. But every once in a while you might feel the need to send a method as a parameter to another method, and that's when you'll need delegates. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DelegateApp {

  /// <summary>
  /// A class to define a person
  /// </summary>
  public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
  }

  class Program {
    //Our delegate
    public delegate bool FilterDelegate(Person p);

    static void Main(string[] args) {

      //Create 4 Person objects
      Person p1 = new Person() { Name = "John", Age = 41 };
      Person p2 = new Person() { Name = "Jane", Age = 69 };
      Person p3 = new Person() { Name = "Jake", Age = 12 };
      Person p4 = new Person() { Name = "Jessie", Age = 25 };

      //Create a list of Person objects and fill it
      List<Person> people = new List<Person>() { p1, p2, p3, p4 };

      //Invoke DisplayPeople using appropriate delegate
      DisplayPeople("Children:", people, IsChild);
      DisplayPeople("Adults:", people, IsAdult);
      DisplayPeople("Seniors:", people, IsSenior);

      Console.Read();
    }

    /// <summary>
    /// A method to filter out the people you need
    /// </summary>
    /// <param name="people">A list of people</param>
    /// <param name="filter">A filter</param>
    /// <returns>A filtered list</returns>
    static void DisplayPeople(string title, List<Person> people, FilterDelegate filter) {
      Console.WriteLine(title);

      foreach (Person p in people) {
        if (filter(p)) {
          Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
        }
      }

      Console.Write("\n\n");
    }

    //==========FILTERS===================
    static bool IsChild(Person p) {
      return p.Age < 18;
    }

    static bool IsAdult(Person p) {
      return p.Age >= 18;
    }

    static bool IsSenior(Person p) {
      return p.Age >= 65;
    }
  }
}

Output:

Children:
Jake, 12 years old


Adults:
John, 41 years old
Jane, 69 years old
Jessie, 25 years old


Seniors:
Jane, 69 years old

I agree with everything that is said already, just trying to put some other words on it.

A delegate can be seen as a placeholder for a/some method(s).

By defining a delegate, you are saying to the user of your class, "Please feel free to assign, any method that matches this signature, to the delegate and it will be called each time my delegate is called".

Typical use is of course events. All the OnEventX delegate to the methods the user defines.

Delegates are useful to offer to the user of your objects some ability to customize their behavior. Most of the time, you can use other ways to achieve the same purpose and I do not believe you can ever be forced to create delegates. It is just the easiest way in some situations to get the thing done.


Say you want to write a procedure to integrate some real-valued function f (x) over some interval [a, b]. Say we want to use the 3-Point Gaussian method to do this (any will do, of course).

Ideally we want some function that looks like:

// 'f' is the integrand we want to integrate over [a, b] with 'n' subintervals.
static double Gauss3(Integrand f, double a, double b, int n) {
  double res = 0;

  // compute result
  // ...

  return res;
}

So we can pass in any Integrand, f, and get its definite integral over the closed interval.

Just what type should Integrand be?

Without Delegates

Well, without delegates, we'd need some sort of interface with a single method, say eval declared as follows:

// Interface describing real-valued functions of one variable.
interface Integrand {
  double eval(double x);
}

Then we'd need to create a whole bunch of classes implementing this interface, as follows:

// Some function
class MyFunc1 : Integrand {
  public double eval(double x) {
    return /* some_result */ ;
  }
}

// Some other function
class MyFunc2 : Integrand {
  public double eval(double x) {
    return /* some_result */ ;
  }
}

// etc

Then to use them in our Gauss3 method, we need to invoke it as follows:

double res1 = Gauss3(new MyFunc1(), -1, 1, 16);
double res2 = Gauss3(new MyFunc2(), 0, Math.PI, 16);

And Gauss3 needs to do the look like the following:

static double Gauss3(Integrand f, double a, double b, int n) {
  // Use the integrand passed in:
  f.eval(x);
}

So we need to do all that just to use our arbitrary functions in Guass3.

With Delegates

public delegate double Integrand(double x);

Now we can define some static (or not) functions adhering to that prototype:

class Program {
   public delegate double Integrand(double x);   
   // Define implementations to above delegate 
   // with similar input and output types
   static double MyFunc1(double x) { /* ... */ }
   static double MyFunc2(double x) { /* ... */ }
   // ... etc ...

   public static double Gauss3(Integrand f, ...) { 
      // Now just call the function naturally, no f.eval() stuff.
      double a = f(x); 
      // ...
   }

   // Let's use it
   static void Main() {
     // Just pass the function in naturally (well, its reference).
     double res = Gauss3(MyFunc1, a, b, n);
     double res = Gauss3(MyFunc2, a, b, n);    
   }
}

No interfaces, no clunky .eval stuff, no object instantiation, just simple function-pointer like usage, for a simple task.

Of course, delegates are more than just function pointers under the hood, but that's a separate issue (function chaining and events).

Tags:

C#

.Net

Delegates

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