C# Tutorial Lesson 14: Delegates

A delegate is a reference type that points to a method. This is how MSDN documentation describes a delegate:

"Once a delegate is assigned a method, it behaves exactly like that method. The delegate method can be used like any other method, with parameters and a return value."

Take a look at this delegate:

public delegate int SumPtr(int x,int y);

Let's make a method that could be called by the delegate:

int Add(int x,int y)
{
  return x+y;
}

This is how we use a delegate to call this method:

SumPtr sPtr=new SumPtr(Add);
Console.WriteLine(sPtr(34,6));//prints 40

In .NET 2.0 we can also use an annonymous method with a delegate:

SumPtr sPtr2 = delegate(int x,int y)
{
  return x+y;
}

.method private hidebysig static int32 '<.ctor>b__0'(int32 x, int32 y) cil managed { .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 ) //

Let's consider a problem of calculating a numerical integral. We would like to have a method which takes different functions and outputs corresponding values of the integral of these functions. In C++ the code would look like this:

The method integral takes a delegate instead of a pointer to a function. An instance of a delegate new Integral.Function(f1) must be passed to the method integral.

using System;
//calculate the integral of f(x) between x=a and x=b by splitting the interval in step_number steps
class Integral
{
    public delegate double Function(double x); //declare a delegate that takes a double and returns a double
    public static double integral(Function f,double a, double b,int step_number)
    {
         double sum=0;
          double step_size=(b-a)/step_number;
         for(int i=1;i<=step_number;i++)
        sum=sum+f(a+i*step_size)*step_size; //divide the area under f(x) into  step_number rectangles and sum their areas
          return sum;
    }
}
class Test
{
    //simple functions to be integrated
    public static double f1( double x)
    {
       return x*x;
    }
    public static double f2(double x)
    {
       return x*x*x;
    }
    public static void Main()
    {//output the value of the integral.
       Console.WriteLine(Integral.integral(new Integral.Function(f1),1,10,20));
       Console.WriteLine(Integral.integral(new Integral.Function(f2),1,10,20));
    }
}