Question

From MSDN doc:

A delegate is a type that safely encapsulates a method, similar to a
function pointer in C and C++. Unlike C function pointers, delegates
are object-oriented, type safe, and secure.

I know what it is and how to use it. But I wonder whether or not it is written base on delegate pattern that I know (from wikipedia) .
What is the difference between them?

Answer 1

The C# (not the pattern) delegate might be useful when you are implementing the delegate pattern just look at this delegate pattern implementation from wikipedia with my changes:

//NOTE: this is just a sample, not a suggestion to do it in such way

public interface I
{
    void F();
    void G();
}

public static class A
{
    public static void F() { System.Console.WriteLine("A: doing F()"); }
    public static void G() { System.Console.WriteLine("A: doing G()"); }
}

public static class B
{
    public static void F() { System.Console.WriteLine("B: doing F()"); }
    public static void G() { System.Console.WriteLine("B: doing G()"); }
}

public class C : I
{
    // delegation 
    Action iF = A.F;
    Action iG = A.G;

    public void F() { iF(); }
    public void G() { iG(); }

    // normal attributes
    public void ToA() { iF = A.F; iG = A.G; }
    public void ToB() { iF = B.F; iG = B.G; }
}

public class Program
{
    public static void Main()
    {
        C c = new C();
        c.F();     // output: A: doing F()
        c.G();     // output: A: doing G()
        c.ToB();
        c.F();     // output: B: doing F()
        c.G();     // output: B: doing G()
    }
}

Again delegate might be useful here, but it isn’t for it was introduced. You should look at it like on the low-level construction rather then the pattern. In the pair with the events it could be used to implement publisher/subscriber(observer) pattern – just look at this article, or it could sometimes help you to implement visitor pattern – this is actively used in the LINQ:

public void Linq1() 
{ 
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 

    // n => n < 5 is lambda function, creates a delegate here
    var lowNums = numbers.Where(n => n < 5); 

    Console.WriteLine("Numbers < 5:"); 
    foreach (var x in lowNums) 
    { 
        Console.WriteLine(x); 
    } 
} 

To summarize: a language delegate is not the pattern itself, it just allows you to operate functions as the first class objects.