Advantages of using delegates

enum TaskStatus
{
   Started,
   StillProcessing,
   Finished
}

delegate void CallbackDelegate(Task t, TaskStatus status);

class Task
{
    public void Start(CallbackDelegate callback)
    {
        callback(this, TaskStatus.Started);

        // calculate PI to 1 billion digits
        for (...)
        {
            callback(this, TaskStatus.StillProcessing);
        }

        callback(this, TaskStatus.Finished);
    }
}

class Program
{
    static void Main(string[] args)
    {
        Task t = new Task();
        t.Start(new CallbackDelegate(MyCallbackMethod));
    }

    static void MyCallbackMethod(Task t, TaskStatus status)
    {
        Console.WriteLine("The task status is {0}", status);
    }
}

private delegate void UpdateHandler(string message);
private UpdateHandler Update;

public void Register(IObserver observer)
{
    Update+=observer.Update;
}

public void Unregister(IObserver observer)
{
    Update-=observer.Update;
}

public void Notify(string message)
{
    Update(message);
}

public Subject()
{
    observers = new List<IObserver>();
}

public void Register(IObserver observer)
{
    observers.Add(observer);
}

public void Unregister(IObserver observer)
{
    observers.Remove(observer);
}

public void Notify(string message)
{
    // call update method for every observer
    foreach (IObserver observer in observers)
    {
    	observer.Update(message);
    }
}

public delegate void UpdateHandler(string message);
private UpdateHandler Update;

public void Register(UpdateHandler observerRoutine)
{
    Update+=observerRoutine;
}

public void Unregister(UpdateHandler observerRoutine)
{
    Update-=observerRoutine;
}

public void Notify(string message)
{
    Update(message);
}

public delegate string TestDelegate();
protected void Page_Load(object sender, EventArgs e)
{
    TestDelegate TD1 = new TestDelegate(DiaplayMethodD1);
    TestDelegate TD2 = new TestDelegate(DiaplayMethodD2);
    TD2 = TD1 + TD2; // Make TD2 as multi-cast delegate
    lblDisplay.Text =  TD1(); // invoke delegate
    lblAnotherDisplay.Text = TD2();


    // Note: Using a delegate allows the programmer to encapsulate a reference
    //       to a method inside a delegate object. Its like the function pointer
    //       in C or C++.
}
//the Signature has to be same.
public string DiaplayMethodD1()
{
    //lblDisplay.Text = "Multi-Cast Delegate on EXECUTION"; // Enable on multi-cast
    return "This is returned from the first method of delegate explanation";
}
// The Method can be static also
public static string DiaplayMethodD2()
{
    return " Extra words from second method";
}