Advantages of using delegates?

Question

I\'m looking to implement the Observer pattern in VB.NET or C# or some other first-class .NET language. I\'ve heard that delegates can be used for this, but can\'t figure ou

Answer 1

When you can directly call a method, you don't need a delegate.

A delegate is useful when the code calling the method doesn't know/care what the method it's calling is -- for example, you might invoke a long-running task and pass it a delegate to a callback method that the task can use to send notifications about its status.

Here is a (very silly) code sample:

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);
    }
}

As you can see, the Task class doesn't know or care that -- in this case -- the delegate is to a method that prints the status of the task to the console. The method could equally well send the status over a network connection to another computer. Etc.

Answer 2

Here is something that i can write down as a reason of using delegate. The following code is written in C# And please follow the comments.

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";
}

Best Regards, Pritom Nandy, Bangladesh