Callback Methods through Delegates in C#

Question

June 26 2007 Hi Guys Callback Methods through Delegates in C# is explained in the following website but it seems heavy example for the C# beginners. Does anyone know any other websites which provide simple explanation about callback Methods. Please help me http://www.aspfree.com/c/a/C-Sharp/C-Sharp-Delegates-Explained/4/ Thank you

Maha · Answer

Thank you Jan Montano

Jan Montano · Answer

In addition the term callback was used way back in C programming. I'm just not sure if it started with C. Here's a detailed description of callback worth reading. :) http://en.wikipedia.org/wiki/Callback_(computer_science)

Maha · Answer

Thank you very much for your explanation Alan Thank you very much for your example Jibasingh. I understood it

Alan · Answer

Hi maha First of all I hadn't noticed jebasingh's excellent example of delegate callback which I think he must have posted as I was typing mine in as he beat me by 16 seconds ;) To answer your question you only need to use += when adding a new delegate to the invocation list of a pre-existing delegate. As I wasn't using multicast delegates in my example I didn't need to use it. I just used 'Callback' when naming the class and delegate for want of a better name - you can call it anything you like. When used like this a delegate is called a callback because the method which accepts it as a parameter uses the delegate to 'call back' the method it points to and pass it some information and/or to obtain some information from that method.

Maha · Answer

June 27 2007 Thank you Jibasingh. I am trying to understand your example. Thank you Alan for providing simple example. += sign is used when assigning more delegate objects. Here you didn’t use it. I don’t know why. Perhaps all implementations are in the same class. Please explain if my reason is not correct. Anyhow program is working well. I can use your example as a launch pad to understand more complicated examples. Is there any reason why it is called “callback”. cbd = new CallbackDelegate(Cube);

Alan · Answer

TBH I couldn't find a more understandable example than this because the 'real world' use of callbacks is not really a beginner's topic. However this very simple console application may help you to understand the mechanism. The point here is that the Power() method neither knows nor cares whether it is supposed to square or cube a given number. Instead it is passed a delegate which points to the method which will do the appropriate work. The result is then returned to Power() which returns it to the calling code. using System; //define delegate class delegate int CallbackDelegate(int result); class Callback { static void Main() { int number = 2; CallbackDelegate cbd = new CallbackDelegate(Square); Console.WriteLine({0} squared is {1} number Power(number cbd)); cbd = new CallbackDelegate(Cube); number++; Console.WriteLine({0} cubed is {1} number Power(number cbd)); Console.ReadLine(); } static int Power (int number CallbackDelegate cbd) { return cbd(number); // call the 'callback' method and return the result } static int Square(int number) { return number * number; } static int Cube(int number) { return number * number * number; } }

jebasingh · Answer

hope  you can understand the concept of delegate callback from here... best of luck using System; // declare a delegate type // delegate references a method that accept a parameter of type string public delegate void del(string message); // class sample delegate class delegateSample { //static method reference by the delegate public static void DelegateMethod(string message) { Console.WriteLine(message); } // here the delegate callback is actually happen public void process(del callback) { for(int i=1; i { callback(C# corner+ i); } } } // Main class  since i created this as console application.... class mainclass { static void Main() { delegateSample delSam = new delegateSample(); del handler = new del(delegateSample.DelegateMethod); delSam.process(handler); } }