What is a callback?

Question

What's a callback and how is it implemented in C#?

Answer 1

If you referring to ASP.Net callbacks:

In the default model for ASP.NET Web pages, the user interacts with a page and clicks a button or performs some other action that results in a postback. The page and its controls are re-created, the page code runs on the server, and a new version of the page is rendered to the browser. However, in some situations, it is useful to run server code from the client without performing a postback. If the client script in the page is maintaining some state information (for example, local variable values), posting the page and getting a new copy of it destroys that state. Additionally, page postbacks introduce processing overhead that can decrease performance and force the user to wait for the page to be processed and re-created.

To avoid losing client state and not incur the processing overhead of a server roundtrip, you can code an ASP.NET Web page so that it can perform client callbacks. In a client callback, a client-script function sends a request to an ASP.NET Web page. The Web page runs a modified version of its normal life cycle. The page is initiated and its controls and other members are created, and then a specially marked method is invoked. The method performs the processing that you have coded and then returns a value to the browser that can be read by another client script function. Throughout this process, the page is live in the browser.

Source: http://msdn.microsoft.com/en-us/library/ms178208.aspx

If you are referring to callbacks in code:

Callbacks are often delegates to methods that are called when the specific operation has completed or performs a sub-action. You'll often find them in asynchronous operations. It is a programming principle that you can find in almost every coding language.

More info here: http://msdn.microsoft.com/en-us/library/ms173172.aspx

Answer 2

A callback is a function pointer that you pass in to another function. The function you are calling will 'callback' (execute) the other function when it has completed.

Check out this link.

Answer 3

In computer programming, a callback is executable code that is passed as an argument to other code.

—Wikipedia: Callback (computer science)

C# has delegates for that purpose. They are heavily used with events, as an event can automatically invoke a number of attached delegates (event handlers).

Answer 4

A callback lets you pass executable code as an argument to other code. In C and C++ this is implemented as a function pointer. In .NET you would use a delegate to manage function pointers.

A few uses include error signaling and controlling whether a function acts or not.

Wikipedia

Answer 5

Definition

A callback is executable code that is passed as an argument to other code.

Implementation

// Parent can Read
public class Parent
{
    public string Read(){ /*reads here*/ };
}

// Child need Info
public class Child
{
    private string information;
    // declare a Delegate
    delegate string GetInfo();
    // use a instance of the declared Delegate
    public GetInfo GetMeInformation;

    public void ObtainInfo()
    {
        // Child will use the Parent capabilities via the Delegate
        information = GetMeInformation();
    }
}

Usage

Parent Peter = new Parent();
Child Johny = new Child();

// say Johny from where to obtain info
Johny.GetMeInformation = Peter.Read;

Johny.ObtainInfo(); // here Johny 'ask' Peter to read

Links

• more details for C#.

Answer 6

Probably not the dictionary definition, but a callback usually refers to a function, which is external to a particular object, being stored and then called upon a specific event.

An example might be when a UI button is created, it stores a reference to a function which performs an action. The action is handled by a different part of the code but when the button is pressed, the callback is called and this invokes the action to perform.

C#, rather than use the term 'callback' uses 'events' and 'delegates' and you can find out more about delegates here.

Answer 7

A callback is a function that will be called when a process is done executing a specific task.

The usage of a callback is usually used in a asynchronous logic.

To create a callback in C#, you need to store a function address inside a variable. This is achieved using a delegate or the new lambda schemantic Func or Action.

    public delegate void WorkCompletedCallBack(string result);

    public void DoWork(WorkCompletedCallBack callback)
    {
        callback("Hello world");
    }

    public void Test()
    {
        WorkCompletedCallBack callback = TestCallBack; // Notice that I am referencing a method without its parameter
        DoWork(callback);
    }

    public void TestCallBack(string result)
    {
        Console.WriteLine(result);
    }

In today C#, this could be done using lambda like:

    public void DoWork(Action<string> callback)
    {
        callback("Hello world");
    }

    public void Test()
    {
        DoWork((result) => Console.WriteLine(result));
    }