What is a callback?

Question

I just met you,
And this is crazy,
But here's my number (delegate),
So if something happens (event),
Call me, maybe (callback)?

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).

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 in 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 semantic 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));
    }

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 an 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();

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

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

Links

• more details for C#.

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.