What is a callback?

Question

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

Answer 1

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 callback)
    {
        callback("Hello world");
    }

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