为什么IHttpAsyncHandler提供extraData参数？

Question

A bit not relevant for today's technology but I saw another way of working with Tasks in APM environment , besides the Task.FromAsync

Async Handler in asp.net :

public class Handler : IHttpAsyncHandler
{

 public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
      //...
    }

    public void EndProcessRequest(IAsyncResult result)
    {
     //...
    }
  }

• The context parameter is the actual context which I can access / (or pass to another beginXXX operation) request and response.
• cb is for me to execute / (or pass to another beginXXX operation) when the operation finished.

Question

But what is the object extraData is doing in the signature of the method ?

It is not that I get some state from the framework , on the contrary - I create state and pass it forward so that my EndXXX could cast result.AsyncState to T and use that data.

So why is it there?

Answer 1

In short, it is required by the APM Pattern , which IHttpAsyncHandler is following. You don't need it here, but there are cases (of pattern usage, not handler usage) when it is useful to correlate callbacks.

Update:

Here's the recommended way to use Task s in APM:

public IAsyncResult BeginCalculate(int decimalPlaces, AsyncCallback ac, object state)
{
    Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
    Task<string> f = Task<string>.Factory.StartNew(_ => Compute(decimalPlaces), state);
    if (ac != null) f.ContinueWith((res) => ac(f));
    return f;
}

public string Compute(int numPlaces)
{
    Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId);

    // Simulating some heavy work.
    Thread.SpinWait(500000000);

    // Actual implemenation left as exercise for the reader.
    // Several examples are available on the Web.
    return "3.14159265358979323846264338327950288";
}

public string EndCalculate(IAsyncResult ar)
{
    Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
    return ((Task<string>)ar).Result;
}

Please note that the state is passes to task factory, and the resulted task is used as both the argument to the callback and the return value.