CODEWITHSUNDEEP
c#asp.netasynchronous
Royi Namir
Royi Namir

Reputation: 148664

Why does the IHttpAsyncHandler provide extraData parameter?

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)
    {
     //...
    }
  }

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?

Upvotes: 0

Views: 342

Answers (1)

Sergei Rogovtcev
Sergei Rogovtcev

Reputation: 5832

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 Tasks 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.

Upvotes: 3

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