Dec 16, 2012

Understanding async keyword in c#

Posted in programming, c#

Understanding how async keyword works in c#

So, c# 5 introduced a new keyword to simplify asynchronous programming by the introduction of await keyword. Functionally this keyword will wait for the completion of asynchronous operation that was started.

Technically though it has a complex contract, take a look at the same code below

internal class Class3
{
    public async Task<PersonDetails[]> GetPersonDetails(string personName)
    {
      var personGoogleDetails = CollectPersonDetailFromGoogle(personName);
      DoSomeComplexWork();
      PersonDetails[] personDetails = await personGoogleDetails;
      CallSomeSimpleWork();

      return personDetails;
    }

    private CustomAwaitable CollectPersonDetailFromGoogle(string personName)
    {
      return null;
    }

    private void CallSomeSimpleWork()
    {
      throw new NotImplementedException();
    }

    private void DoSomeComplexWork()
    {
      throw new NotImplementedException();
    }    
}

internal class CustomAwaitable
{
  public CustomAwaiter<PersonDetails[]> GetAwaiter()
  {
    return new CustomAwaiter<PersonDetails[]>();
  }
}

internal class CustomAwaiter<T> : System.Runtime.CompilerServices.INotifyCompletion
{
  public bool IsCompleted { get; set; }
  public void OnCompleted(Action continuation)
  {
    throw new NotImplementedException();
  }

  public T GetResult()
  {
    return default(T);
  }
}

internal class PersonDetails
{
}

There are few things, that I want to bring it up,

GetPersonDetails() method’s signature says it will return Task<PersonDetails> however the return statement just returns PersonDetails[] and the compiler is happy about it.
Look at the dis assembler screen shot and see <GetPersonDetails>d_0 which is an anonymous class created. Any idea why?</span>
Look at use of “await” keyword, though my method CollectPersonDetailFromGoogle is not a Task<T> or Task, the compiler is still happy with that.

humblelistener is a software engineering professional and a human.

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