July 2012 - Posts

One of main characteristics of MediaTypeFormatter’s in ASP.NET Web API is that they leverage the Task Parallel Library (TPL) for reading or writing an model into an stream. When you derive your class from the base class MediaTypeFormatter, you have to either implement the WriteToStreamAsync or ReadFromStreamAsync methods for writing or reading a model from a stream respectively.

These two methods return a Task, which internally does all the serialization work, as it is illustrated bellow.

public abstract class MediaTypeFormatter
  public virtual Task WriteToStreamAsync(Type type, object value, 
     Stream writeStream, HttpContent content, TransportContext transportContext);
  public virtual Task<object> ReadFromStreamAsync(Type type, Stream readStream, 
     HttpContent content, IFormatterLogger formatterLogger);

However, most of the times, serialization is a safe operation that can be done synchronously. In fact, many of the serializer classes you will find in the .NET framework only provide sync methods. So the question is, how you can transform that synchronous work into a Task ?.

Creating a new task using the method Task.Factory.StartNew for doing all the serialization work would be probably the typical answer. That would work, as a new task is going to be scheduled. However, that might involve some unnecessary context switches, which are out of our control and might be affect performance on server code specially.  

If you take a look at the source code of the MediaTypeFormatters shipped as part of the framework, you will notice that they actually using another pattern, which uses a TaskCompletionSource class.

public Task WriteToStreamAsync(Type type, object value, Stream writeStream, 
HttpContent content, TransportContext transportContext)
  var tsc = new TaskCompletionSource<AsyncVoid>();
  //Do all the serialization work here synchronously
  return tsc.Task;
/// <summary>
/// Used as the T in a "conversion" of a Task into a Task{T}
/// </summary>
private struct AsyncVoid

They are basically doing all the serialization work synchronously and using a TaskCompletionSource for returning a task already done.

To conclude this post, this is another approach you might want to consider when using serializers that are not compatible with an async model.

Update: Henrik Nielsen from the ASP.NET team pointed out the existence of a built-in media type formatter for writing sync formatters. BufferedMediaTypeFormatter http://t.co/FxOfeI5x

Posted by cibrax
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