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);
}
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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>();
tsc.SetResult(default(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
{
}
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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
