ASP.NET Web API: HttpContext.Current is null inside Task

A few days ago, a colleague asked my help to solve an issue he had with ASP.NET Web API. His code looked like the following:

	public class WebApiController:ApiController
	{
	public async Task Post([FromBody]string value)
	{
	await DoSomething();
	Trace.WriteLine(RequestInfo.Instance.Data);
	Debug.Assert(HttpContext.Current!=null); //Fails in ASP.NET MVC/Web API 4
	}
	private Task DoSomething()
	{
	return Task.Factory.StartNew(() =>
	{
	Trace.WriteLine(HttpContext.Current.Request.FilePath);
	}).ContinueWith((t) =>
	{
	return Task.Delay(3000);
	});
	}
	}

view raw ApiController.cs hosted with ❤ by GitHub

The problem was that HttpContext.Current was always null after switching to another task.

When I tried to reproduce the issue by building a similar sample on my computer, it just worked. The only difference between his code and mine was that I created an ASP.NET Web API 4.5 application, where he was still using ASP.NET 4.

So switching to .NET 4.5 solved the problem. But why? It seems that in ASP.NET 4.5, a task friendly synchronization context got introduced. This synchronization context ensures that the originel context is restored after leaving the await block.

So make sure that you either:

• Set httpRuntime.targetFramework to 4.5, or
• In your appSettings, set aspnet:UseTaskFriendlySynchronizationContext to true.

Some more information can be found here.