< as described in [The Nature of TaskCompletionSource<TResult>](http://blogs.msdn.com/b/pfxteam/archive/2009/06/02/9685804.aspx):
The TaskCompletionSource<TResult> type serves two related purposes, both alluded to by its name: it is a source for creating a task, and the source for that task’s completion. In essence, a TaskCompletionSource<TResult> acts as the producer for a Task<TResult> and its completion. You create a TaskCompletionSource<TResult> and hand the underlying Task<TResult> it’s created, accessible from its Task property. Unlike Tasks created by Task.Factory.StartNew, the Task handed out by TaskCompletionSource<TResult> does not have any scheduled delegate associated with it
[CancellationToken](https://msdn.microsoft.com/en-us/library/system.threading.cancellationtoken.aspx)
To me, a classic scenario for using TaskCompletionSource is when it's possible that my method won't necessarily have to do a time consuming operation. What it allows us to do is to choose the specific cases where we'd like to use a new thread.
A good example for this is when you use a cache. You can have a GetResourceAsync method, which looks in the cache for the requested resource and returns at once (without using a new thread, by using TaskCompletionSource) if the resource was found. Only if the resource wasn't found, we'd like to use a new thread and retrieve it using Task.Run().
```
using System.Threading;
using System.Threading.Tasks;
public class AsyncManualResetEvent
{
private TaskCompletionSource<bool> taskCompletionSource = new TaskCompletionSource<bool>();
public Task WaitAsync()
{
return taskCompletionSource.Task;
}
public void Set()
{
TaskCompletionSource<bool> tcs = taskCompletionSource;
Task.Factory.StartNew(s => ((TaskCompletionSource<bool>) s).TrySetResult(true),
tcs,
CancellationToken.None,
TaskCreationOptions.PreferFairness,
TaskScheduler.Default);
tcs.Task.Wait();
}
public void Reset()
{
while (true)
{
TaskCompletionSource<bool> tcs = taskCompletionSource;
if (!tcs.Task.IsCompleted ||
Interlocked.CompareExchange(ref taskCompletionSource, new TaskCompletionSource<bool>(), tcs) == tcs)
{
return;
}
}
}
}
public static class TaskExtensions
{
public static Task AsAwaitable(this CancellationToken token)
{
var ev = new AsyncManualResetEvent();
token.Register(() => ev.Set());
return ev.WaitAsync();
}
}
```
Source
------
* [How to await a CancellationToken](https://socialeboladev.wordpress.com/2013/03/05/how-to-await-a-cancellationtoken/) by SocialEbola
* [Building Async Coordination Primitives, Part 1: AsyncManualResetEvent](http://blogs.msdn.com/b/pfxteam/archive/2012/02/11/10266920.aspx) by Stephen Toub
* [Real life scenarios for using TaskCompletionSource<T>](http://stackoverflow.com/questions/15316613/real-life-scenarios-for-using-taskcompletionsourcet) from StackOverflow
* [Diving deep with WinRT and await](http://blogs.msdn.com/b/windowsappdev/archive/2012/04/24/diving-deep-with-winrt-and-await.aspx) by Stephen Toub from [Parallel Extensions Team](http://blogs.msdn.com/b/pfxteam/)
]]>