Cold Tasks and TaskExtensions.Unwrap

问题:

I've got a caching class that uses cold (unstarted) tasks to avoid running the expensive thing multiple times.

public class AsyncConcurrentDictionary<TKey, TValue> : System.Collections.Concurrent.ConcurrentDictionary<TKey, Task<TValue>>
{
    internal Task<TValue> GetOrAddAsync(TKey key, Task<TValue> newTask)
    {
        var cachedTask = base.GetOrAdd(key, newTask);

        if (cachedTask == newTask && cachedTask.Status == TaskStatus.Created) // We won! our task is now the cached task, so run it 
            cachedTask.Start();

        return cachedTask;
    }
}

This works great right up until your task is actually implemented using C#5's await, ala

cache.GetOrAddAsync("key", new Task(async () => {
  var r = await AsyncOperation();
  return r.FastSynchronousTransform();
}));)`

Now it looks like TaskExtensions.Unwrap() does exactly what I need by turning Task<Task<T>> into a Task<T>, but it seems that wrapper it returns doesn't actually support Start() - it throws an exception.

TaskCompletionSource (my go to for slightly special Task needs) doesn't seem to have any facilities for this sort of thing either.

Is there an alternative to TaskExtensions.Unwrap() that supports "cold tasks"?

回答1:

All you need to do is to keep the Task before unwrapping it around and start that:

public Task<TValue> GetOrAddAsync(TKey key, Func<Task<TValue>> taskFunc)
{
    Task<Task<TValue>> wrappedTask = new Task<Task<TValue>>(taskFunc);
    Task<TValue> unwrappedTask = wrappedTask.Unwrap();

    Task<TValue> cachedTask = base.GetOrAdd(key, unwrappedTask);

    if (cachedTask == unwrappedTask)
        wrappedTask.Start();

    return cachedTask;
}

Usage:

cache.GetOrAddAsync(
    "key", async () =>
    {
        var r = await AsyncOperation();
        return r.FastSynchronousTransform();
    });