Cold Tasks and TaskExtensions.Unwrap

Question

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

public class AsyncConcurrentDictionary

        

Answer 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();
    });