Parallel.ForEach using Thread.Sleep equivalent

Question

So here\'s the situation: I need to make a call to a web site that starts a search. This search continues for an unknown amount of time, and the only way I know if the searc

Answer 1

It's quite easy to implement with tasks and async/await, as noted by @KevinS in the comments:

async Task<ItemData> ProcessItemAsync(Item item)
{
    while (true)
    {
        if (await isSearchFinishedAsync(item))
            break;
        await Task.Delay(30 * 1000);
    }
    return await downloadDataAsync(item);
}

// ...

var items = getItems();
var tasks = items.Select(i => ProcessItemAsync(i)).ToArray();
await Task.WhenAll(tasks);
var data = tasks.Select(t = > t.Result);

This way, you don't block ThreadPool threads in vain for what is mostly a bunch of I/O-bound network operations. If you're not familiar with async/await, the async-await tag wiki might be a good place to start.

I assume you can convert your synchronous methods isSearchFinished and downloadData to asynchronous versions using something like HttpClient for non-blocking HTTP request and returning a Task<>. If you are unable to do so, you still can simply wrap them with Task.Run, as await Task.Run(() => isSearchFinished(item)) and await Task.Run(() => downloadData(item)). Normally this is not recommended, but as you have hundreds of items, it sill would give you a much better level of concurrency than with Parallel.ForEach in this case, because you won't be blocking pool threads for 30s, thanks to asynchronous Task.Delay.

Answer 2

You can also write a generic function using TaskCompletionSource and Threading.Timer to return a Task that becomes complete once a specified retry function succeeds.

public static Task RetryAsync(Func<bool> retryFunc, TimeSpan retryInterval)
{
    return RetryAsync(retryFunc, retryInterval, CancellationToken.None);
}

public static Task RetryAsync(Func<bool> retryFunc, TimeSpan retryInterval, CancellationToken cancellationToken)
{
    var tcs = new TaskCompletionSource<object>();

    cancellationToken.Register(() => tcs.TrySetCanceled());

    var timer = new Timer((state) =>
    {
        var taskCompletionSource = (TaskCompletionSource<object>) state;

        try
        {                   
            if (retryFunc())
            {
                taskCompletionSource.TrySetResult(null);
            }
        }
        catch (Exception ex)
        {
            taskCompletionSource.TrySetException(ex);
        }
    }, tcs, TimeSpan.FromMilliseconds(0), retryInterval);

    // Once the task is complete, dispose of the timer so it doesn't keep firing. Also captures the timer
    // in a closure so it does not get disposed.
    tcs.Task.ContinueWith(t => timer.Dispose(),
                          CancellationToken.None,
                          TaskContinuationOptions.ExecuteSynchronously,
                          TaskScheduler.Default);

    return tcs.Task;
}

You can then use RetryAsync like this:

var searchTasks = new List<Task>();

searchTasks.AddRange(items.Select(
        downloadItem => RetryAsync( () => isSearchFinished(downloadItem),  TimeSpan.FromSeconds(2))  // retry timout
        .ContinueWith(t => downloadData(downloadItem), 
                      CancellationToken.None, 
                      TaskContinuationOptions.OnlyOnRanToCompletion, 
                      TaskScheduler.Default)));

await Task.WhenAll(searchTasks.ToArray());

The ContinueWith part specifies what you do once the task has completed successfully. In this case it will run your downloadData method on a thread pool thread because we specified TaskScheduler.Default and the continuation will only execute if the task ran to completion, i.e. it was not canceled and no exception was thrown.