具有指定结果的任务并行库WaitAny

我能想到的最好的办法是为每个Task指定一个ContinueWith，检查结果，如果true取消其他任务。对于取消任务，您可能需要使用CancellationToken。

var tasks = servers
    .Select(s => Task.Run(...)
        .ContinueWith(t =>
            if (t.Result) {
                // cancel other threads
            }
        )
    ).ToArray();

更新：另一种解决方案是WaitAny，直到完成正确的任务（但它也有一些缺点，例如从列表中删除已完成的任务并从剩余任务中创建新的数组是一项相当繁重的操作）：

List<Task<bool>> tasks = servers.Select(s => Task<bool>.Factory.StartNew(server => CallServer((string)server), s)).ToList();
bool result;
do {
    int idx = Task.WaitAny(tasks.ToArray());
    result = tasks[idx].Result;
    tasks.RemoveAt(idx);
} while (!result && tasks.Count > 0);
// cancel other tasks

更新2：现在我会用Rx:

[Fact]
public async Task AwaitFirst()
{
    var servers = new[] { "server1", "server2", "server3", "server4" };
    var server = await servers
        .Select(s => Observable
            .FromAsync(ct => CallServer(s, ct))
            .Where(p => p)
            .Select(_ => s)
        )
        .Merge()
        .FirstAsync();
    output.WriteLine($"Got result from {server}");
}
private async Task<bool> CallServer(string server, CancellationToken ct)
{
    try
    {
        if (server == "server1")
        {
            await Task.Delay(TimeSpan.FromSeconds(1), ct);
            output.WriteLine($"{server} finished");
            return false;
        }
        if (server == "server2")
        {
            await Task.Delay(TimeSpan.FromSeconds(2), ct);
            output.WriteLine($"{server} finished");
            return false;
        }
        if (server == "server3")
        {
            await Task.Delay(TimeSpan.FromSeconds(3), ct);
            output.WriteLine($"{server} finished");
            return true;
        }
        if (server == "server4")
        {
            await Task.Delay(TimeSpan.FromSeconds(4), ct);
            output.WriteLine($"{server} finished");
            return true;
        }
    }
    catch(OperationCanceledException)
    {
        output.WriteLine($"{server} Cancelled");
        throw;
    }
    throw new ArgumentOutOfRangeException(nameof(server));
}

在我的机器上测试需要3.32秒（这意味着它没有等待第四台服务器），我得到了以下输出：

server1 finished
server2 finished
server3 finished
server4 Cancelled
Got result from server3

您可以使用AsyncEx库中的OrderByCompletion()，该库在任务完成时返回任务。你的代码可能看起来像：

var tasks = servers
    .Select(s => Task.Factory.StartNew(server => CallServer((string)server), s))
    .OrderByCompletion();
foreach (var task in tasks)
{
    if (task.Result)
    {
        Console.WriteLine("found");
        break;
    }
    Console.WriteLine("not found yet");
}
// cancel any outstanding tasks since the correct server has been found

使用InterlockedCompareExchange将做到这一点，只有一个任务能够在服务器ReturedData 上写入

    public void SearchServers()
        {
            ResultClass serverReturnedData = null;
            var servers = new[] {"server1", "server2", "server3", "server4"};
            var tasks = servers.Select(s => Task<bool>.Factory.StartNew(server => 
            {
               var result = CallServer((string)server), s);
               Interlocked.CompareExchange(ref serverReturnedData, result, null);
            }).ToArray();
            Task.WaitAny(tasks); //how do I say "WaitAny where the result is true"?
        //
        // use serverReturnedData as you want.
        // 
        }

EDIT：正如Jasd所说，上面的代码可以在变量serverReturnedData具有有效值之前返回（如果服务器返回null值，则可能发生这种情况），以确保您可以将结果包装在自定义对象中。

下面是一个基于svick答案的通用解决方案：

public static async Task<T> GetFirstResult<T>(
this IEnumerable<Func<CancellationToken, Task<T>>> taskFactories, 
Action<Exception> exceptionHandler,
Predicate<T> predicate)
{
    T ret = default(T);
    var cts = new CancellationTokenSource();
    var proxified = taskFactories.Select(tf => tf(cts.Token)).ProxifyByCompletion();
    int i;
    for (i = 0; i < proxified.Length; i++)
    {
        try
        {
            ret = await proxified[i].ConfigureAwait(false);
        }
        catch (Exception e)
        {
            exceptionHandler(e);
            continue;
        }
        if (predicate(ret))
        {
            break;
        }
    }
    if (i == proxified.Length)
    {
        throw new InvalidOperationException("No task returned the expected value");
    }
    cts.Cancel(); //we have our value, so we can cancel the rest of the tasks
    for (int j = i+1; j < proxified.Length; j++)
    {
        //observe remaining tasks to prevent process crash 
        proxified[j].ContinueWith(
         t => exceptionHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted)
                   .Forget();
    }
    return ret;
}

其中ProxifyByCompletion实现为：

public static Task<T>[] ProxifyByCompletion<T>(this IEnumerable<Task<T>> tasks)
{
    var inputTasks = tasks.ToArray();
    var buckets = new TaskCompletionSource<T>[inputTasks.Length];
    var results = new Task<T>[inputTasks.Length];
    for (int i = 0; i < buckets.Length; i++)
    {
        buckets[i] = new TaskCompletionSource<T>();
        results[i] = buckets[i].Task;
    }
    int nextTaskIndex = -1;
    foreach (var inputTask in inputTasks)
    {
        inputTask.ContinueWith(completed =>
        {
            var bucket = buckets[Interlocked.Increment(ref nextTaskIndex)];
            if (completed.IsFaulted)
            {
                Trace.Assert(completed.Exception != null);
                bucket.TrySetException(completed.Exception.InnerExceptions);
            }
            else if (completed.IsCanceled)
            {
                bucket.TrySetCanceled();
            }
            else
            {
                bucket.TrySetResult(completed.Result);
            }
        }, CancellationToken.None, 
           TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
    }
    return results;
}

Forget是抑制CS4014:的空方法

public static void Forget(this Task task) //suppress CS4014
{
}