如何使用谓词实现Task.WhenAny()

Question

我想异步执行几个任务,每个任务都会运行http请求,可以抛出异常或安全结束.我需要在第一个任务成功完成或所有任务都失败时完成.请指教.

Answer 1

等待任何任务并在满足条件时返回任务.否则再等待其他任务,直到没有其他任务要等待.

public static async Task<Task> WhenAny( IEnumerable<Task> tasks, Predicate<Task> condition )
{
    var tasklist = tasks.ToList();
    while ( tasklist.Count > 0 )
    {
        var task = await Task.WhenAny( tasklist );
        if ( condition( task ) )
            return task;
        tasklist.Remove( task );
    }
    return null;
}

简单检查一下

var tasks = new List<Task> {
    Task.FromException( new Exception() ),
    Task.FromException( new Exception() ),
    Task.FromException( new Exception() ),
    Task.CompletedTask, };

var completedTask = WhenAny( tasks, t => t.Status == TaskStatus.RanToCompletion ).Result;

if ( tasks.IndexOf( completedTask ) != 3 )
    throw new Exception( "not expected" );

Answer 2

public static Task<T> GetFirstResult<T>(
    ICollection<Func<CancellationToken, Task<T>>> taskFactories, 
    Predicate<T> predicate) where T : class
{
    var tcs = new TaskCompletionSource<T>();
    var cts = new CancellationTokenSource();

    int completedCount = 0;
    // in case you have a lot of tasks you might need to throttle them 
    //(e.g. so you don't try to send 99999999 requests at the same time)
    // see: http://stackoverflow.com/a/25877042/67824
    foreach (var taskFactory in taskFactories)
    {
        taskFactory(cts.Token).ContinueWith(t => 
        {
            if (t.Exception != null)
            {
                Console.WriteLine($"Task completed with exception: {t.Exception}");
            }
            else if (predicate(t.Result))
            {
                cts.Cancel();
                tcs.TrySetResult(t.Result);
            }

            if (Interlocked.Increment(ref completedCount) == taskFactories.Count)
            {
                tcs.SetException(new InvalidOperationException("All tasks failed"));
            }

        }, cts.Token);
    }

    return tcs.Task;
}

使用示例：

using System.Net.Http;
var client = new HttpClient();
var response = await GetFirstResult(
    new Func<CancellationToken, Task<HttpResponseMessage>>[] 
    {
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
    }, 
    rm => rm.IsSuccessStatusCode);
Console.WriteLine($"Successful response: {response}");