如何运行使用相同外部变量的任务？

Question

我有收到一个时期的web服务（ExternalWebService）（开始和结束日期）并返回所有日志此期间，我想为这项服务经过长时间的电话。问题在于，此服务只允许每个请求发送少量数据，而长时间意味着大量数据，这是导致错误的原因。因此，我决定循环遍历作为参数传递的期间的几个月，并使用任务并行调用此服务，并在任务执行结束时连接结果。下面是代码：

public List<object> GetList(DateTime start, DateTime end) 
{ 
    List<object> finalList = new List<object>(); 
    object lockList = new object(); 

    DateTime current = start; 

    List<Task> threads = new List<Task>(); 

    do 
    { 
     current = new DateTime(Math.Min(current.AddMonths(1).Ticks, end.Ticks)); 

     Task thread = Task.Run(() => { 
      List<object> partialList = ExternalWebService.GetListByPeriod(from: start, to: current); 
      lock (lockList) 
      { 
       finalList = finalList.Concat(partialList).ToList(); 
      } 
     }); 

     threads.Add(thread); 

     start = current; 
    } 
    while (current < end); 

    Task.WhenAll(threads).Wait(); 

    return finalList; 
} 

此代码的工作，但有一个意想不到的结果，因为变量线程内使用前开始和当前变化。那么，我能做些什么来保证开始和目前内部使用的日期Task.Run具有相同的值，它们在创建线程时具有相同的值？

Answer 1

您可以创建接收你想要的DateTime的方法，并返回委托它传递给Task.Run方法，是这样的：

private Action GetMethodAction(DateTime current) 
{ 
    return() => { /* your code here */ } 
} 

这样的current价值必将给您的行动返回。希望能帮助到你。

Answer 2

最好不要共享和改变你的日期。您可以启动一组异步查询Web服务的任务，并将结果展平。

public class GetData { 

    public async Task<IEnumerable<object>> GetDataAsync(DateTime startDate, DateTime endDate) { 
     var daysPerChunk = 28; 
     var totalChunks = (int)Math.Ceiling((endDate - startDate).TotalDays/daysPerChunk); 
     var chunks = Enumerable.Range(0, totalChunks); 

     var dataTasks = chunks.Select(chunkIndex => { 
      var start = startDate.AddDays(chunkIndex * daysPerChunk); 
      var end = new DateTime(Math.Min(start.AddDays(daysPerChunk).Ticks, endDate.Ticks)); 
      return ExternalWebService.GetListByPeriodAsync(from: start, to: end); 
     }); 
     var results = await Task.WhenAll(dataTasks); 

     var data = results.SelectMany(_ => _); 
     return data.ToList(); 
    } 
} 

public class ExternalWebService { 

    private static HttpClient Client { 
     get; 
    } = new HttpClient(); 


    public async static Task<IEnumerable<object>> GetListByPeriodAsync(DateTime from, DateTime to) { 
     var response = await Client.GetAsync("GetListByPeriodFromToUri"); 
     if (response != null && response.IsSuccessStatusCode) { 
      using (var stream = await response.Content.ReadAsStreamAsync()) { 
       using (var reader = new StreamReader(stream)) { 
        var str = reader.ReadToEnd(); 
        return JsonConvert.DeserializeObject<IEnumerable<object>>(str); 
       } 
      } 
     } 
     return Enumerable.Empty<object>(); 
    } 
} 

Answer 3

这里是为我工作的代码：

protected override List<object> GetList(DateTime start, DateTime end) 
{ 
    List<object> list = new List<object>(); 
    object lockList = new object(); 

    DateTime current = start; 

    List<Task> threads = new List<Task>(); 

    do 
    { 
     current = new DateTime(Math.Min(current.AddMonths(1).Ticks, end.Ticks)); 

     Task thread = Task.Run(GetMethodFunc(start, current)).ContinueWith((result) => 
     { 
      lock (lockList) 
      { 
       list = list.Concat(result.Result).ToList(); 
      } 
     }); 

     threads.Add(thread); 

     start = current; 
    } 
    while (current < end); 

    Task.WhenAll(threads).Wait(); 

    return list; 
} 

private Func<List<object>> GetMethodFunc(DateTime start, DateTime end) 
{ 
    return() => { 
     List<object> partialList = ExternalWebService.GetListByPeriod(from: start, to: end); 
     return partialList; 
    }; 
}