C# Task.WhenAll

1、有时候我们需要同时执行一些操作，然后把这些操作的结果进行汇总，以达到用异步处理降低操作耗时的效果，此时我们会考虑使用Task，而Task.WhenAll则排上了用场。

public void IssueStatistics(Msg msg)
        {
            string guid = Util.GetGuid();

            try
            {
                var beginTime = msg.ToString(0);
                var endTime = msg.ToString(1);

                var task = GetIssueStatisticsAsync(beginTime, endTime);
                var dtRet = task.Result;

            }
        }

 private async Task<DataTable> GetIssueStatisticsAsync(string beginTime, string endTime)
        {
            var taskList = new List<Task<DataTable>>
            {
                GetIssueDataAsync(beginTime, endTime),
                GetB2CIssueDataAsync(beginTime, endTime)
            };

            var tasks = taskList.ToArray();
            var tables = await Task.WhenAll(tasks).ConfigureAwait(false);

            var retDt = orderDAL.CreateIssueDataTable();
            for (int index = 0; index < tables.Length; index++)
            {
                for (int k = 0; k < tables[index].Rows.Count; k++)
                {
                    retDt.ImportRow(tables[index].Rows[k]);
                }
            }
            return retDt;
        }


        private Task<DataTable> GetIssueDataAsync(string beginTime, string endTime)
        {
            return Task.Run(() =>
            {
                var dt = orderDAL.GetIssueDataOfOrder(beginTime, endTime);
                return dt;
            });
        }


        private Task<DataTable> GetB2CIssueDataAsync(string beginTime, string endTime)
        {
            return Task.Run(() =>
            {
                var dt = orderDAL.GetIssueDataOfB2C(beginTime, endTime);
                return dt;
            });
        }

这里有个地方需要注意的，如果不是在控制台应用程序，那么需要在Task.WhenAll后面加 ConfigureAwait(false)，不然无法返回当前线程。

当然，按照官网的实例写法，你也可以这样写：

private async Task<int> ExecuteDataAsync(bool isJit, int topRows, bool isRetry)
        {
            var dtRecord = _dao.GetDataOfStockOut(topRows, isRetry);
            var dataList = _comDao.ToSyncDataList(dtRecord);
            if (dataList == null) return 0;

            var tasksQuery = from data in dataList select DoRunIssueDataAsync(data);
            var tasks = tasksQuery.ToArray();

            int[] flags = await Task.WhenAll(tasks);
            int total = flags.Sum();
            return total;
        }

        private async Task<int> DoRunIssueDataAsync(SyncDataM data)
        {
            return await Task.Run(() =>
            {
               //要处理的事情return 1;
            });
        }

这样调用就可以了：

var task = ExecuteDataAsync(true, 20, true);
task.Wait();