如何使用单个“await Task.WhenAll”简化多个等待？

Question

我假设我必须在下面的代码中使用Task.WhenAll但无法弄清楚它应该正确实现。

请帮忙。

 public async void UpdateData()
        {
            var month = (cbMonths.SelectedItem as MonthView).ID;
            var year = (cbYears.SelectedItem as YearView).ID;
            var deviceTypeID = (int)DeviceType;
            var calendar = await GetCalendar(month, year, deviceTypeID);
            var workTypes = await GetWorkTypes(); 

            if (calendar != null && workTypes != null) // Task.WhenAll ???
            {
                //...
            }
        }


 private async Task<List<WorkTypeItem>> GetWorkTypes()
        {
            try
            {
                HttpClient client = new HttpClient();

                var url = Properties.Settings.Default.ServerBaseUrl + @"/api/staff/WorkTypes";

                HttpResponseMessage response = await client.GetAsync(url);
                if (response.IsSuccessStatusCode)    // Check the response StatusCode
                {
                    var serSettings = new JsonSerializerSettings() { TypeNameHandling = TypeNameHandling.All };    
                    string responseBody = await response.Content.ReadAsStringAsync();

                    return JsonConvert.DeserializeObject<List<MSOCommon.WorkTypeItem>>(responseBody, serSettings); 
                }              
                else
                {
                    logger.Error(Properties.Resources.DATACannotGetWorkTypes);
                }
            }
            catch (Exception ex)
            {
                logger.Error(Properties.Resources.DATACannotGetWorkTypes + " " + ex.Message);
            }

            return null;
        }

Answer 1

如果您希望两个任务同时执行，则不要await这些方法。 而是将他们的任务传递给变量并在Task.WhenAll调用它们

public async Task UpdateData() {
    var month = (cbMonths.SelectedItem as MonthView).ID;
    var year = (cbYears.SelectedItem as YearView).ID;
    var deviceTypeID = (int)DeviceType;
    var task1 = GetCalendar(month, year, deviceTypeID);
    var task2 = GetWorkTypes(); 

    await Task.WhenAll(task1, task2);

    var calendar = task1.Result;
    var workTypes = task2.Result;
}

另请注意，您应该避免使用async void方法。

Answer 2

var calendarTask = GetCalendar(month, year, deviceTypeID);
var workTypesTask = GetWorkTypes(); 

Task.WaitAll(calendarTask, workTypesTask);
var calendar = await calendarTask;
var workTypes = await workTypesTask;

要回答 @crazyGamer ，您这样做的原因是为了让两个任务可以同时运行。 否则，您甚至在开始第二个任务之前就在等待第一个任务。 当然，如果他们相互依赖，那是一件好事。 否则，这会在 MP 系统上运行得更快。