如何从非异步方法调用异步方法

问题没有提供测试解决方案所需的完整源代码。

回答你关于在非异步方法中调用可等待函数的问题。你可以使用

CallWebService().Wait(optional timeout); 

或

CallWebService().GetAwaiter().GetResult(); 

你也应该改变你的函数定义

async void CallWebService(int Month, int Year)

async Task CallWebService(int Month, int Year);

正确处理异常和线程切换

如果你可以在不阻塞的情况下调用CallWebService，那么你也可以执行

CallWebService(1,2).ContinueWith((task) =>
{
});

尝试以下

protected async Task FillCalendarWindows()
        {
            try
            {
                for (int r = 0; r < 6; r++)
                {
                    for (int c = 0; c < 7; c++)
                    {
                        if (r == 0)
                        {
                            labels.Add(new Label
                            {
                                HorizontalOptions = LayoutOptions.Center,
                                VerticalOptions = LayoutOptions.Center,
                                TextColor = Color.Black,
                                FontSize = 18,
                                FontAttributes = FontAttributes.Bold
                            });
                            DayLabels.Children.Add(labels.Last(), c, r);
                        }
                        buttons.Add(new CalendarButton
                        {
                            BorderRadius = 0,
                            BorderWidth = BorderWidth,
                            BorderColor = BorderColor,
                            FontSize = DatesFontSize,
                            BackgroundColor = DatesBackgroundColor,
                            HorizontalOptions = LayoutOptions.FillAndExpand,
                            VerticalOptions = LayoutOptions.FillAndExpand
                        });
                        buttons.Last().Clicked += DateClickedEvent;
                        MainCalendar.Children.Add(buttons.Last(), c, r);
                    }
                }
                flag = 1;
                //Device.BeginInvokeOnMainThread(() => CallWebService(StartDate.Month, StartDate.Year));
                await CallWebService(StartDate.Month, StartDate.Year);
                //CallServiceInNewTask(StartDate.Month, StartDate.Year);
                //Device.BeginInvokeOnMainThread(() => await ChangeCalendar(CalandarChanges.All));
            }
            catch (Exception e)
            {
            }
        }

你的Webservice应该像

public async Task CallWebService(int Month, int Year)
    {
        try
        {
            var response = await GetResponseFromWebService.GetResponse<ServiceClasses.RootObject_AttendanceTable>(ServiceURL.GetAttendanceTableList + "Month=" + Month + "&Year=" + Year + "&EmpCd=" + _empCode);
            if (response.Flag == true)
            {
                if (ListObjAttendanceTblList == null)
                {
                    ListObjAttendanceTblList = new List<LstAttendanceDtl>();
                }
                for (int i = 0; i < response.lstAttendanceDtl.Count; i++)
                {
                    var objAttendanceTableList = new LstAttendanceDtl();
                    objAttendanceTableList.AttendanceDt = response.lstAttendanceDtl[i].AttendanceDt;
                    objAttendanceTableList.EarlyDeparture = response.lstAttendanceDtl[i].EarlyDeparture;
                    objAttendanceTableList.InTime = response.lstAttendanceDtl[i].EarlyDeparture;
                    objAttendanceTableList.LateArrival = response.lstAttendanceDtl[i].EarlyDeparture;
                    objAttendanceTableList.OutTime = response.lstAttendanceDtl[i].OutTime;
                    objAttendanceTableList.OverTime = response.lstAttendanceDtl[i].OverTime;
                    objAttendanceTableList.Reason = response.lstAttendanceDtl[i].Reason;
                    objAttendanceTableList.Remark = response.lstAttendanceDtl[i].Remark;
                    objAttendanceTableList.Shift = response.lstAttendanceDtl[i].Shift;
                    objAttendanceTableList.WorkingHrs = response.lstAttendanceDtl[i].WorkingHrs;
                    ListObjAttendanceTblList.Add(objAttendanceTableList);
                }
            }
            else
            {
            }
            if (flag == 1)
            {
               await ChangeCalendar(CalandarChanges.All);
            }
            else
            {
               await ChangeCalendar(CalandarChanges.StartDate);
            }
        }
        catch (WebException e)
        {
        }
    } 

ChnageCalender方法应该是

protected async Task ChangeCalendar(CalandarChanges changes)
    {
        try
        {
            if (changes.HasFlag(CalandarChanges.StartDate))
            {
                Device.BeginInvokeOnMainThread(() => CenterLabel.Text = StartDate.ToString(TitleLabelFormat));
            }
            var start = CalendarStartDate;
            var beginOfMonth = false;
            var endOfMonth = false;
            for (int i = 0; i < buttons.Count; i++)
            {
                endOfMonth |= beginOfMonth && start.Day == 1;
                beginOfMonth |= start.Day == 1;
                LstAttendanceDtl objAttendanceDtl = ListObjAttendanceTblList.Find(s => s.AttendanceDt.Equals(start.Date.ToString("dd/MM/yyyy")));
                string remarks = string.Empty;
                if (i < 7 && WeekdaysShow && changes.HasFlag(CalandarChanges.StartDay))
                {
                    Device.BeginInvokeOnMainThread(() => labels[i].Text = start.ToString(WeekdaysFormat));
                    //labels[i].Text = start.ToString(WeekdaysFormat);
                    //DateTime d = Convert.ToDateTime(objAttendanceDtl.AttendanceDt).Date; 
                }
                if (changes.HasFlag(CalandarChanges.All))
                {
                    Device.BeginInvokeOnMainThread(()=>buttons[i].Text = string.Format("{0}", start.Day));
                    //buttons[i].Text = string.Format("{0}", start.Day);
                }
                else
                {
                    Device.BeginInvokeOnMainThread(() => buttons[i].TextWithoutMeasure = string.Format("{0}", start.Day));
                }
                buttons[i].Date = start;
                var isInsideMonth = beginOfMonth && !endOfMonth;
                if (objAttendanceDtl != null)
                {
                    remarks = objAttendanceDtl.Remark;
                    if ((remarks.ToLower()).Trim() == stringFullDay.ToLower().Trim())
                    {
                        SetButtonPresent(buttons[i], isInsideMonth);
                    }
                    else if (remarks.ToLower().Trim() == stringAbsent.ToLower().Trim())
                    {
                        SetButtonAbsent(buttons[i], isInsideMonth);
                    }
                    else if (remarks.ToLower().Trim() == stringWeekOff.ToLower().Trim())
                    {
                        SetButtonWeekendMood(buttons[i], isInsideMonth);
                    }
                    else if (remarks.ToLower().Trim() == stringHolidays.ToLower().Trim())
                    {
                        SetButtonHolidays(buttons[i], isInsideMonth);
                    }
                    else if (remarks.ToLower().Trim() == stringSecondhalfAbsent.ToLower().Trim() ||
                        remarks.ToLower().Trim() == stringFirsthalfAbsent.ToLower().Trim())
                    {
                        SetButtonHalfDayMood(buttons[i], isInsideMonth);
                    }
                    else
                    {
                        SetButtonDisabled(buttons[i]);
                    }
                }
                else
                {
                    SetButtonOutSideMonth(buttons[i]);
                }
                SpecialDate sd = null;
                if (SpecialDates != null)
                {
                    sd = SpecialDates.FirstOrDefault(s => s.Date.Date == start.Date);
                }
                if (sd != null)
                {
                    SetButtonSpecial(buttons[i], sd);
                }
                else if (SelectedDate.HasValue && start.Date == SelectedDate.Value.Date)
                {
                    SetButtonSelected(buttons[i], isInsideMonth);
                }
                start = start.AddDays(1);
            }
        }
        catch (Exception e)
        {
        }
    }

我假设FillCalendarWindows()方法是从UI线程调用的，但你不希望UI线程等待并冻结控件，而外部操作GetResponseFromWebService。GetResponse发生，这就是为什么在CallWebService()中等待异步调用的原因。

我认为使用异步CallWebService的一个更好的方法是使用"异步所有的方式"，所以你将不得不改变CallWebService上游的所有方法为异步方法。这有一个额外的好处，你不需要做Device。由于async/await捕获了调用者的同步上下文，因此ChangeCalendar将在UI线程上被调用。

async void SomeEventHandler()
{
// called from the UI thread (or its equivalent in Xamarin)
    await FillCalendarWindows();
}
protected async Task FillCalendarWindows()
    {
        try
        {
           //create 7*6 = 42 labels and buttons
            await CallWebService(StartDate.Month, StartDate.Year);
        }
        catch (Exception e)
        {
        }
    }
public async Task CallWebService(int Month, int Year)
    {
        try
        {
            await GetResponseFromWebService.GetResponse... ;
            // .... same code as in your example 
            ChangeCalendar(....);
        }
        catch /*... */
        {
        }
    } 
protected void ChangeCalendar(int changes)
    {
        try
        {
            /* no need to do Device.BeginInvokeOnMainThread () so you can replace all that with normal calls*/
        }
        catch (Exception e)
        {
            /* ... */
        }
    }

不确定系统如何。ArgumentOutOfRangeException会被引发，我无法在github上找到正确的代码版本，所以我无法调查那个特定的错误。我的猜测是你有多个线程修改"按钮"集合，当你调用设备。你可能会发现集合的元素比预期的少。

TL;DR:始终使用async/await而不是以同步方式调用async方法，这应该更容易找到问题的原因

我从你的评论中至少看到了两个问题

1. "If(response.flag==true)(控制超出函数)并直接调用ChangeCalendar() -显然是因为你没有等待。

2. 你的按钮数量(42)与标签数量(7)不一样，所以当你尝试用相同的"i"去标签[i]和按钮[i]时，你会得到标签索引的ArgumentOutOfRangeException。注:if(r==0)限制标签数量为7。

           for (int r = 0; r < 6; r++)
           {
               for (int c = 0; c < 7; c++)
               {
                   if (r == 0)
                   {
                       labels.Add(new Label