async task cancellation c# xamarin.ios (native)

async task cancellation

this my code
async Task GetStudentOrStaffPhoto(string type, string studentStaffId)
{
try
{
if (Reachability.IsInternetReachable())
{
// Create Singleton instance of the account factory class
SearchFactory searchFactory = SearchFactory.GetInstance;
// Create appropriate account facade instance
var searchFacade = searchFactory.CreateSearchInstance(Entity.Common.SearchTypeEnum.RegularSearch);
var schlCode = NSUserDefaults.StandardUserDefaults.StringForKey(AppConstants.LastSelSchoolCodeUDKey);

                var studentStaffPhotoResponse = await searchFacade.GetStudentOrStaffPhoto(schlCode, type, ids);
                if (studentStaffPhotoResponse.IsResponseSuccess)
                {
                    if (studentStaffPhotoResponse.Response != null)
                    {
                        var indexToUpdate = pageIndex;
                        //foreach (var item in studentStaffPhotoResponse.Response)
                        //{
                        for (int i = 0; i < studentStaffPhotoResponse.Response.Count; i++)
                        {
                            if (studentStaffPhotoResponse.Response[i].Image != null)
                            {
                                byte[] sPhoto = Convert.FromBase64String(studentStaffPhotoResponse.Response[i].Image);
                                if (sPhoto != null)
                                {
                                    searchResults[indexToUpdate].Photo = sPhoto;

                            InvokeOnMainThread(() =>
                            {
                                var visibleRows = SearchResultTableView.IndexPathsForVisibleRows;
                                var lastVisibleRowIndex = visibleRows[visibleRows.Count() - 1].Row;
                                if (indexToUpdate <= lastVisibleRowIndex && pageIndex == 0)
                                {
                                    var cell = (SearchResultCell)SearchResultTableView.CellAt(NSIndexPath.FromRowSection(indexToUpdate, 0));
                                    if (cell != null)
                                    {
                                        //var staffImageView = (UIImageView)cell.ContentView.ViewWithTag(indexToUpdate + 2000);
                                        var studentStaffImageView = cell.ContentView.ViewWithTag(indexToUpdate + 2000) as UIImageView;
                                        if (searchResults[indexToUpdate].Photo != null)
                                        {
                                            studentStaffImageView.Image = new UIImage(NSData.FromArray(searchResults[indexToUpdate].Photo));
                                        }
                                    }
                                }
                            });
                            indexToUpdate += 1;
                        }
                    }
                        }
                    }
                    pageIndex += pageSize;
                    if (pageIndex <= searchResults.Count )
                    {
                        if (Reachability.IsInternetReachable())
                        {
                            await GetStudentOrStaffPhoto( type,  ids);
                        }
                        else
                        {
                            var alert = UIAlertController.Create(SharedConstants.NoInternetTitle, SharedConstants.NoInternetMessage, UIAlertControllerStyle.Alert);
                            alert.AddAction(UIAlertAction.Create(AppConstants.OK, UIAlertActionStyle.Cancel, null));
                            PresentViewController(alert, true, null);
                        }
                    }
                }
                else if (studentStaffPhotoResponse.IsUnauthorized)
                {
                    InvokeOnMainThread(() =>
                    {
                        CommonHelper.ShowUnauthorizedAlert(this);
                    });
                }
                else
                {
                    InvokeOnMainThread(() =>
                    {
                        var errMsg = !string.IsNullOrEmpty(studentStaffPhotoResponse.ResponseMessage) ? studentStaffPhotoResponse.ResponseMessage : SharedConstants.ServerErrorMessage;
                        var alert = UIAlertController.Create(SharedConstants.ServerErrorTitle, errMsg, UIAlertControllerStyle.Alert);
                        alert.AddAction(UIAlertAction.Create(AppConstants.OK, UIAlertActionStyle.Cancel, (obj) =>
                        {
                            NavigationController.PopViewController(true);
                        }));
                        PresentViewController(alert, true, null);
                    });
                }
            }
            else
            {
                InvokeOnMainThread(() =>
                {
                    var alert = UIAlertController.Create(NSBundle.MainBundle.GetLocalizedString(AppConstants.NoInternetTitleLKey, ""),
                                        NSBundle.MainBundle.GetLocalizedString(AppConstants.NoInternetMsgLKey, ""),
                                                UIAlertControllerStyle.Alert);
                    alert.AddAction(UIAlertAction.Create(NSBundle.MainBundle.GetLocalizedString(AppConstants.OKLKey, ""),
                                     UIAlertActionStyle.Cancel, (obj) =>
                                     {
                                         NavigationController.PopViewController(true);
                                     }));
                    PresentViewController(alert, true, null);
                });
            }

        }
        catch (Exception ex)
        {
            CommonHelper.LogException(ex, this);
        }
    }