UICollectionViewCell GetCell

Question

在我向下滚动的UICollectionViewCell GetCell方法中，我没有遇到任何问题。但是，当我向上滚动时，我得到一个对象引用空指针。我有点困惑，我理解错误信息。但是，获取错误消息的原因令人费解。我应该指出，我不熟悉iOS生态系统，也许我在错误的地方进行故障排除。错误消息如下图所示：

GetCell方法的代码

        public override UICollectionViewCell GetCell (UICollectionView collectionView, NSIndexPath indexPath)
        {
            UIView cell;



            if (questionDataModel[indexPath.Section].ComplianceIndicator) 
            {
                cell = collectionView.DequeueReusableCell (QuestionUICollectionViewDelegateDataSource.complianceQuestionCellId, indexPath);
            }
            else
            {
                cell = collectionView.DequeueReusableCell (QuestionUICollectionViewDelegateDataSource.questionCellId, indexPath);
            }

            int row = indexPath.Row;

            UILabel lblQuestionAnswer = (UILabel)cell.ViewWithTag (1);
            UITableViewRowSwitch btnQuestionAnswer = (UITableViewRowSwitch)cell.ViewWithTag (2);

            //  btnQuestionAnswer.ValueChanged -= btnQuestionAnswer_Click;

            //  btnQuestionAnswer.ValueChanged -= btnQuestionAnswer_Click;

            //  btnQuestionAnswer.ValueChanged += btnQuestionAnswer_Click;

            if (row < questionDataModel[indexPath.Section].QuestionAnswers.Count)
            {


                    lblQuestionAnswer.Text = questionDataModel[indexPath.Section].QuestionAnswers[indexPath.Row].Answer;


                    btnQuestionAnswer.section = indexPath.Section;
                    btnQuestionAnswer.row = indexPath.Row;

                    btnQuestionAnswer.Tag = indexPath.Row;


                    btnQuestionAnswer.On = questionDataModel[indexPath.Section].QuestionAnswers[indexPath.Row].isTicked;

                    //----------------TODO----------------//
                    // ---- 
                    //  btnQuestionAnswer.ValueChanged += btnQuestionAnswer_Click;

                    //  btnQuestionAnswer.ValueChanged -= btnQuestionAnswer_Click;
                    //if (!btnQuestionAnswer.hasEvent)
                    {
                        btnQuestionAnswer.ValueChanged -= btnQuestionAnswer_Click;
                        btnQuestionAnswer.ValueChanged -= btnQuestionAnswer_Click;
                        btnQuestionAnswer.ValueChanged += btnQuestionAnswer_Click;

                        //btnQuestionAnswer.hasEvent = true;
                    }

                    btnQuestionAnswer.relatedView = collectionView;
                    if (questionDataModel[indexPath.Section].isLocked)
                    {
                        btnQuestionAnswer.Enabled = false;
                    }
                    else
                    {
                        btnQuestionAnswer.Enabled = true;
                    }

                    lblQuestionAnswer.Hidden = false;
                    btnQuestionAnswer.Hidden = false;

                }

            else
            {
                lblQuestionAnswer.Hidden = true;
                btnQuestionAnswer.Hidden = true;
            }

            if (controller.loggedInUser.UserType != "Inspector")
            {
                btnQuestionAnswer.Enabled = false;
            }

            return (UICollectionViewCell)cell;
        }

如果添加以下代码下面的代码      在下面的if语句中 if（indexPath.Section＆gt; 0）

if row< questionDataModel[indexPath.Section].QuestionAnswers.Count)
{

我可以向上滚动没有问题，但现在CellDispleyEnded方法存在潜在问题，请参见下图：

CellDisplayEnded的代码如下：

    public override void CellDisplayingEnded (UITableView tableView, UITableViewCell cell, NSIndexPath indexPath)
    {
        UITableViewRowSwitch btnCompliant = (UITableViewRowSwitch)cell.ViewWithTag (4);
        //btnCompliant.relatedViewObserver.Dispose ();
        btnCompliant.relatedView.RemoveObserver ((NSObject)btnCompliant.relatedViewObserver, "contentSize");
        btnCompliant.ValueChanged -= btnCompliant_Click;
        btnCompliant.relatedView = null;

        UITableViewRowButton btnSave = (UITableViewRowButton)cell.ViewWithTag (5);
        btnSave.TouchUpInside -= btnSave_Click;

        UITableViewRowButton btnPicture = (UITableViewRowButton)cell.ViewWithTag (7);
        btnPicture.TouchUpInside -= btnPicture_Click;

        UITableViewRowButton btnComment = (UITableViewRowButton)cell.ViewWithTag (6);
        btnComment.TouchUpInside -= btnComment_Click;

    }