Question

我在INotifyCollectionChanged的自定义实现上触发CollectionChanged事件时遇到此异常：

类型＆＃39; System.InvalidOperationException＆＃39;的例外情况发生在   PresentationFramework.dll但未在用户代码中处理

其他信息：＆＃39; 25＆＃39;集合更改事件中的索引不是   适用于收集大小＆＃39; 0＆＃39;。

XAML Datagrid作为ItemsSource绑定到集合。

如何避免此异常发生？

代码如下：

Xamarin.forms

错误发生在以下行：

public class MultiThreadObservableCollection<T> : ObservableCollection<T>
{
    private readonly object lockObject;

    public MultiThreadObservableCollection()
    {
        lockObject = new object();
    }

    private NotifyCollectionChangedEventHandler myPropertyChangedDelegate;


    public override event NotifyCollectionChangedEventHandler CollectionChanged
    {
        add
        {
            lock (this.lockObject)
            {
                myPropertyChangedDelegate += value;
            }
        }
        remove
        {
            lock (this.lockObject)
            {
                myPropertyChangedDelegate -= value;
            }
        }
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
            var eh = this.myPropertyChangedDelegate;
            if (eh != null)
            {
                Dispatcher dispatcher;
                lock (this.lockObject)
                {
                    dispatcher = (from NotifyCollectionChangedEventHandler nh in eh.GetInvocationList()
                                  let dpo = nh.Target as DispatcherObject
                                  where dpo != null
                                  select dpo.Dispatcher).FirstOrDefault();
                }

                if (dispatcher != null && dispatcher.CheckAccess() == false)
                {
                    dispatcher.Invoke(DispatcherPriority.DataBind, (Action)(() => this.OnCollectionChanged(e)));
                }
                else
                {
                    lock (this.lockObject)
                    {
                            foreach (NotifyCollectionChangedEventHandler nh in eh.GetInvocationList())
                            {
                                nh.Invoke(this, e);
                            }
                    }
                }
            }           
    }

谢谢！

Answer 1

关键在于（按设计）nh.Invoke（this，e）;被异步调用。当集合绑定在XAML中并且集合发生更改时，将调用System.Windows.Data.ListCollectionView的私有方法AdjustBefore。这里，ListCollectionView检查eventArgs中提供的索引是否属于集合;如果没有，则抛出主题中的异常。

在问题中报告的实现中，NotifyCollectionChangedEventHandler在延迟时间被调用，此时集合可能已经被更改，并且eventArgs中提供的索引可能不再属于它。

避免ListCollectionView执行此检查的一种方法是使用新的eventargs替换eventargs，而不是报告添加或删除的项目，只需要重置操作（当然，效率会丢失！）。

这是一个有效的实施方案：

public class MultiThreadObservableCollection<T> : ObservableCollectionEnh<T>
{
    public override event NotifyCollectionChangedEventHandler CollectionChanged;

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        var eh = CollectionChanged;
        if (eh != null)
        {
            Dispatcher dispatcher = (from NotifyCollectionChangedEventHandler nh in eh.GetInvocationList()
                                     let dpo = nh.Target as DispatcherObject
                                     where dpo != null
                                     select dpo.Dispatcher).FirstOrDefault();

            if (dispatcher != null && dispatcher.CheckAccess() == false)
            {
                dispatcher.Invoke(DispatcherPriority.DataBind, (Action)(() => this.OnCollectionChanged(e)));
            }
            else
            {
                // IMPORTANT NOTE:
                // We send a Reset eventargs (this is inefficient).
                // If we send the event with the original eventargs, it could contain indexes that do not belong to the collection any more,
                // causing an InvalidOperationException in the with message like:
                // 'n2' index in collection change event is not valid for collection of size 'n2'.
                NotifyCollectionChangedEventArgs notifyCollectionChangedEventArgs = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);

                foreach (NotifyCollectionChangedEventHandler nh in eh.GetInvocationList())
                {
                    nh.Invoke(this, notifyCollectionChangedEventArgs);
                }
            }
        }
    }
}

参考文献： https://msdn.microsoft.com/library/system.windows.data.listcollectionview(v=vs.110).aspx

https://msdn.microsoft.com/library/ms752284(v=vs.110).aspx

System.InvalidOperationException＆＃39; n＆＃39;集合更改事件中的索引对于大小的收集无效＆＃39; 0＆＃39;

1 个答案: