这里是一段代码，有时它会引发异常：
Pendings = ClientCode.PendingOrders.Select(x => new DisplayPending()
{
ItemCode = ClientCode.Items.First(y => y.Id == x.ItemCode).Id,
ItemName = ClientCode.Items.First(y => y.Id == x.ItemCode).Name,
OrderNo = x.BuyOrderNo == 0 ? x.SellOrderNo : x.BuyOrderNo,
OrderType = x.OrderType == OrderType.Buy ? "Buy" : "Sell",
PartyCode = x.PartyCode,
Price = x.Price,
Quantity = x.Quantity
});
这是消息：
System.NullReferenceException: 'Object reference not set to an instance of an object.'
x was null.
[PendingOrders是一个ObservableCollection，并且有500多个项目。
编辑
这是我扩展的ObservableCollection的填充方式：
public class AsyncObsetion<T> : ObservableCollection<T>
{
SynchronizationContext context = SynchronizationContext.Current;
readonly object _lock = new object();
public AsyncObsetion() { BindingOperations.EnableCollectionSynchronization(this, _lock); }
public AsyncObsetion(IEnumerable<T> list) : base(list) { BindingOperations.EnableCollectionSynchronization(this, _lock); }
void RaiseCollectionChanged(object param) => base.OnCollectionChanged((NotifyCollectionChangedEventArgs)param);
void RaisePropertyChanged(object param) => base.OnPropertyChanged((PropertyChangedEventArgs)param);
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (SynchronizationContext.Current == context) RaiseCollectionChanged(e);
else context.Send(RaiseCollectionChanged, e);
}
protected override void OnPropertyChanged(PropertyChangedEventArgs e)
{
if (SynchronizationContext.Current == context) RaisePropertyChanged(e);
else context.Send(RaisePropertyChanged, e);
}
public void InsertRange(IEnumerable<T> items)
{
this.CheckReentrancy();
foreach (var item in items)
this.Items.Add(item);
this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}
}
[Client连接到Server时，它发送6个byte[]，它们被添加到名为AsyncObsetion<T>，Items，NewsCollection，Issued，BuyOrders，SellOrders的8个PendingOrders中。 ，ExecutedOrders和MyExecutedOrders在客户端。这段代码负责处理客户端从服务器获取的byte[]：
var tasks = new List<Task>();
tasks.Add(Task.Run(() =>
{
for (int i = 0; i < header.ItemSize; i += Constants.itemSize)
{
var item = PacMan<ItemStruct>.Unpack(itemArray.Skip(i).Take(Constants.itemSize).ToArray());
Items.Add(new Item()
{
Id = item.Id,
Name = item.Name,
Cap = item.Cap,
Floor = item.Floor,
Mid = item.Floor + ((item.Cap - item.Floor) / 2),
Securities = item.Securities,
Owners = item.Owners,
Govt = item.Govt,
Institutions = item.Institutions,
Foreign = item.Foreign,
Public = item.Public,
PerSecurity = PacMan<PerSecurityFinancialsStruct>.ArrayToList<PerSecurityFinancials>(item.PerSecurity),
Dividends = PacMan<DividendStruct>.ArrayToList<Dividend>(item.Dividends),
InitialSecurity = item.InitialSecurity
});
}
}).ContinueWith(t =>
{
if (header.HasExecuted) GetExOrders(header.ExecutedSize, execArray);
if (header.HasNews) AddNews(newsArray.ToArray());
}));
tasks.Add(Task.Run(() => { if (header.HasBuy) GetOrders(header.BuySize, buyArray, "buy"); }));
tasks.Add(Task.Run(() => { if (header.HasSell) GetOrders(header.SellSize, sellArray, "sell"); }));
tasks.Add(Task.Run(() => AddIssue(issueArray.ToArray())));
Task.WaitAll(tasks.ToArray());
hasReceivedData = true;
App.Current.Dispatcher.Invoke(() => CommandManager.InvalidateRequerySuggested());
OnConnected();
OrderVM.ItemCode = Items.First().Id;
e.Completed += Receive;
e.SetBuffer(headerBuffer, 0, headerBuffer.Length);
if (!e.AcceptSocket.ReceiveAsync(e)) Receive(null, e);
OnConnected()是一个事件，当它被触发时，客户端开始从Pendings中创建PendingOrders的过程，该过程由buyArray和sellArray组成。我认为这是问题所在，我相信有时OnConnected()会在Task.WaitAll(tasks.ToArray())之前被解雇。如果有人感兴趣，这是GetOrders的作用：
void GetOrders(int size, IEnumerable<byte> array, string type)
{
var orderList = new List<AllOrder>();
var pendingList = new List<AllOrder>();
for (int i = 0; i < size; i += Constants.orderSize)
{
var order = PacMan<AllOrderStruct>.Unpack(array.Skip(i).Take(Constants.orderSize).ToArray());
AddInitNewOrder(order, orderList, pendingList);
}
if (type == "buy") CheckNumberAndAdd(orderList, BuyOrders);
else CheckNumberAndAdd(orderList, SellOrders);
CheckNumberAndAdd(pendingList, PendingOrders);
}
这里是AddInitNewOrder：
void AddInitNewOrder(AllOrderStruct order, List<AllOrder> orders, List<AllOrder> pendingOrders)
{
var o = new AllOrder();
o.Action = order.Action;
o.OrderType = order.OrderType;
o.ItemCode = order.ItemCode;
o.BrokerName = order.BrokerName;
o.PartyCode = order.PartyCode;
o.Price = order.Price;
o.Quantity = order.Quantity;
o.BuyOrderNo = order.BuyOrderNo;
o.SellOrderNo = order.SellOrderNo;
orders.Add(o);
if (o.BrokerName == BrokerName) pendingOrders.Add(o);
}
这里是CheckNumberAndAdd：
void CheckNumberAndAdd<T>(List<T> normList, AsyncObsetion<T> obsList)
{
var count = normList.Count;
if(count > 50)
{
obsList.InsertRange(normList.Take(count - 50));
var remaining = normList.Skip(count - 50).ToList();
for (int i = 0; i < remaining.Count; i++) obsList.Insert(0, remaining[i]);
}
else for (int i = 0; i < count; i++) obsList.Insert(0, normList[i]);
}
如果在GetOrders函数中设置brekpoint，我会看到pendingList从sellArray中获得483个项目，从buyArray中获得494个项目，所以我总共有977个项目。因此，我应该始终在PendingOrders中获得977个项目，但是如果我在GetOrders中删除brekpoint并在Subscribe方法中设置了断点，则该事件已与该事件挂钩ClientCode.OnConnected += Subscribe;，我看到PendingOrders有时会少于977个项目。
如@BionicCode所述，您在集合中只有一些null元素。
只需用.Where(x != null)或类似的方法过滤掉它们。尽管这可以解决问题，但是通常未检测到的空引用是有关代码中星期数的提示。
考虑加强您的空值检查策略，以避免将来出现这些错误。使用相同的框架，您可以对可为空的对象执行检查，从而使代码更简洁，更健壮。
解决问题的一种方法是像这样包装GetOrders方法的最后三行：
App.Current.Dispatcher.Invoke(() =>
{
if (type == "buy") CheckNumberAndAdd(orderList, BuyOrders);
else CheckNumberAndAdd(orderList, SellOrders);
CheckNumberAndAdd(pendingList, PendingOrders);
});
这可行。