SignalR Hub 抛出 NRE

Question

我对空引用异常有疑问，不明白为什么会发生这种情况。这是我的中心

public class UserHub : Hub
    {
        private static List<Connection> Users = new List<Connection>();

        public override async Task OnConnected()
        {
            string id = Context.ConnectionId;
            string userName = Context.User.Identity.Name;

            if (Users == null)
                Users = new List<Connection>();

            if (!Users.ToArray().Any(_ => _.UserName == userName))
                Clients.AllExcept(Context.ConnectionId).onNewUserConnected(id, userName);

            if (!Users.ToArray().Any(x => x.ConnectionId == Context.ConnectionId))
                Users.Add(new Connection { ConnectionId = id, UserName = userName });

            Clients.Caller.onConnected(id, userName, Users.DistinctBy(_ => _.UserName).ToList());

            await base.OnConnected();
        }


        public override async Task OnReconnected()
        {
            string id = Context.ConnectionId;
            string userName = Context.User.Identity.Name;

            if (Users == null)
                Users = new List<Connection>();

            if (!Users.Any(_ => _.UserName == userName))
                Clients.AllExcept(Context.ConnectionId).onNewUserConnected(id, userName);

            if (!Users.Any(x => x.ConnectionId == id))
                Users.Add(new Connection { ConnectionId = id, UserName = userName });

            Clients.Caller.onConnected(id, userName, Users.DistinctBy(_ => _.UserName).ToList());

            await base.OnReconnected();
        }

        public override async Task OnDisconnected(bool stopCalled)
        {
            if (Users == null)
                Users = new List<Connection>();

            var item = Users.FirstOrDefault(x => x.ConnectionId == Context.ConnectionId);
            if (item != null)
                Users.Remove(item);


            Clients.All.onUserDisconnected(Context.ConnectionId, item.UserName);

            await base.OnDisconnected(stopCalled);
        }
    }

问题出现在 onConnected 方法中，在这一行：

if (!Users.ToArray().Any(_ => _.UserName == userName))

我尝试在上面添加对 null 的检查，但它不起作用。怎么了？该问题随机出现。

堆栈跟踪：

at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
   at App.Web.Hubs.UserHub.<OnConnected>d__1.MoveNext() in E:\Company\App\App\Presentation\App.Web\Hubs\UserHub.cs:line 49
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
...

Answer 1

您的代码不是线程安全的，因此您在访问Users 列表时会看到各种问题。多个连接可以同时启动或关闭，它们都在访问静态 Users 属性。访问列表时考虑使用并发数据结构或锁。