客户端调用函数时的 c# .NET Core SignalR 异常（并不总是发生）

Question

我有以下服务器SignalR Hub：

 public class AlertHub : Hub
    {
        static ConcurrentDictionary<string, string> _users = new ConcurrentDictionary<string, string>();

        public Task SendMessage(string user, string message)
        {
            return Clients.All.SendAsync("BroadcastMessage", user, message);
        }

        public Task SendMessageToUser(string userUid, string user, string message, string totalMessages)
        {
            var clientId = _users.FirstOrDefault(x => x.Value == userUid).Key;
            return Clients.Client(clientId).SendAsync("BroadcastMessage", user, message, totalMessages);
        }

        public static void ClearState()
        {
            _users.Clear();
        }

        public override Task OnConnectedAsync()
        {
            _users.TryAdd(Context.ConnectionId, Context.ConnectionId);

            return base.OnConnectedAsync();
        }

        public override Task OnDisconnectedAsync(Exception exception)
        {
            string userUid;
            _users.TryRemove(Context.ConnectionId, out userUid);

            return base.OnDisconnectedAsync(exception);
        }

        public void SetUserUid(string userUid)
        {
            _users[Context.ConnectionId] = userUid;
        }
    }

在我的客户端上，就在await _signalRConnection.StartAsync(); 之前，我将userUid 设置为我的_users 字典：

//Send user name for this client, so we won't need to send it with every message
                await _signalRConnection.InvokeAsync("SetUserUid", this.State.UserUID);

而且效果很好，没有例外。

然后在我的客户端上，当我尝试发送消息时，我会执行以下操作：（这通常有效，异常​​是间歇性的）

await _signalRConnection.InvokeAsync("SendMessageToUser", model.UserUid.ToString().ToLower(), this.State.UserName, $"{model.UserName} has sent you a message.", countMessagesString);

这正常工作，但有时它开始崩溃，在服务器中显示以下错误：

Microsoft.AspNetCore.SignalR.Internal.DefaultHubDispatcher.? [?] - MESSAGE: Failed to invoke hub method 'SendMessageToUser'.
 System.ArgumentNullException: Value cannot be null. (Parameter 'connectionId')

那是因为当尝试获取 clientId 时，它会在此处返回 null：

 var clientId = _users.FirstOrDefault(x => x.Value == userUid).Key; <-- HERE
return Clients.Client(clientId).SendAsync("BroadcastMessage", user, message, totalMessages);

所以我不知道为什么 _users 字典有时没有密钥。这很奇怪。

关于如何挖掘或解决它的任何线索？

** 更新 **

这就是我所做的：

  public class AlertHubDictionary
    {
        static ConcurrentDictionary<string, string> _users;

        public AlertHubDictionary()
        {
            _users = new ConcurrentDictionary<string, string>();
        }

        public ConcurrentDictionary<string, string> UsersDictionary { get { return _users; } }

        public void ClearUsersDictionary()
        {
            _users.Clear();
        }
    }

然后这个类注册为Singleton DI：

   services.AddSingleton<AlertHubDictionary>();

最后对 Hub 类进行替换：

public class AlertHub : Hub
    {
        private readonly ILogger<AlertHub> _logger;
        private AlertHubDictionary _hubDictionary;
        static ConcurrentDictionary<string, string> _users = new ConcurrentDictionary<string, string>();

        public AlertHub(ILogger<AlertHub> logger, AlertHubDictionary hubDictionary)
        {
            _logger = logger;
            _hubDictionary = hubDictionary;
        }
        public Task SendMessage(string user, string message)
        {
            //Clients.Client("").SendAsync("BroadcastMessage", user, message);
            return Clients.All.SendAsync("BroadcastMessage", user, message);
        }

        public Task SendMessageToUser(string userUid, string user, string message, string totalMessages)
        {
            //var clientId = _users.FirstOrDefault(x => x.Value == userUid).Key;
            var clientId = _hubDictionary.UsersDictionary.FirstOrDefault(x => x.Value == userUid).Key;

            using (var scope = _logger.BeginScope("SCOPED_VALUE"))
            {
                _logger.LogInformation($"Send message to userUid {userUid} - {user} with clientId {clientId}.");

            }


            if (clientId == null)
            {
                // the user is not connected
                return Task.FromResult(0);
            }
            else
            {

                return Clients.Client(clientId).SendAsync("BroadcastMessage", user, message, totalMessages);
            }
        }

        public void ClearState()
        {
            _hubDictionary.ClearUsersDictionary();
            //_users.Clear();

            using (var scope = _logger.BeginScope("SCOPED_VALUE"))
            {
                _logger.LogInformation("_users cleared");

            }

        }

        public override Task OnConnectedAsync()
        {
            _hubDictionary.UsersDictionary.TryAdd(Context.ConnectionId, Context.ConnectionId);
            //_users.TryAdd(Context.ConnectionId, Context.ConnectionId);

            using (var scope = _logger.BeginScope("SCOPED_VALUE"))
            {
                _logger.LogInformation($"ConnectionId {Context.ConnectionId} connected.");

            }

            return base.OnConnectedAsync();
        }

        public override Task OnDisconnectedAsync(Exception exception)
        {
            string userUid;
            _hubDictionary.UsersDictionary.TryRemove(Context.ConnectionId, out userUid);

            using (var scope = _logger.BeginScope("SCOPED_VALUE"))
            {
                _logger.LogInformation($"UserUid {userUid} disconnected.");

            }


            return base.OnDisconnectedAsync(exception);
        }

        public void SetUserUid(string userUid)
        {
            _hubDictionary.UsersDictionary[Context.ConnectionId] = userUid;

            using (var scope = _logger.BeginScope("SCOPED_VALUE"))
            {
                _logger.LogInformation($"Connection {Context.ConnectionId} linked to userUid {userUid}");

            }

            //ClientNameChanged?.Invoke(Context.ConnectionId, userName);
        }
    }

Answer 1

根据您捕获用户 ID 的方式，您似乎正在尝试将 SignalR 集线器视为单例。根据 Microsoft 的文档，集线器是一个瞬态类：

https://docs.microsoft.com/en-us/aspnet/core/signalr/hubs?view=aspnetcore-3.1#create-and-use-hubs

集线器是暂时的：

不要将状态存储在集线器类的属性中。每个集线器方法 调用在新的集线器实例上执行。

因此，如果您想保留已注册用户 ID 的共享实例，您可能应该将其作为一个单独的类进行管理，该类注册为单例并注入到您的集线器类中，以便可以独立于集线器维护状态.