use hash set

Author: datwelk

src/SignalR/server/Core/src/DefaultHubLifetimeManager.cs

@@ -226,8 +226,7 @@ public override Task SendGroupsAsync(IReadOnlyList<string> groupNames, string me
     {
         // Each task represents the list of tasks for each of the writes within a group
         List<Task>? tasks = new List<Task>();
-
-        var connections = new ConcurrentDictionary<string, HubConnectionContext>();
+        var connections = new HashSet<string>();
 
         foreach (var groupName in groupNames)
         {
@@ -241,14 +240,14 @@ public override Task SendGroupsAsync(IReadOnlyList<string> groupNames, string me
             {
                 foreach (var connection in group)
                 {
-                    connections.TryAdd(connection.Key, connection.Value);
+                    connections.Add(connection.Key);
                 }
             }
         }
 
-        foreach (var connection in connections)
+        foreach (var connectionId in connections)
         {
-            tasks.Add(SendConnectionAsync(connection.Key, methodName, args, cancellationToken));
+            tasks.Add(SendConnectionAsync(connectionId, methodName, args, cancellationToken));
         }
 
         if (tasks != null)