public class ConnectionMapping
 {
   private readonly Dictionary _connections =
     new Dictionary();

    public int Count
    {
     get {  return _connections.Count; }
    }

     public void Add(T key, string connectionId)
     {
      lock (_connections)
       {
        HashSet connections;
        if (!_connections.TryGetValue(key, out connections))
        {
          connections = new HashSet();
         _connections.Add(key, connections);
        }

      lock (connections)
       {
         connections.Add(connectionId);
       }
      }
     }

     public IEnumerable GetConnections(T key)
     {
       HashSet connections;
       if (_connections.TryGetValue(key, out connections))
        {
         return connections;
        }
       return Enumerable.Empty();
     }

     public void Remove(T key, string connectionId)
     {
      lock (_connections)
      {
       HashSet connections;
       if (!_connections.TryGetValue(key, out connections))
        {
         return;
        }
        lock (connections)
        {
         connections.Remove(connectionId);
         if (connections.Count == 0)
         {
         _connections.Remove(key);
         }
        }
       }
    }
  }