ClientApiPoC/Shared/SignalR/ClientTracker.cs

using System.Collections.Concurrent;

namespace ClientApiPoC.Shared.SignalR {
    public class ClientTracker {
        private ConcurrentDictionary<string, string> _knownClientIds = new();

        public ICollection<string> CurrentClientIds => _knownClientIds.Values;

        public bool TryAddClientId(string clientId) {
            if (string.IsNullOrWhiteSpace(clientId)) throw new ArgumentNullException(nameof(clientId));
            return _knownClientIds.TryAdd(clientId, clientId);
        }

        public bool TryRemoveClientId(string clientId) {
            return _knownClientIds.TryRemove(clientId, out _);
        }
    }
}