source: TFP-WebServer/CommandExtensions/src/PrivateMessageConnections.cs@ 459

using System.Collections.Generic;

namespace CommandExtensions {
        public static class PrivateMessageConnections {
                private static readonly Dictionary<PlatformUserIdentifierAbs, PlatformUserIdentifierAbs> senderOfLastPM = new Dictionary<PlatformUserIdentifierAbs, PlatformUserIdentifierAbs> ();

                public static void SetLastPMSender (ClientInfo _sender, ClientInfo _receiver) {
                        senderOfLastPM [_receiver.InternalId] = _sender.InternalId;
                }

                public static ClientInfo GetLastPMSenderForPlayer (ClientInfo _player) {
                        if (!senderOfLastPM.TryGetValue (_player.InternalId, out PlatformUserIdentifierAbs recUserId)) {
                                return null;
                        }

                        ClientInfo recInfo = ConnectionManager.Instance.Clients.ForUserId (recUserId);
                        return recInfo;
                }
        }
}