ConcurrentDictionary AddOrUpdate

var sessionId = a.Session.SessionID.ToString();
userDic.AddOrUpdate(
  authUser.UserId,
  sessionId,
  (key, oldValue) => sessionId);

static class ExtensionMethods
{
    // Either Add or overwrite
    public static void AddOrUpdate<K, V>(this ConcurrentDictionary<K, V> dictionary, K key, V value)
    {
        dictionary.AddOrUpdate(key, value, (oldkey, oldvalue) => value);
    }
}

userDic[authUser.UserId] = sessionId;