var queue = new System.Collections.Concurrent.ConcurrentQueue<string>();

queue.Enqueue("First");

string item;
if (queue.TryPeek(out item))
{
     Console.WriteLine("First item was " + item);
}
else
{
     Console.WriteLine("Queue was empty.");
}

if (queue.TryDequeue(out item))
{
     Console.WriteLine("Dequeued first item " + item);
}

var queue = new ConcurrentQueue<string>();

// adding to queue is much the same as before
queue.Enqueue("First");

var iterator = queue.GetEnumerator();

queue.Enqueue("Second");
queue.Enqueue("Third");

// only shows First
while (iterator.MoveNext()

using System.Collections.Concurrent;
static ConcurrentDictionary<string, MemoryDBUserAuth> Cache = new ConcurrentDictionary<string, MemoryDBUserAuth>();

Cache.TryGetValue(userID, out oldAuthInfo) == false)

Cache.TryAdd(userID, newAuthInfo);

var oldToken = oldAuthInfo.GameAuthToken;
Cache.TryUpdate(userID, newAuthInfo, oldAuthInfo);

MemoryDBUserAuth userInfo;
Cache.TryRemove(userID, out userInfo);