TryDequeueSlow(out result); // slow path that needs to fix up segments
/// <summary>Tries to dequeue an item, removing empty segments as needed.</summary>
private bool TryDequeueSlow(out T item)
{
while (true)
{
// Get the current head
ConcurrentQueueSegment <T> head = _head;
// Try to take. If we're successful, we're done.
if (head.TryDequeue(out item))
{
return(true);
}
// Check to see whether this segment is the last. If it is, we can consider
// this to be a moment-in-time empty condition (even though between the TryDequeue
// check and this check, another item could have arrived).
if (head._nextSegment == null)
{
item = default(T);
return(false);
}
// At this point we know that head.Next != null, which means
// this segment has been frozen for additional enqueues. But between
// the time that we ran TryDequeue and checked for a next segment,
// another item could have been added. Try to dequeue one more time
// to confirm that the segment is indeed empty.
Debug.Assert(head._frozenForEnqueues);
if (head.TryDequeue(out item))
{
return(true);
}
// This segment is frozen (nothing more can be added) and empty (nothing is in it).
// Update head to point to the next segment in the list, assuming no one's beat us to it.
lock (_crossSegmentLock)
{
if (head == _head)
{
_head = head._nextSegment;
}
}
}
}
TryDequeueSlow(out result); // slow path that needs to fix up segments
/// <summary>Tries to dequeue an item, removing empty segments as needed.</summary>
private bool TryDequeueSlow([MaybeNullWhen(false)] out T item)
{
while (true)
{
// Get the current head
ConcurrentQueueSegment<T> head = _head;
// Try to take. If we're successful, we're done.
if (head.TryDequeue(out item))
{
return true;
}
// Check to see whether this segment is the last. If it is, we can consider
// this to be a moment-in-time empty condition (even though between the TryDequeue
// check and this check, another item could have arrived).
if (head._nextSegment == null)
{
item = default!;
/// <summary>
/// Attempts to remove and return the object at the beginning of the <see
/// cref="ConcurrentQueue{T}"/>.
/// </summary>
/// <param name="result">
/// When this method returns, if the operation was successful, <paramref name="result"/> contains the
/// object removed. If no object was available to be removed, the value is unspecified.
/// </param>
/// <returns>
/// true if an element was removed and returned from the beginning of the
/// <see cref="ConcurrentQueue{T}"/> successfully; otherwise, false.
/// </returns>
public bool TryDequeue(out T result) =>
_head.TryDequeue(out result) || // fast-path that operates just on the head segment
TryDequeueSlow(out result); // slow path that needs to fix up segments