Preventing access to concurrent queue while rebuilding

Question

I have a ConcurrentQueue which holds an object with some helpers methods to enqueue and dequeue from the queue. There is a need to remove items from the queue which match some criteria and then rebuild the queue since they might not be in the first item dequeued.

How can I do this safely?

ConcurrentQueue is supposed to be thread safe, but I want to also prevent items from being added/removed from the queue when the removal and rebuilding is happening.

I'm trying this, but I'm not sure if it's correct.

public class EmailRepository
{
    private readonly object _lockObj _lockObj = new object();
    private ConcurrentQueue<EmailMessage> _emailMessages = new ConcurrentQueue<EmailMessage>();

    public ConcurrentQueue<EmailMessage> EmailMessages => _emailMessages;

    public void AddEmailMessage(string subject, string body)
    {
        _emailMessages.Enqueue(new EmailMessage(subject, body));
    }

    public void AddEmailMessage(EmailMessage message)
    {
        _emailMessages.Enqueue(message);
    }

    public bool RemoveEmailMessage(out EmailMessage message)
    {
        return _emailMessages.TryDequeue(out message);
    }

    public void RemoveSiteEmailsAndRebuildQueue(int key)
    {
        for (int i = 0; i < _emailMessages.Count; i++)
        {
            RemoveEmailMessage(out EmailMessage message);
            if (message.KeyValue.Equals(key))
            {
                continue;
            }

            AddEmailMessage(message);
        }
    }
}

I'm exposing _emailMessages but nothing is actually modifying the collection unless they call the appropriate methods.

I think I should be putting something around the logic for RemoveSiteEmailsAndRebuildQueue to ensure nothing will enqeueue or dequeue until that method is done executing, but I'm not sure what that something is.

looks like that adding a lock statement (learn.microsoft.com/en-us/dotnet/csharp/language-reference/…) to all helper functions will be enough. — Serg
– Serg, Commented Mar 24, 2021 at 18:35

Amir · Accepted Answer · 2021-03-24 18:34:53Z

You can even don't use ConcurrentQueue and use a simple lock object like this:

public class EmailRepository
{
    private readonly object _lockObj = new object();
    private Queue<EmailMessage> _emailMessages = new Queue<EmailMessage>();

    public Queue<EmailMessage> EmailMessages => _emailMessages;

    public void AddEmailMessage(string subject, string body)
    {
        lock (_lockObj)
        {
            _emailMessages.Enqueue(new EmailMessage(subject, body));
        }
    }

    public void AddEmailMessage(EmailMessage message)
    {
        lock (_lockObj)
        {
            AddWithoutLock(message);
        }
    }

    private void AddWithoutLock(EmailMessage message)
    {
        _emailMessages.Enqueue(message);

    }

    public bool RemoveEmailMessage(out EmailMessage message)
    {
        lock (_lockObj)
        {
            return _emailMessages.TryDequeue(out message);
        }
    }

    public void RemoveSiteEmailsAndRebuildQueue(int key)
    {
        lock (_lockObj)
        {
            for (int i = 0; i < _emailMessages.Count; i++)
            {
                RemoveEmailMessage(out EmailMessage message);
                if (message.KeyValue.Equals(key))
                {
                    continue;
                }

                AddWithoutLock(message);
            }
        }
    }
}

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