C#:如何使IEnumerable <T>线程安全？

private readonly object _lock = new object();
private IEnumerable<T> _items;

public void AddItem(T item)
{
    lock (_lock)
    {
        var list = _items.ToList();
        list.Add(item);
        _items = list.AsEnumerable();
    }
}

public IEnumerator<T> GetEnumerator()
{
    lock (_lock)
    {
        return _items.GetEnumerator();
    }
}

private ConcurrentBag<T> _items = new ConcurrentBag<T>();

public void AddItem(T item)
{
    _items.Add(item);
}

public IEnumerator<T> GetEnumerator()
{
    return _items.GetEnumerator();
}

private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();
private IEnumerable<T> _items;

public void AddItem(T item)
{
    _lock.EnterWriteLock();
    try
    {
        var list = _items.ToList();
        list.Add(item);
        _items = list.AsEnumerable();
    }
    finally
    {
        _lock.ExitWriteLock();
    }
}

public IEnumerator<T> GetEnumerator()
{
    _lock.EnterReadLock();
    try
    {
        return _items.GetEnumerator();
    }
    finally
    {
        _lock.ExitReadLock();
    }
}