c# - 并发堆栈 : How to check if contains object?

Question

如何检查堆栈中是否包含特定对象？

private ConcurrentStack<int> cs = new ConcurrentStack<int>();
cs.Push(1);

Answer 1

方法 Stack<T>.Contains 在 ConcurrentStack<T> -class 中不可用.我猜是因为那不是线程安全的。

所以如果你需要它，你必须使用锁，然后你可以使用 Enumerable.Contains :

private ConcurrentStack<int> cs = new ConcurrentStack<int>();
private Object csLockObject  = new Object();

...

bool contains = false;
lock (csLockObject)
{
    contains = cs.Contains(1);
}

但是当您枚举此快照时，另一个线程可能会向堆栈添加或从堆栈中删除项目。如果您想防止这种情况发生，您还需要在添加/删除的地方加一把锁。

I want to avoid duplicates

好吧，您可以使用像这样的类，它使用 ConcurrentDictionary检查它是否唯一:

public class ConcurrentUniqueStack<T>
{
    private readonly ConcurrentDictionary<T, int> _itemUnique; // there is no ConcurrentHashSet so we need to use a Key-only dictionary
    private readonly ConcurrentStack<T> _stack;

    public ConcurrentUniqueStack() : this(EqualityComparer<T>.Default)
    {
    }
    public ConcurrentUniqueStack(IEqualityComparer<T> comparer)
    {
        _stack = new ConcurrentStack<T>();
        _itemUnique = new ConcurrentDictionary<T, int>(comparer);
    }

    public bool TryPush(T item)
    {
        bool unique = _itemUnique.TryAdd(item, 1);
        if (unique)
        {
            _stack.Push(item);
        }

        return unique;
    }
    public bool TryPop(out T result)
    {
        bool couldBeRemoved = _stack.TryPop(out result);
        if (couldBeRemoved)
        {
            _itemUnique.TryRemove(result, out int whatever);
        }
        return couldBeRemoved;
    }

    public bool TryPeek(out T result) => _stack.TryPeek(out result);
}