c# - 如何防止以下 C# 代码中的死锁？

Question

以下 C# 类用于多线程环境。我删除了很多实际代码。几乎同时调用 MethodA 和 MethodB 时会出现问题。 IsDepleted 属性中的锁定顺序不能解决问题。从 IsDepleted 属性中删除 lock(WaitingQueue) 解决了死锁，但是当另一个线程在 WaitingQueue.Count == 0 和 Processing.Count == 0 语句之间从 WaitingQueue 添加/删除项目时，此解决方案会导致问题。

using System.Collections.Generic;

class Example
{
    bool IsDepleted
    {
        get
        {
            lock (Processing)
            {
                lock (WaitingQueue)
        {
                    return WaitingQueue.Count == 0
             && Processing.Count == 0;
        }
            }
        }
    }

    private readonly List<object> Processing = new List<object>();
    private readonly Queue<object> WaitingQueue = new Queue<object>();

    public void MethodA(object item)
    {
        lock (WaitingQueue)
        {
            if (WaitingQueue.Count > 0)
            {
                if (StartItem(WaitingQueue.Peek()))
                {
                    WaitingQueue.Dequeue();
                }
            }
        }
    }

    public void MethodB(object identifier)
    {
        lock (Processing)
        {
            Processing.Remove(identifier);
            if (!IsDepleted)
            {
                return;
            }
        }
    //Do something...
    }

    bool StartItem(object item)
    {
        //Do something and return a value
    }
}

Answer 1

这取决于您是想要快速修复还是严格修复。

一种快速解决方法是在所有情况下只使用一个锁对象。

例如private readonly object _lock = new object();

然后锁定它。但是，根据您的情况，这对性能的影响可能超出您的接受范围。

即你的代码会变成这样:

using System.Collections.Generic;

class Example
{
    private readonly object _lock = new object();

    bool IsDepleted
    {
        get
        {
            lock (_lock)
            {
                return WaitingQueue.Count == 0
                 && Processing.Count == 0;
            }
        }
    }

    private readonly List<object> Processing = new List<object>();
    private readonly Queue<object> WaitingQueue = new Queue<object>();

    public void MethodA(object item)
    {
        lock (_lock)
        {
            if (WaitingQueue.Count > 0)
            {
                if (StartItem(WaitingQueue.Peek()))
                {
                    WaitingQueue.Dequeue();
                }
            }
        }
    }

    public void MethodB(object identifier)
    {
        lock (_lock)
        {
            Processing.Remove(identifier);
            if (!IsDepleted)
            {
                return;
            }
        }
        //Do something...
    }

    bool StartItem(object item)
    {
        //Do something and return a value
    }
}