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c# - 更少锁定的线程安全

转载 作者：行者123 更新时间：2023-11-30 18:37:24 25 4
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Possible Duplicate:
C# Threading & Blocking

我正在尝试有效地确定哪种方法更好:

目前，我有一个单例实例，它公开以延迟加载方式加载的实体。我列出了三种方法，每种方法都有一些优点。第一种方法完全依赖双锁模式来确保线程安全。第二种方法不使用锁定，但它有可能在比赛中使用双倍负载。第三种方法确实使用了一种我越来越喜欢的解决方案。 (System.Lazy)。

出于某种原因，我觉得第二种方法 (System.Thread.InterLocked) 有问题，但我无法指出它。是否有理由支持一种方法而不是另一种方法？我在上一篇文章中确实介绍了这一点，我觉得第三种选择是从现在开始的方式。

我将代码剥离到准系统，以便能够解释设计。 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TPLDemo
{
  public class SomeEntity
  {
  }

  public class MultiThreadedManager
  {
    private static readonly System.Lazy<MultiThreadedManager> instance = new Lazy<MultiThreadedManager>(() => { return new MultiThreadedManager(); });
    private readonly object _syncRoot = new object();
    private List<SomeEntity> _inMemoryEntities = null;
    private List<SomeEntity> _inMemoryEntitiesUsingLockFreeApproach = null;
    private System.Lazy<List<SomeEntity>> _inMemoryUsingLazy = new Lazy<List<SomeEntity>>(() => { return MultiThreadedManager.Instance.LoadFromSomewhere(); });

    public static MultiThreadedManager Instance
    {
      get { return instance.Value; }
    }


    public IEnumerable<SomeEntity> LazyEntities
    {
      get
      {
        return _inMemoryUsingLazy.Value;
      }
    }

    public IEnumerable<SomeEntity> LocklessEntities
    {
      get
      {
        if (_inMemoryEntitiesUsingLockFreeApproach == null)
        {
          do
          {
          // Is it possible multiple threads hit this at the same time?
          } while (System.Threading.Interlocked.CompareExchange<List<SomeEntity>>(ref                _inMemoryEntitiesUsingLockFreeApproach, this.LoadFromSomewhere(), null) != null);
        }

        return _inMemoryEntitiesUsingLockFreeApproach;
      }
    }


    /// <summary>
    /// This is thread safe but it involved some locking. 
    /// </summary>
    public IEnumerable<SomeEntity> Entities
    {
      get
      {
        if (_inMemoryEntities == null)
        {
          lock (_syncRoot)
          {
            if (_inMemoryEntities == null)
            {
              List<SomeEntity> list = this.LoadFromSomewhere();
              _inMemoryEntities = list;
            }
          }
        }

        return _inMemoryEntities;
      }
    }

    private List<SomeEntity> LoadFromSomewhere()
    {
      return new List<SomeEntity>();
    }

    public void ReloadEntities()
    {
      // This is sufficient becasue any subsequent call will reload them safely. 
      _inMemoryEntities = null;

      // This is sufficient becasue any subsequent call will reload them safely. 
      _inMemoryEntitiesUsingLockFreeApproach = null;

      // This is necessary becasue _inMemoryUsingLazy.Value is readonly.
      _inMemoryUsingLazy = new Lazy<List<SomeEntity>>(() => { return MultiThreadedManager.Instance.LoadFromSomewhere(); });
    }
  }
}

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