C#:使用共享资源的最佳方式?

我需要序列化对共享资源(即缓存)的访问。我使用后面描述的模式,但有时,特别是前两个线程,会加载两次数据。问题在哪里?

public class Entity { }

public class CacheManager
{
    public static CacheManager Instance = new CacheManager();

    private CacheManager()
    {
        _thisLock = new Object();
        _cache = new Dictionary<int, Entity>();
    }

    private readonly Object _thisLock;
    private readonly IDictionary<int, Entity> _cache;

    public Entity GetEntity(int id)
    {
        Entity entity;
        if ( !_cache.TryGetValue(id, out entity) ) {
            /* Only one thread, at a time, can go inside lock statement.
             * So, if threads [1] and [2] came here at the same time, .NET shoud pass thread [1] inside and [2] waits. */
            lock ( _thisLock ) {
                if ( !_cache.TryGetValue(id, out entity) ) { /* If we are [2] : check(and loads from cache) if [1] did work for us. */
                    /* we are [1] so let's load entity from repository and add it to cache */
                    entity = new Entity(); // simulate repository access
                    _cache.Add(id, entity);
                }
            }
        }
        return entity;
    }
}

为什么两个线程都在lock语句中执行步骤?

非常感谢。