在C#中将方法转换为异步的正确方法？

// Synchronous version
public class ThingCache
{
    private static readonly object _lockObj;
    // ... other stuff

    public Thing Get(string key,Func<Thing> cacheMissResolver)
    {
        if (cache.Contains(key))
            return cache[key];

        Thing item;

        lock(_lockObj)
        {
            if (cache.Contains(key))
                return cache[key];

            item = cacheMissResolver();    
            cache.Add(key,item);
        }

        return item;
    }
}

关于消费异步方法有很多关于在线的材料,但我在生产它们时发现的建议似乎不那么明确.鉴于这是为了成为图书馆的一部分,我的下列任何一种尝试都是正确的吗？

// Asynchronous attempts
public class ThingCache
{
    private static readonly SemaphoreSlim _lockObj = new SemaphoreSlim(1);
    // ... other stuff

    // attempt #1
    public async Task<Thing> Get(string key,Func<Thing> cacheMissResolver)
    {
        if (cache.Contains(key))
            return await Task.FromResult(cache[key]);

        Thing item;

        await _lockObj.WaitAsync();

        try
        {
            if (cache.Contains(key))
                return await Task.FromResult(cache[key]);

            item = await Task.Run(cacheMissResolver).ConfigureAwait(false);
            _cache.Add(key,item);
        }
        finally
        {
            _lockObj.Release();
        }

        return item;
    }

    // attempt #2
    public async Task<Thing> Get(string key,Func<Task<Thing>> cacheMissResolver)
    {
        if (cache.Contains(key))
            return await Task.FromResult(cache[key]);

        Thing item;

        await _lockObj.WaitAsync();

        try
        {
            if (cache.Contains(key))
                return await Task.FromResult(cache[key]);

            item = await cacheMissResolver().ConfigureAwait(false);
            _cache.Add(key,item);
        }
        finally
        {
            _lockObj.Release();
        }

        return item;
    }
}

使用SemaphoreSlim是否是在异步方法中替换lock语句的正确方法？ (我无法在锁定声明的正文中等待.)

我应该使用类型为Func< Task< Thing>>的cacheMissResolver参数代替？虽然这会增加确保解析器func在调用者上是异步的负担(包装在Task.Run中,但我知道如果需要很长时间,它将被卸载到后台线程).

谢谢.