小编Alb*_*ert的帖子

我一直在阅读MSDN上的这篇异步文章,但我无法理解Lazy<T>给定示例的目的.

public class AsyncCache<TKey, TValue>
{
    private readonly Func<TKey, Task<TValue>> _valueFactory;
    private readonly ConcurrentDictionary<TKey, Lazy<Task<TValue>>> _map;

    public AsyncCache(Func<TKey, Task<TValue>> valueFactory)
    {
        if (valueFactory == null) throw new ArgumentNullException("loader");
        _valueFactory = valueFactory;
        _map = new ConcurrentDictionary<TKey, Lazy<Task<TValue>>>();
    }

    public Task<TValue> this[TKey key]
    {
        get
        {
            if (key == null) throw new ArgumentNullException("key");
            return _map.GetOrAdd(key, toAdd => 
                new Lazy<Task<TValue>>(() => _valueFactory(toAdd))).Value;
        }
    }
}

根据我的理解,当你调用.Value时Lazy<T>,它将调用内部的构造函数.从示例中,它立即被调用,为什么要添加Lazy<T>？