c# - 为什么这个异步 lambda 函数调用不能在 C# 中编译？

Question

在我的理解中，我似乎遗漏了一些非常基本的东西。我正在尝试调用一个需要 Func<T> 的异步函数由调用者传入，以防异步函数在缓存中找不到值。我真的很困惑为什么我的代码不能编译。

我的调用代码如下

static void Main(string[] args)
{
    Func<Task<string>> dataFetcher = async () =>
    {
        string myCacheValue = await new HttpClient().GetStringAsync("http://stackoverflow.com/");
        return myCacheValue;
    };

    MyCache<string> cache = new MyCache<string>();

    string mValue = cache.GetOrCreateAsync("myCacheKey", dataFetcher).Result;

}

MyCache如下

internal class MyCache<T> where T: class
{
    private Cache localCache = null;

    public MyCache()
    {
        localCache = new Cache();
    }
    public T GetOrCreate(string cacheKey, Func<T> doWork)
    {
        T cachedObject = null;
        if (string.IsNullOrEmpty(cacheKey) || string.IsNullOrWhiteSpace(cacheKey))
            return cachedObject;

        cachedObject = localCache.Get(cacheKey) as T;

        if (null == cachedObject)
        {
            cachedObject = doWork();
        }

        localCache.Add(cacheKey, cachedObject, null, DateTime.Now.AddMinutes(5), Cache.NoSlidingExpiration, CacheItemPriority.Normal, null);

        return cachedObject;
    }

    public async Task<T> GetOrCreateAsync(string cacheKey, Func<T> doWork)
    {
        T cachedObject = null;

        if (string.IsNullOrEmpty(cacheKey) || string.IsNullOrWhiteSpace(cacheKey))
            return cachedObject;

        try
        {
            cachedObject = await Task.Factory.StartNew<T>(doWork);
            localCache.Add(cacheKey, cachedObject, null, DateTime.Now.AddMinutes(5), Cache.NoSlidingExpiration, CacheItemPriority.Normal, null);
        }
        catch (Exception)
        {
            cachedObject = null;
        }
        finally
        {
        }

        return cachedObject;
    }
}

在调用代码行

string mValue = cache.GetOrCreateAsync("myCacheKey", dataFetcher).Result;

给我一​​个编译时错误 Argument 2: cannot convert from System.Func<System.Threading.Tasks.Task<string>> to System.Func<string>

我不知道我错过了什么？如果有人可以提供帮助，那就太好了!

谢谢~KD

Answer 1

使用重载方法并传递 Func<Task<T>>论证它。然后您可以等待该结果(即 Task<T> )。

public async Task<T> GetOrCreateAsync(string cacheKey, Func<Task<T>> doWorkAsync)
{
    T cachedObject = null;

    if (string.IsNullOrEmpty(cacheKey) || string.IsNullOrWhiteSpace(cacheKey))
        return cachedObject;

    try
    {
        cachedObject = await doWorkAsync();
        localCache.Add(cacheKey, cachedObject, null, DateTime.Now.AddMinutes(5), Cache.NoSlidingExpiration, CacheItemPriority.Normal, null);
    }
    catch (Exception)
    {
        cachedObject = null;
    }
    finally
    {
    }

    return cachedObject;
}