未等待嵌套的异步 lambda

Question

以下代码不会返回它正在迭代的整个集合。每次运行时返回的数组具有任意长度。怎么了？

public async Task<IHttpActionResult> GetClients()
{
    var clientInfoCollection = new ConcurrentBag<ClientInfoModel>();

    await _client.Iterate(async (client) =>
    {
        clientInfoCollection.Add(new ClientInfoModel
        {
            name = client.name,
            userCount = await _user.Count(clientId)
        });
    });

    return Ok(clientInfoCollection.ToArray());
}

以下代码使用新的异步 MongoDB C# 驱动程序

public async Task Iterate(Action<TDocument> processor)
{
    await _collection.Find<TDocument>(_ => true).ForEachAsync(processor);
}

Answer 1

您看到任意数量的值的原因是事实上Iterate接收了一个 type 的委托Action<T>，它等效于async void，有效地使其成为“即发即忘”的执行风格。

内部方法实际上并不知道异步委托已传递给它，因此它迭代集合而不实际异步等待每个项目完成。

您需要做的是使方法参数成为类型的委托Func<TDocument, Task> 并使用正确的重载ForEachAsync：

public Task Iterate(Func<TDocument, Task> processor)
{
    return _collection.Find<TDocument>(_ => true).ForEachAsync(processor);
}

你可以在这里看到来源：

public static async Task ForEachAsync<TDocument>(
                    this IAsyncCursor<TDocument> source, 
                    Func<TDocument, int, Task> processor,
                    CancellationToken cancellationToken = default(CancellationToken))
{
    Ensure.IsNotNull(source, "source");
    Ensure.IsNotNull(processor, "processor");

    // yes, we are taking ownership... assumption being that they've
    // exhausted the thing and don't need it anymore.
    using (source)
    {
        var index = 0;
        while (await source.MoveNextAsync(cancellationToken).ConfigureAwait(false))
        {
            foreach (var document in source.Current)
            {
                await processor(document, index++).ConfigureAwait(false);
                cancellationToken.ThrowIfCancellationRequested();
            }
        }
    }
}