不能在 Parallel.Foreach 中使用 await。错误：- 'await' 运算符只能在异步 lambda 表达式中使用

Question

我的 asp.net mvc-5 Web 应用程序中有以下方法：-

public async Task <List<Details>> Get()
{
    Parallel.ForEach(resourcesinfo.operation.Details,new ParallelOptions { MaxDegreeOfParallelism = 20 }, (c) =>
    {
        ResourceAccountListInfo resourceAccountListInfo = new ResourceAccountListInfo();
        using (WebClient wc = new WebClient()) 
        {
            string url = currentURL + "resources/" + c.RESOURCEID + "/accounts?AUTHTOKEN=" + pmtoken;
            string tempurl = url.Trim();

            var json =  await wc.DownloadStringTaskAsync(tempurl);
            resourceAccountListInfo = JsonConvert.DeserializeObject<ResourceAccountListInfo>(json);
        }
        //code goes here

但我收到此错误：-

“await”运算符只能在异步 lambda 表达式中使用。考虑用 'async' 修饰符标记这个 lambda 表达式

请问有人可以推荐吗？

Answer 1

Parallel.ForEeach 不是为使用异步函数而设计的，您需要使用更现代的类，例如TPL Dataflow 中的类。您可以通过将 NuGet 包安装到您的项目Microsoft.Tpl.Dataflow 来获得它。您可以将以前的代码重新创建为

private const int MAX_PARALLELISM = 20

public async Task <List<Details>> Get()
{
    var block = new ActionBlock<Entry>(async (c) => 
        {
            ResourceAccountListInfo resourceAccountListInfo = new ResourceAccountListInfo();
            using (WebClient wc = new WebClient()) 
            {
                string url = currentURL + "resources/" + c.RESOURCEID + "/accounts?AUTHTOKEN=" + pmtoken;
                string tempurl = url.Trim();

                var json =  await wc.DownloadStringTaskAsync(tempurl);
                resourceAccountListInfo = JsonConvert.DeserializeObject<ResourceAccountListInfo>(json);
            }
            //code goes here
        }
        ,new ExecutionDataflowBlockOptions 
                   { 
                         MaxDegreeOfParallelism = MAX_PARALLELISM
                   });

    foreach(var entry in resourcesinfo.operation.Details)
    {
        await block.SendAsync(entry);
    }

    block.Complete();
    await block.Completion;

    //More code here    
}

---

经过一番思考，这里有一个稍微复杂一点的版本，它完成了从读取记录到从Get()返回结果的整个管道

private const int MAX_PARALLELISM = 20

public async Task<List<Details>> Get()
{
    List<Details> result = new List<Details>();

    var getRecordBlock = new TransformBlock<Entry, ResourceAccountListInfo>(async (c) =>
        {
            ResourceAccountListInfo resourceAccountListInfo = new ResourceAccountListInfo();
            using (WebClient wc = new WebClient())
            {
                string url = currentURL + "resources/" + c.RESOURCEID + "/accounts?AUTHTOKEN=" + pmtoken;
                string tempurl = url.Trim();

                var json = await wc.DownloadStringTaskAsync(tempurl);
                resourceAccountListInfo = JsonConvert.DeserializeObject<ResourceAccountListInfo>(json);
            }
            return resourceAccountListInfo;
        }
        , new ExecutionDataflowBlockOptions
        {
            MaxDegreeOfParallelism = MAX_PARALLELISM
        });

    //Defaults to MaxDegreeOfParallelism = 1
    var addToListBlock = new ActionBlock<ResourceAccountListInfo>(info =>
    {
        Details detail = TurnResourceAccountListInfoInToDetails(info);
        result.Add(detail);
    });

    getRecordBlock.LinkTo(addToListBlock, new DataflowLinkOptions { PropagateCompletion = true});

    foreach (var entry in resourcesinfo.operation.Details)
    {
        await getRecordBlock.SendAsync(entry);
    }

    getRecordBlock.Complete();
    await addToListBlock.Completion;

    return result;
}