Parallel.Foreach()没有结果 [英] Parallel.Foreach() yields no result

问题描述

我正在尝试使用Parallel.Foreach()查询mongo-db并行性，但没有得到任何结果.但是，当我尝试在常规的foreach循环中运行相同的东西时，我能够执行预期的任务.

var exceptions = new ConcurrentQueue<Exception>();
var secondaryObjectsDictionaryCollection = new Dictionary<string, List<JObject>>();

// This works
foreach(var info in infos)
{
    try
    {
        name = await commonValidator.ValidateAsync(name);
        await commonValidator.ValidateIdAsync(name, id);
        var list = await helper.ListRelatedObjectsAsync(name, id, info, false);

        secondaryObjectsDictionaryCollection.Add(info.PrimaryId, secondaryObjectsList.ToList());
    }
    catch (Exception ex)
    {
        exceptions.Enqueue(ex);
    }
}

//This does not
Parallel.ForEach(infos, async info =>
{
    try
    {
        name = await commonValidator.ValidateAsync(name);
        await commonValidator.ValidateIdAsync(name, id);
        var list = await helper.ListRelatedObjectsAsync(name, id, info, false);

        secondaryObjectsDictionaryCollection.Add(info.PrimaryId, secondaryObjectsList.ToList());
    }
    catch (Exception ex)
    {
        exceptions.Enqueue(ex);
    }
});

我只想并行执行此任务，因为涉及到不同的mongodb集合，而且还减少了响应时间.

我无法弄清楚并行循环中出了什么问题. 并行执行这些任务的任何其他方法也将起作用.

解决方案

让我们看一下说明相同问题的更简单示例

您有与此类似的代码

var results = new Dictionary<int, int>();

Parallel.ForEach(Enumerable.Range(0, 5), async index =>
{
  var result = await DoAsyncJob(index);
  results.TryAdd(index, result);
});

您的代码因为表达式而无法运行

async index => {...}

返回未等待

的任务

像这样

Parallel.ForEach(Enumerable.Range(0, 5), index => new Task());

顺便说一句，当您像示例中那样使用多线程时，在进行并行更新以避免错误和死锁时应使用 ConcurrentDictionary 而不是Dictionary >

此处的最佳解决方案不是使用并行循环，而是使用 Task.WhenAll

var tasks = Enumerable.Range(0, 5).Select(async index =>
{
  var result = await DoAsyncJob(index);
  results.TryAdd(index, result);
});

await Task.WhenAll(tasks);

I am trying to query a mongo-db parallely using Parallel.Foreach() but I am not getting any results. But when I try to run the same thing in regular foreach loop I am able to perform the expected tasks.

var exceptions = new ConcurrentQueue<Exception>();
var secondaryObjectsDictionaryCollection = new Dictionary<string, List<JObject>>();

// This works
foreach(var info in infos)
{
    try
    {
        name = await commonValidator.ValidateAsync(name);
        await commonValidator.ValidateIdAsync(name, id);
        var list = await helper.ListRelatedObjectsAsync(name, id, info, false);

        secondaryObjectsDictionaryCollection.Add(info.PrimaryId, secondaryObjectsList.ToList());
    }
    catch (Exception ex)
    {
        exceptions.Enqueue(ex);
    }
}

//This does not
Parallel.ForEach(infos, async info =>
{
    try
    {
        name = await commonValidator.ValidateAsync(name);
        await commonValidator.ValidateIdAsync(name, id);
        var list = await helper.ListRelatedObjectsAsync(name, id, info, false);

        secondaryObjectsDictionaryCollection.Add(info.PrimaryId, secondaryObjectsList.ToList());
    }
    catch (Exception ex)
    {
        exceptions.Enqueue(ex);
    }
});

I want to perform this task in parallel only since different mongodb collections are involved and also to reduce the response time.

I am not able to figure out what's getting wrong in my parallel loop. Any other approach to perform these tasks in parallel will also work.

解决方案

Lets take a look at more simple example that illustrates same problems

You have code similar to this

var results = new Dictionary<int, int>();

Parallel.ForEach(Enumerable.Range(0, 5), async index =>
{
  var result = await DoAsyncJob(index);
  results.TryAdd(index, result);
});

Your code doesn't run because expression

async index => {...}

returns Task that is not awaited

like this

Parallel.ForEach(Enumerable.Range(0, 5), index => new Task());

By the way, when you work with multithreading like in your example you should use ConcurrentDictionary instead of Dictionary, when you make parallel updates to avoid errors and deadlocks

Best solution here not to use Parallel loop, but instead use Task.WhenAll

var tasks = Enumerable.Range(0, 5).Select(async index =>
{
  var result = await DoAsyncJob(index);
  results.TryAdd(index, result);
});

await Task.WhenAll(tasks);

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