c# - 如果源为空，Parallel.ForEach 是否启动线程？

Question

我以这种方式使用 Parallel.ForEach:

public void myMethod(IEnumerable<MyType> paramIeCollection)
{
    Parallel.Foreach(paramIeCollection,
          (iterator) =>
          {
              //Do something
          });
}

我想知道当 paramIeCollection 为空时，Parallel.ForEach 无论如何都会启动并从线程池中获取线程并消耗资源，或者它是否首先检查是否有项目在集合中。

如果它不检查，为了避免这种情况，我在这段代码中考虑:

if(paramIeCollection.count > 0)
{
    //run Parallel.Foreach
}

所以问题是，在调用 Parallel.ForEach 之前检查集合是否有项是一个好习惯，还是不需要？

Answer 1

说实话，它确实会检查。但是，如果您查看源代码，它会在弄清楚之前进行一些其他检查和平衡。

如果您有处理器指令强制症，像if(list.count > 0) 这样的简单事先检查可能会为您节省大量 IL。然而在现实世界中它不会有太大的不同

System.Threading.Tasks Reference Source

例如，对于简单的 IEnumerable 重载，您可以通过 E.g 跟踪源代码

public static ParallelLoopResult ForEach<TSource>(IEnumerable<TSource> source, Action<TSource> body)

电话 ForEachWorker

private static ParallelLoopResult ForEachWorker<TSource, TLocal>(
        IEnumerable<TSource> source,
        ParallelOptions parallelOptions,
        Action<TSource> body,
        Action<TSource, ParallelLoopState> bodyWithState,
        Action<TSource, ParallelLoopState, long> bodyWithStateAndIndex,
        Func<TSource, ParallelLoopState, TLocal, TLocal> bodyWithStateAndLocal,
        Func<TSource, ParallelLoopState, long, TLocal, TLocal> bodyWithEverything,
        Func<TLocal> localInit, Action<TLocal> localFinally)

内联电话

// This is an honest-to-goodness IEnumerable.  Wrap it in a Partitioner and defer to our
        // ForEach(Partitioner) logic.
return PartitionerForEachWorker<TSource, TLocal>(Partitioner.Create(source), parallelOptions, body, bodyWithState,
            bodyWithStateAndIndex, bodyWithStateAndLocal, bodyWithEverything, localInit, localFinally);

PartitionerForEachWorker

// Main worker method for Parallel.ForEach() calls w/ Partitioners.
private static ParallelLoopResult PartitionerForEachWorker<TSource, TLocal>(
        Partitioner<TSource> source, // Might be OrderablePartitioner
        ParallelOptions parallelOptions,
        Action<TSource> simpleBody,
        Action<TSource, ParallelLoopState> bodyWithState,
        Action<TSource, ParallelLoopState, long> bodyWithStateAndIndex,
        Func<TSource, ParallelLoopState, TLocal, TLocal> bodyWithStateAndLocal,
        Func<TSource, ParallelLoopState, long, TLocal, TLocal> bodyWithEverything,
        Func<TLocal> localInit,
        Action<TLocal> localFinally)

最终由谁来检查

while (myPartition.MoveNext())