在C＃中将Parallel.ForEach与XmlNodeList一起使用

Question

I already done basic foreach loop with XmlNodeList as given bellow.

Sample XML File (books.xml)

XmlDocument doc = new XmlDocument();
doc.Load("books.xml");
XmlNodeList xnList = doc.SelectNodes("catalog/book");
foreach (XmlNode node in xnList)
{
   Console.WriteLine(node["author"].InnerText);
} 

How do I convert this loop into Parallel.ForEach ?

I've try with this code.But it's didn't work.

Parallel.ForEach(xnList, (XmlNode node) =>
{
   Console.WriteLine(node["author"].InnerText);
});

It's says Error 2

Argument 1: cannot convert from System.Xml.XmlNodeList to System.Collections.Generic.IEnumerable<System.Xml.XmlNode>

Answer 1

XmlNodeList implements the non-generic IEnumerable . You'll need to cast it first in order to work with an IEnumerable<XmlNode> , as that is what Parallel.ForEach operates on:

Parallel.ForEach(xnList.Cast<XmlNode>(), (XmlNode node) =>
{
   Console.WriteLine(node["author"].InnerText);
});

Answer 2

Another tip, you can set how many parallel processes you want:

 Parallel.ForEach(nodes.Cast<XmlNode>(), new ParallelOptions { MaxDegreeOfParallelism = 
 Environment.ProcessorCount },
 (XmlNode node) =>
 { 
      string value = node.InnerText;
      //Some other task
 });

In above Envorinment.ProcessorCount refers to the number of logical cores the CPU reports to Windows. I strongly suggest keeping below this number. I have i9 28-cores, and set to 28, pretty much locks up your machine (just leave 1-2 free).

Another use for this is that when you are debugging, it is hard to track the multi-threads breaking at the same location when you have many threads running. Setting this to 1 will make it work like a regular Loop.

Be mindful of impacts of this. If you are interacting with public items, remember you will have 20+ parallels calls in one hit. I have found that trying to add unique Dictionary terms for some bizarre reason tells me there are duplicates when running multithread.. works fine with just 1.