Below is a file parser for old style nuget package configuration files. The Parallel.ForEach is probably overkill; in most cases its fast enough without. However, when using the parallel loop, can I avoid the need for the lock / shared list?

class Program
    private static readonly object _lock = new Object();
    private static readonly List<Package> _packages = new List<Package>();

    static void Main(string[] args)
        string[] files = Directory.GetFiles(Directory.GetCurrentDirectory(), "packages.config", SearchOption.AllDirectories);

        Parallel.ForEach(files, GetPackagesForFile);

        IEnumerable<IGrouping<string, Package>> idGroups =
            _packages.GroupBy(p => p.Id).OrderBy(g => g.Key);

        foreach (IGrouping<string, Package> idGroup in idGroups)
            IEnumerable<IGrouping<string, Package>> versionGroups = idGroup.GroupBy(p => p.Version);
            Console.WriteLine($"{idGroup.Key} ({versionGroups.Count()})");

            foreach (IGrouping<string, Package> versionGroup in versionGroups)

    private static void GetPackagesForFile(string filepath)
        var filePackages = XDocument.Load(filepath).Root.Elements("package").Select(GetPackageFromElement);
        lock(_lock) _packages.AddRange(filePackages);

    private static Package GetPackageFromElement(XElement element) =>
        new Package(
  • 1
    \$\begingroup\$ I don't think there is a thread safe List so yes I think you need the lock. Since it is one set of read/write heads not sure how much you gain here. XMLreader may work better here. \$\endgroup\$ – paparazzo May 1 '18 at 14:22
  • \$\begingroup\$ @paparazzo a thread-safe list is called ConcurrentBag. \$\endgroup\$ – t3chb0t May 1 '18 at 18:03
  • \$\begingroup\$ @t3chb0t If it does not implement IList it is not a List in my book. \$\endgroup\$ – paparazzo May 1 '18 at 18:29
  • \$\begingroup\$ @t3chb0t A ConcurrentBag has problems with accessing items from different threads. I'd use a ConcurrentDictionary<Package, byte> and just have the byte be 0. \$\endgroup\$ – Hosch250 May 1 '18 at 18:37
  • \$\begingroup\$ @Hosch250 mhmm, this would make it pretty usless then... but I think retrieving items is usually less of a problem than collecting them. Personally I wouldn't use it because it's actually rarely useful... I just mentioned it for the sake of completness because it's not entirely true that there is no thread-safe list, or at least something similar. \$\endgroup\$ – t3chb0t May 1 '18 at 18:41

If you want to do it this way then you need a lock but since the rest of your code is using Linq I would suggest you convert it over to PLINQ instead of using Parallel.ForEach,

Also you are iterating over the enumerable versionGroups twice. In your specific case it shouldn't cause an issue but that's not best practices with an IEnumerable.

First I'm going to change the GetPackagesForFile to return an array of Packages instead of using the static _packages field.

private static Package[] GetPackagesForFile(string filepath)
    return XDocument.Load(filepath).Root?.Elements("package").Select(GetPackageFromElement).ToArray() ??
           new Package[0];

I also changed it so to not throw a NullException if root not defined. That's up to you if you want the exception there to tell you something was wrong or not. So you might want to remove the ? and the ?? part if you want the exception to be thrown.

Now if you want to use PLINQ we just need to use the AsParallel() extension method. I'm going to use an anonymous class but you could create one if that's what you prefer. Since I'm using an anonymous class I can't specify the type so going to use the var keyword, which doesn't match your coding style.

static void Main(string[] args)

    string[] files = Directory.GetFiles(Directory.GetCurrentDirectory(), "packages.config",

    var idGroups = files.AsParallel()
        .GroupBy(p => p.Id)
        .Select(g => new
            Id = g.Key,
            Version = new HashSet<string>(g.Select(p => p.Version))
        }).OrderBy(p => p.Id);

    foreach (var idGroup in idGroups)
        Console.WriteLine($"{idGroup.Id} ({idGroup.Version.Count})");

        foreach (string version in idGroup.Version)

Since in your code all you care about is the distinct list of versions for each ID and the count I put them in a hashset instead of doing the grouping. I could have used the g.Select(p => p.Version).Distinct().ToList() but just adding them to a hashset seems simpler code.

And in closing like you already stating making this parallel might be overkill. If you want to test it compared to not being paralleled then you just need to remove the AsParallel() and the rest of the code can stay the same.


I also prefer the PLINQ solution showed by @CharlesNRice but as far as clean-code is concerned you could change a couple things.

GetPackagesForFile method should not have side-effects. This means, as far as possible, it's always better to use pure methods.

Applied to your code GetPackagesForFile should return some result, e.g IEnumerable<Package>

private static IEnumerable<Package> GetPackages(string filepath)
    return XDocument.Load(filepath).Root.Elements("package").Select(GetPackageFromElement);

that you add to _packages inside Parallel.ForEach

    source: files,
    body: file =>

Another good habit is to always use {}. It makes your code less error-prone.

If you want to make your code fully lazy, then use EnumerateFile instead of the eager GetFiles

Using vars instead of explicit types would make your code less verbose.


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