Question

Update 2011-05-20 12:49AM: The foreach is still 25% faster than the parallel solution for my application. And don’t use the collection count for max parallelism, use somthing closer to the number of cores on your machine.

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I have an IO bound task that I would like to run in parallel. I want to apply the same operation to every file in a folder. Internally, the operation results in a Dispatcher.Invoke that adds the computed file info to a collection on the UI thread. So, in a sense, the work result is a side effect of the method call, not a value returned directly from the method call.

This is the core loop that I want to run in parallel

foreach (ShellObject sf in sfcoll)
    ProcessShellObject(sf, curExeName);

The context for this loop is here:

        var curExeName = Path.GetFileName(Assembly.GetEntryAssembly().Location);
        using (ShellFileSystemFolder sfcoll = ShellFileSystemFolder.FromFolderPath(_rootPath))
        {
            //This works, but is not parallel.
            foreach (ShellObject sf in sfcoll)
                ProcessShellObject(sf, curExeName);

            //This doesn't work.
            //My attempt at PLINQ.  This code never calls method ProcessShellObject.

            var query = from sf in sfcoll.AsParallel().WithDegreeOfParallelism(sfcoll.Count())
                        let p = ProcessShellObject(sf, curExeName)
                        select p;
        }

    private String ProcessShellObject(ShellObject sf, string curExeName)
    {
        String unusedReturnValueName = sf.ParsingName
        try
        {
            DesktopItem di = new DesktopItem(sf);
            //Up date DesktopItem stuff
            di.PropertyChanged += new PropertyChangedEventHandler(DesktopItem_PropertyChanged);
            ControlWindowHelper.MainWindow.Dispatcher.Invoke(
                (Action)(() => _desktopItemCollection.Add(di)));
        }
        catch (Exception ex)
        {
        }
        return unusedReturnValueName ;
    }

Thanks for any help!

+tom

Answer 1

Your query object created via LINQ is an IEnumerable. It gets evaluated only if you enumerate it (eg. via foreach loop):

        var query = from sf in sfcoll.AsParallel().WithDegreeOfParallelism(sfcoll.Count())
                    let p = ProcessShellObject(sf, curExeName)
                    select p;
        foreach(var q in query) 
        {
            // ....
        }
        // or:
        var results = query.ToArray(); // also enumerates query