c# - Parallel LINQ UnauthorizedAccessException when Aggregating results

Question

I am searching in parallel using LINQ to find pattern matching files.

public class ParallelLinq
{
    public IList<string> SearchFolders = new List<string>
    {
        @"C:\Windows" //can be multiple
    };

    protected virtual IEnumerable<string> GetFiles(string path, string[] searchPatterns, SearchOption searchOption = SearchOption.AllDirectories)
    {
        return searchPatterns.AsParallel()
            .SelectMany(searchPattern =>
            {
                try
                {
                    return Directory.EnumerateFiles(path, searchPattern, searchOption);
                }
                catch (Exception ex) //catch UnauthoizedException/IOExceptions
                {
                    return Enumerable.Empty<string>();
                }
            });
    }

    public IEnumerable<string> Find(IList<string> patterns)
    {
        var testResultFiles = Enumerable.Empty<string>();

        if (!SearchFolders.Any() || !patterns.Any())
        {
            return testResultFiles;
        }

        testResultFiles = SearchFolders.AsParallel().Aggregate(testResultFiles, (current, folder) => current.Union(GetFiles(folder, patterns.ToArray())));

        return testResultFiles;
    }
}

However when I try evaluate the values I am running into System.UnauthorizedAccessException: Access to the path 'C:\Windows\appcompat\Programs' is denied.

var plinq = new ParallelLinq();
var res = plinq.Find(new List<string> { "*.dll" });
Console.WriteLine("Linq Count: " + res.Count());

While these exceptions are expected, How can we catch them and continue ahead?

Full Exception:

Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.UnauthorizedAccessException: Access to the path 'C:\Windows\appcompat\Programs' is denied. at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileSystemEnumerableIterator1.AddSearchableDirsToStack(SearchData localSearchData) at System.IO.FileSystemEnumerableIterator1.MoveNext() at System.Linq.Parallel.SelectManyQueryOperator3.SelectManyQueryOperatorEnumerator1.MoveNext(TOutput& currentElement, Pair2& currentKey) at System.Linq.Parallel.PipelineSpoolingTask2.SpoolingWork() at System.Linq.Parallel.SpoolingTaskBase.Work() at System.Linq.Parallel.QueryTask.BaseWork(Object unused) at System.Linq.Parallel.QueryTask.<>c.<.cctor>b__10_0(Object o) at System.Threading.Tasks.Task.InnerInvoke() at System.Threading.Tasks.Task.Execute() --- End of inner exception stack trace --- at System.Linq.Parallel.QueryTaskGroupState.QueryEnd(Boolean userInitiatedDispose) at System.Linq.Parallel.AsynchronousChannelMergeEnumerator1.MoveNextSlowPath() at System.Linq.Parallel.AsynchronousChannelMergeEnumerator1.MoveNext()
at System.Linq.Parallel.QueryOpeningEnumerator1.MoveNext() at System.Linq.Enumerable.<UnionIterator>d__671.MoveNext() at System.Linq.Enumerable.Count[TSource](IEnumerable`1 source)

标签： c#linqexceptionplinq

Answer 1

public class ParallelLinq
{
    public IList<string> SearchFolders = new List<string>
    {
        @"C:\Windows" //can be multiple
    };

    private static string[] TryGetTopDirectories(string path)
    {
        try
        {
            return Directory.GetDirectories(path, "*", SearchOption.TopDirectoryOnly);
        }
        catch
        {
            return new string[0];
        }
    }

    private static IEnumerable<string> GetSubfolders(string path, SearchOption searchOption)
    {
        if (searchOption == SearchOption.TopDirectoryOnly)
        {
            return TryGetTopDirectories(path);
        }
        else
        {
            var topFolders = TryGetTopDirectories(path);
            return topFolders.Concat(
                topFolders.SelectMany(subFolder => GetSubfolders(subFolder, searchOption)));
        }
    }

    protected virtual ParallelQuery<string> GetFiles(string path, string[] searchPatterns, SearchOption searchOption = SearchOption.AllDirectories)
    {
        return GetSubfolders(path, searchOption).AsParallel()
            .SelectMany(subfolder =>
            {
                try
                {
                    return searchPatterns.SelectMany(searchPattern => Directory.EnumerateFiles(subfolder, searchPattern)).ToArray();
                }
                catch (Exception ex) //catch UnauthoizedException/IOExceptions
                {
                    return Enumerable.Empty<string>();
                }
            });
    }

    public IEnumerable<string> Find(IList<string> patterns)
    {
        var testResultFiles = Enumerable.Empty<string>();

        if (!SearchFolders.Any() || !patterns.Any())
        {
            return testResultFiles;
        }

        testResultFiles = SearchFolders.AsParallel().Aggregate(testResultFiles, (current, folder) => current.Union(GetFiles(folder, patterns.ToArray())));

        return testResultFiles;
    }
}