Question

I had a loop like this:

        this.results = new List<Tuple<int, IEnumerable<Thing>>>();

        var utcNow = DateTime.UtcNow;
        var resultsLocker = new object();

        Parallel.ForEach(
            this.dataHelper.GetActiveIds(),
            id =>
            {
                var result = new Tuple<int, IEnumerable<Thing>>(
                    id,
                    this.dataHelper.GetThing(id, this.PossibleLastRunTime, utcNow));

                lock (resultsLocker)
                {
                    this.results.Add(result);
                }
            });

and, using this answer, translated to a more compact and understandable:

        this.results = this.dataHelper.GetActiveIds()
            .AsParallel()
            .Select(id => new Tuple<int, IEnumerable<Thing>>(
                id,
                this.dataHelper.GetThing(id, this.PossibleLastRunTime, utcNow)))
            .ToList();

Now I have a more complex nested loop as such:

        var measuresLocker = new object();
        var measures = new List<Tuple<int, object, object>>();

        Parallel.ForEach(
            this.results,
            result =>
            {
                foreach (var measuredValue in result.Item2.Select(destination =>
                    new Tuple<int, object, object>(
                        result.Item1,
                        destination.Message,
                        destination.DestinationName)))
                {
                    lock (measuresLocker)
                    {
                        measures.Add(measuredValue);
                    }
                }
            });

I want to do something similar, but I’m getting stuck with this code:

        measures = this.results
            .AsParallel()
            .Select(result => result.Item2.Select(destination =>
                new Tuple<int, object, object>(
                    result.Item1,
                    destination.Message,
                    destination.DestinationName)).ToList()).ToList();

I seem to be getting a list of lists and I simply want the one list as per my original code. Can this be done pretty succinctly using LINQ? And if so, how?

Answer 1

To flatten a list of lists into a single list, use SelectMany() instead of Select().

   measures = this.results
        .AsParallel()
        .SelectMany(result => result.Item2.Select(destination =>
            new Tuple<int, object, object>(
                result.Item1,
                destination.Message,
                destination.DestinationName)).ToList()).ToList();