using System;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        string[] documents = new[] { 'n', 'a', 's', 'j', 'a', 'l', 'v', 'g', 'a', 'f', 'f' }

           .Select((item, index) => $"{item}-{index}")

           .ToArray();

        Console.WriteLine($"Documents: [{String.Join(", ", documents)}]");

​

        var grouped = documents.GroupBy(item => item[0]); // Group by the first char

​

        ParallelOptions options = new()

        {

            MaxDegreeOfParallelism = Environment.ProcessorCount

        };

​

        Parallel.ForEach(grouped, options, grouping =>

        {

            foreach (var document in grouping)

            {

                Console.WriteLine($"Processing: {document}");

                Thread.Sleep(500);

            }

        });

    }

}