using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<FileList> fileList = new List<FileList>

            {

                new FileList { ID =1, Path = "C:\\7.pdf", Extension = "pdf"},

                new FileList { ID =2, Path = "C:\\7.pdf", Extension = "png"},

                new FileList { ID =3, Path = "C:\\7.pdf", Extension = "mp3"},

                new FileList { ID =4, Path = "C:\\7.pdf", Extension = "pdf"},

                new FileList { ID =4, Path = "C:\\7.pdf", Extension = "jpg"}

            };

​

            List<ExtensionsCategory> extensionCat = new List<ExtensionsCategory>

            {

                new ExtensionsCategory { ID =1, Extension = "mp3", Category = "Multimedia"},

                new ExtensionsCategory { ID =2, Extension = "pdf", Category = "Document"},

                new ExtensionsCategory { ID =3, Extension = "png", Category = "Photo"},

                new ExtensionsCategory { ID =3, Extension = "jpg", Category = "Photo"}

            };

​

            var result = extensionCat.GroupBy(x => x.Category)

                          .Select(x =>

                          {

                              var files = fileList.Where(f => x.Select(z => z.Extension).Contains(f.Extension)).Count();

                              return new

                              {

                                  Category = x.Key,

                                  TotalFileCount = files

                              };

                          });

         foreach (var item in result)

            {

                Console.WriteLine("Category: {0}, TotalFileCount: {1}", item.Category, item.TotalFileCount);

            }

    }

}

​

 public class FileList

        {

            public int ID { get; set; }

            public string Path { get; set; }

            public string Extension { get; set; }

        }

​

        public class ExtensionsCategory

        {

            public int ID { get; set; }

            public string Extension { get; set; }

            public string Category { get; set; }

        }