using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

                public static void Main(string[] args)

                {

                    List<Vacancy> vacancy = new List<Vacancy>

                    {

                        new Vacancy                    

                        {

                            Documents = new List<JobDocumentViewModel>

                            {

                                new JobDocumentViewModel{DocumentTitle = "C",FileName="Somefile"},                        

                                new JobDocumentViewModel{DocumentTitle = "C",FileName="Somefile"},

                                new JobDocumentViewModel{DocumentTitle = "A",FileName="Somefile"}

                            }

                        },

                        new Vacancy

                        {

                            Documents = new List<JobDocumentViewModel>

                            {

                                new JobDocumentViewModel{DocumentTitle = "C",FileName="Somefile"},

                                new JobDocumentViewModel{DocumentTitle = "Z",FileName="Somefile"},

                                new JobDocumentViewModel{DocumentTitle = "X",FileName="Somefile"}

                            }

                        },

                        new Vacancy

                        {

                            Documents = new List<JobDocumentViewModel>

                            {

                                new JobDocumentViewModel{DocumentTitle = "B",FileName="Somefile"},

                                new JobDocumentViewModel{DocumentTitle = "Z",FileName="Somefile"},

                                new JobDocumentViewModel{DocumentTitle = "X",FileName="Somefile"}

                            }

                        }

                    };

                    vacancy.ForEach(x => x.Documents = x.Documents.OrderBy(y=> y.DocumentTitle).ToList());

                    var ordered = vacancy.OrderBy(x=> x.Documents.FirstOrDefault()?.DocumentTitle);

                    foreach(var item in ordered)

                    {

                        Console.WriteLine(item.Documents.First().DocumentTitle);

                    }

                }

            }

            public class Vacancy

            {

                private List<JobDocumentViewModel> _documents;

                public List<JobDocumentViewModel> Documents

                {

                    get

                    {

                        if (_documents == null)

                            _documents = new List<JobDocumentViewModel>();

​

                        return _documents;

                    }

                    set { _documents = value; }

                }

            }

​

            public class JobDocumentViewModel

            {

                public string DocumentTitle { get; set; }

                public string FileName { get; set; }

​

                public JobDocumentViewModel() { }

                public JobDocumentViewModel(

                    string documentTitle,

                    string fileName)

                {

                    DocumentTitle = documentTitle;

                    FileName = fileName;

                }

            }