using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var group1 = Guid.NewGuid();

        var group2 = Guid.NewGuid();

        var questions = new[] {

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice,

                GroupId = group1,

                Order = -1,

            },

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice,

                GroupId = group2,

                Order = 1,

            },

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice,

                GroupId = group1,

            },

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice,

                GroupId = group2,

                Order = 2,

            },

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice

            },

            new SurveyQuestion {

                Id = Guid.NewGuid(),

                Type = SurveyQuestion.QuestionType.MultipleChoice,

                Order = 1000

            },

        };

        var groups = questions.GroupBy (q => q.GroupId);

        foreach (var @group in groups)

        {

            Console.WriteLine ("Group: {0}", @group.Key .HasValue ? @group.Key.ToString() : "<null>");

            Console.WriteLine (new String('*', 50));

            foreach(var question in @group.OrderBy (g => g.Order)) 

            {

                Console.WriteLine ("Id: {0}, Order: {1}", question.Id, question.Order);    

            }

            Console.WriteLine ();

        }

    }

    public class SurveyQuestion

    {

        public Guid Id { get; set; }

        public Guid? GroupId { get; set; }

        public QuestionType Type { get; set; }

        public int Order { get; set; } 

        // other props

        public enum QuestionType {

            MultipleChoice,

            TrueFalse,

            Written,

        }

    }

    // Define other methods and classes here

​

}