using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Linq.Expressions;

using System.Data;

using System.Data.DataSetExtensions;

public class Program

{

    public static void Main()

    {

        var testq = new List<TestQuestion>()

            {

                new TestQuestion

                {

                    Id = 1,

                    QuestionId = 1,

                    SelectedAnswerId = null

                },

                new TestQuestion

                {

                    Id = 2,

                    QuestionId = 2,

                    SelectedAnswerId = 1

                },

                new TestQuestion

                {

                    Id =3,

                    QuestionId = 1,

                    SelectedAnswerId = 1

                },

                new TestQuestion

                {

                    Id = 4,

                    QuestionId = 3,

                    SelectedAnswerId = 5

                },

                new TestQuestion

                {

                    Id = 5,

                    QuestionId = 1,

                    SelectedAnswerId = 2

                },

                new TestQuestion

                {

                    Id = 6,

                    QuestionId = 3,

                    SelectedAnswerId = 3

                },

                new TestQuestion

                {

                    Id =7,

                    QuestionId = 4,

                    SelectedAnswerId = null

                },

                new TestQuestion

                {

                    Id =8,

                    QuestionId = 5,

                    SelectedAnswerId = null

                },

            };

​

            var result = testq

                //.Where(p => p.SelectedAnswerId.HasValue)

                .GroupBy(p => p.QuestionId)

                .Select(p => 

                {

                    var grouped = p.ToList();

                    TestQuestion testQuestion = grouped.FirstOrDefault(x => x.SelectedAnswerId.HasValue);

                    return testQuestion ?? grouped.FirstOrDefault();

                })

                .ToList();

​

            foreach(var item in result)

                Console.WriteLine(item.Id);

    }

}

​

    public class TestQuestion

    {

        public int Id { get; set; }

        public int QuestionId { get; set; }

        public int? SelectedAnswerId { get; set; }

    }