using System;

using System.Data;

using System.Data.DataSetExtensions;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        DataTable dbSourceTable = new DataTable();

        dbSourceTable.Columns.Add("QuestionID", typeof (int));

        dbSourceTable.Columns.Add("QuestionNo", typeof (int));

        dbSourceTable.Columns.Add("Answer", typeof (string));

        DataTable uiAnswerTable = new DataTable();

        uiAnswerTable.Columns.Add("QuestionID", typeof (int));

        uiAnswerTable.Columns.Add("Answer", typeof (string));

        dbSourceTable.Rows.Add(new object[]{1, 1, "a"});

        dbSourceTable.Rows.Add(new object[]{2, 1, "b"});

        dbSourceTable.Rows.Add(new object[]{3, 1, "c"});

        dbSourceTable.Rows.Add(new object[]{4, 1, "d"});

        dbSourceTable.Rows.Add(new object[]{5, 1, "e"});

        uiAnswerTable.Rows.Add(new object[]{1, "a"});

        uiAnswerTable.Rows.Add(new object[]{1, "c"});

        uiAnswerTable.Rows.Add(new object[]{1, "e"});

        var LeftJoin =

            from dt1row in dbSourceTable.AsEnumerable()join dt2row in uiAnswerTable.AsEnumerable()on dt1row.Field<string>("Answer")equals dt2row.Field<string>("Answer")into table3

            from t3row in table3.DefaultIfEmpty()select new

            {

            QuestionID = dt1row.Field<string>("QuestionID"), QuestionNo = dt1row.Field<string>("QuestionNo"), Answer = dt1row.Field<string>("Answer"), ishecked = t3row != null ? "Yes" : null

            }

​

        ;

        var newList = LeftJoin.ToList();

        foreach (var items in newList)

        {

            Console.WriteLine(items);

        }

    }

}