using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

public class Program

{

    public static void Main()

    {

        DataTable table1 = new DataTable();

        table1.Columns.Add("id", typeof(int));

        table1.Columns.Add("isfav", typeof(int));

        table1.Rows.Add(new object[] {1,0});

        table1.Rows.Add(new object[] {2,0});

        table1.Rows.Add(new object[] {3,0});

        table1.Rows.Add(new object[] {4,0});

        table1.Rows.Add(new object[] {5,0});

        DataTable table2 = new DataTable();

        table2.Columns.Add("id", typeof(int));

        table2.Rows.Add(new object[] {1}); 

        table2.Rows.Add(new object[] {2});

        table1.AsEnumerable()

              .Join( table2.AsEnumerable(),

                    t1 => t1.Field<int>("id"),

                    t2 => t2.Field<int>("id"),

                    (t1, t2) => new { t1 })

              .ToList()

              .ForEach(o => o.t1["isfav"] = 1);

        foreach(DataRow row in table1.Rows)

        {

            Console.WriteLine(string.Join(",", row.ItemArray));

        }

    }

}