using System;

using System.Data;

using System.Linq;

using System.Xml;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        var table = new DataTable();

        table.Columns.Add("Block");

        table.Columns.Add("HouseNo");

        table.Columns.Add("ResidentId", typeof(int));

        table.Rows.Add("A", "101", 1);

        table.Rows.Add("A", "102", 2);

        table.Rows.Add("A", "103", 3);

        table.Rows.Add("B", "201", 4);

        table.Rows.Add("B", "202", 5);

        table.Rows.Add("B", "204", 6);

        // create a new JObject by grouping the data table rows by block and then

        // selecting the groups into JProperties where the block is the property name

        // and the property value is a JArray containing the house numbers in the group

        var obj = new JObject(

            table.Rows.Cast<DataRow>()

                      .GroupBy(r => r["Block"])

                      .Select(g => new JProperty("Block " + g.Key, 

                           new JArray(g.Select(r => 

                               new JObject(

                                   new JProperty(r["ResidentId"].ToString(), r["HouseNo"])

                               )

                           ))

                      ))

        );

        // convert the JObject to a JSON string

        var json = obj.ToString();

        Console.WriteLine(json);

​

    }

}