using System;

using System.Data;

using System.Linq;

using System.Xml;

using Newtonsoft.Json.Linq;

using Formatting = Newtonsoft.Json.Formatting;

public class Program

{

    public static void Main()

    {

        DataTable dt = new DataTable();

        dt.Columns.Add("id");

        dt.Columns.Add("value");

        dt.Rows.Add("1", "hi");

        string json = new JObject(

            dt.Columns.Cast<DataColumn>()

              .Select(c => new JProperty(c.ColumnName, JToken.FromObject(dt.Rows[0][c])))

        ).ToString(Formatting.None);

        Console.WriteLine(json);

    }

}

​