using System;

using System.Collections.Generic;

using System.Data;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        DataTable dt_MappedColumns = new DataTable();

        dt_MappedColumns.Columns.Add("Sr.No");

        dt_MappedColumns.Columns.Add("TColumnName");

        dt_MappedColumns.Columns.Add("AColumnName");

        dt_MappedColumns.Rows.Add("1", "Apple", "Lion");

        dt_MappedColumns.Rows.Add("2", "Orange", "Tiger");

        dt_MappedColumns.Rows.Add("3", "Mango", "Fox");

        dt_MappedColumns.Rows.Add("4", "Orange", "Wolf");

​

        var dictionary1 = new Dictionary<string, object>();

        foreach (DataRow dr in dt_MappedColumns.Rows)

        {

            string key = dr["TColumnName"].ToString();

            string value = dr["AColumnName"].ToString();

            if (!dictionary1.ContainsKey(key))

            {

                // key does not already exist, so add it

                dictionary1.Add(key, value);

            }

            else

            {

                // key exists, get the existing value

                object existingValue = dictionary1[key];

                if (existingValue is string)

                {

                    // replace the existing string value with a list

                    dictionary1[key] = new List<string> { (string)existingValue, value }; 

                }

                else

                {

                    // the existing value is a list, so just add the new value to it

                    ((List<string>)existingValue).Add(value);

                }

            }

        }

​

        string Manjson = JsonConvert.SerializeObject(dictionary1);

        Console.WriteLine(Manjson);

    }

}