.net如何向csv添加一列

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml;
using ConsoleApplication1;
using log4net;
namespace test
{

    static class Program
    {
        static void Main()
        {
         List<string> arr=   File.ReadAllLines(@"D:ektest.csv",Encoding.GetEncoding("Shift_JIS")).ToList();
         List<string> newColumnData = new List<string>() { "345","123" };//添加列值  条数和arr的一样 不够位补null
         arr[0] += ","リファラ"";// 添加的列名
         int index = 1;
         //add new column value for each row.
         arr.Skip(1).ToList().ForEach(line =>
         {
             //-1 for header
             arr[index] += "," + newColumnData[index - 1];
             index++;
         });
         //write the new content
         File.WriteAllLines("D:\test.csv", arr);
        }

    }
}