It iterates through a whole site collection and wherever the list with name “test” it finds, It will do the changes.

   1:  using System;
   2:  using System.Collections.Generic;
   3:  using System.Text;
   4:  using Microsoft.SharePoint;
   5:   
   6:  namespace ConsoleApplication1
   7:  {
   8:      class Program
   9:      {
  10:          static void Main(string[] args)
  11:          {
  12:              using (SPSite site = new SPSite("http://ms9:101"))
  13:              {
  14:                  foreach (SPWeb web in site.AllWebs)
  15:                  {
  16:                      SPList list = web.Lists["test"];
  17:                      SPFieldCollection col = list.Fields;
  18:                      SPField field = col.GetField("Trigger");
  19:                      field.Type = SPFieldType.Choice;
  20:                      field.Update();
  21:                      col.Add("Cost", SPFieldType.Currency, false);
  22:                      
  23:                      col.Add("Due Date", SPFieldType.DateTime, false);
  24:                      list.Update();
  25:                      web.Dispose();
  26:                  }
  27:              }
  28:          }
  29:      }
  30:  }

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