IndexOf(Object, Int32) 方法搜索指定的对象，并返回从指定索引扩展到最后一个元素的ArrayList中的元素范围内的第一次出现的索引。

namespace ConsoleApplication3
{

  class Program
  {
      static void Main(string[] args)
      {

          ArrayList arr = new ArrayList();
          arr.Add("Red");
          arr.Add("Pink");
          arr.Add("Orange");

          arr.Add("Yellow");
          arr.Add("Blue");
          arr.Add("Gray");
          arr.Add("White");

          Console.WriteLine("The elements in the arraylist are:");
          foreach (object obj in arr)
          {
              Console.WriteLine(obj);

          }
          Console.WriteLine("The index value of Red is:" + arr.IndexOf("Red",0));
          Console.WriteLine("The index value of Orange is:" + arr.IndexOf("Orange",0));
          Console.WriteLine("The index value of White is:" + arr.IndexOf("White",2));

          Console.WriteLine("The index value of Blue is:" + arr.IndexOf("Blue",2));
      }
  }
}