您可以使用linq輕松完成所有這些步驟: //1. if the file does not exist in the folder, `the file not exist` is printed if (!files.Any(f => f.Name == fileName)) { Console.WriteLine("the file not exist"); } else { //2 ,3 : sort the list and find file index var indexOfFile = files.OrderBy(item => item.CreationTime) .Select((item, i) => new { Item = item.Name, Index = i }) .First(x => x.Item == fileName).Index; Console.WriteLine("index of this file is: {0}", indexOfFile); }