为什么我不能在IO.File.ReadAllText（）字符串上执行String.Replace（）？

I am using System.IO.FIle.ReadAllText() to get the contents of some template files that I created for email content. Then I want to do a Replace on certain tokens within the files so I can add dynamic content to the template.I am using System.IO.FIle.ReadAllText() to get