System.Environment.CurrentDirectory
Application.StartupPath
https://msdn.microsoft.com/en-us/library/system.windows.forms.application.startuppath(v=vs.110).aspx
System.Environment.CurrentDirectory
https://msdn.microsoft.com/en-us/library/system.environment.currentdirectory(v=vs.110).aspx
百度之后，找到区别
System.Environment.CurrentDirectory的含义是获取或设置当前工作路径，
而Application.StartupPath是获取程序启动路径，
表面上看二者没什么区别，但实际上区别大得很。
先说前者：比如说你程序放在桌面上启动，但是中间你用了一个OpenFileDialog打开了E盘名为abc的文件夹下的某一个文件，那么CurrentDirectory就变成E：abc了，
所以如果你想再获取程序启动文件夹的某一个文件就没用了，
但是Application.StartupPath就不会这样了，无论你中间打开了哪个盘的文件，启动路径都是在桌面那里，一直不会变。
If you want to get the path to the directory under which your executable runs, you should not rely on the Environment.CurrentDirectory,
since it can be changed in a number of ways (shotrtcut settings, etc). Try one of these options instead:
http://stackoverflow.com/questions/1321628/environment-currentdirectory-in-c-net
You shouldn't be using the Environment.CurrentDirectory value as a base for file lookups because it can change and may not always be under your control. e.g. a File Save As to a different folder may change the 'current folder' value. As you can see it can yield unpredictable results.
Use a value that you can control better. e.g. a ResourcesFolderPath value in a configuration (xml?) file that is updated when you install your app.
Desktop 和 DesktopDirectory
https://msdn.microsoft.com/en-us/library/system.environment.specialfolder(v=vs.110).aspx
Desktop ：The logical Desktop rather than the physical file system location.
DesktopDirectory：he directory used to physically store file objects on the desktop.
What's the difference between SpecialFolder.Desktop and SpecialFolder.DesktopDirectory?
A directory is a location in the file system.
A folder is a location in the shell namespace.
A directory is a kind of folder.
A virtual folder is not necessarily backed by a directory.
For example consider libraries or search folders.
The user's desktop directory is a location in the file system.
The desktop folder merges that with virtual items like all users items, recycle bin, shortcut to documents folder etc.