string path = "C:\\dir1\\dir2\\foo.txt";  

string str = "GetFullPath：" + Path.GetFullPath(path) + "\r\n";  
str += "GetDirectoryName：" + Path.GetDirectoryName(path) + "\r\n"; 
str += "GetFileName：" + Path.GetFileName(path) + "\r\n";  
str += "GetFileNameWithoutExtension：" + Path.GetFileNameWithoutExtension(path) + "\r\n";  
str += "GetExtension：" + Path.GetExtension(path) + "\r\n"; 
str += "GetPathRoot：" + Path.GetPathRoot(path) + "\r\n"; 

MessageBox.Show(str); 
结果：  
GetFullPath：C:\dir1\dir2\foo.txt 
GetDirectoryName：C:\dir1\dir2 
GetFileName：foo.txt  
GetFileNameWithoutExtension：foo
GetExtension：.txt 
GetPathRoot：C:\