string htmlFile = "E:\\xyz\\toy\\aa.html";

               FileInfo fi = new FileInfo(htmlFile);
               string fileName=fi.Name;//aa.html
               string directName = fi.DirectoryName;//E:\\xyz\\toy

------解决方案--------------------
C# code

int i=(aa.html).Length //aa.html 长度
string htmlFile = "E:\\xyz\\toy\\aa.html";
string a = htmlFile.Substring(htmlFile.Length-i,i); 
string a = htmlFile.Substring(0,htmlFile.Length-i);