string path = System.Environment.GetFolderPath (Environment.SpecialFolder.InternetCache);
foreach (string f in System.IO.Directory.GetFiles (path))
{
System.IO.FileInfo fi = new System.IO.FileInfo(f);
//fi.Length; //文件大小等其他信息
int i = fn.IndexOf (str);
int sInt = 0; //这是你要的
if (i>0)
{
string strtmp = fn.Substring (str.Length);
sInt = System.Convert.ToInt32 (strtmp); //注意处理异常
}