static void Main(string[] args)
{
    string filepath = @"c:\xx\aaabb.jpg";
    //只要文件名 aaabb
    string result = System.IO.Path.GetFileNameWithoutExtension(filepath);
    Console.WriteLine(result);
    //如果还要前面路径 c:\xx\aaabb
    result = filepath.Remove(filepath.Length - System.IO.Path.GetExtension(filepath).Length);
    Console.WriteLine(result);
    Console.ReadKey();
}