สอบถามครับ ผมจะใช้ OR/AND ได้ยังไง ครับ

System.IO.Directory.GetFiles(folSong, "*.mp3").ToList<string>()
                         .Where(f => new Regex(@"(_\d{1,})", RegexOptions.None).IsMatch(Path.GetFileName(f)))
                         .ToList<string>()
                         .ForEach(_f =>
                         {
                             string __f = folSong + "\\" + new Regex(@"(_\d{1,})", RegexOptions.None).Replace(Path.GetFileName(_f),"");
                             this.Invoke(new Action(() => this.Text = "Check File:" + _f));
                              if (!System.IO.File.Exists(__f))
                              {
                                  System.IO.File.Move(_f, __f);
                              }
                              else
                              {
                                 //check file size ....
                                 long a = new FileInfo(_f).Length;
                                 long b = new FileInfo(__f).Length;
                                 if (a >= b-0.3*b && a <=b+0.3*b)
                                  System.IO.File.Delete(_f);
                              }
                         });