xD_Antlion

static void ChangeNameRecursive(DirectoryInfo dir)
{
  int i = 0;
  foreach(var item in dir.GetFiles())
  {  
    File.Move (item.FullName, $"{name} test_ {i.ToString()} {".pp"}");
    i++;
  }
  foreach(var d in dir.GetDirectories())
  {  
    if(d.Name == "." || d.Name == "..") 
    {
       continue;
    }
    ChangeNameRecursive(d);
  }
}

private readonly string name = @"C:\Путь";
// const PATH = @"C:\Путь";
ChangeNameRecursive(new DirectoryInfo (name));