A very basic example of how the app_code folder can be used to create and home re-usable functions that razorviews can then use. These can use either be .cs or .cshtml files.

app_code - re-usable functions for razorviews (1)

@*
 *  FILE  : /App_Code/StringFunctions.cshtml
 *@

@functions
{
  @*
   *  FUNCTION : ENCODE STRING
   *  ************************
   *  Description : takes an input string and returns it lower-cases and url-encoded
   *  
   *  Example use : building custom urls and/or query-strings
   *@

  public static string Encode(string value)
  {
    return HttpUtility.UrlEncode(value.ToLower());
  }
}

app_code - re-usable functions for razorviews (2)

@*
 *  FILE  : /SiteElements/Razorviews/MyRazorview.cshtml
 *@

@{
  @*
   *  NOTES
   *  *****
   *  To call functions we refer to the file name and function name:
   *  
   *  Example : FileName.FunctionName();
   *@

  string string1 = "This is a string of some sort";
  string string2 = StringFunctions.Encode(string1);
  
  <p>@string2</p>   // would print :  this+is+a+string+of+some+sort
}