Using Type Guards

A type guard can be required in two cases, as follows.

• When assigning to an object reference where the declared class of the expression on the right of the assignment is a superclass of the declared class on the left-hand side; for example:

  countAllClasses(): Integer updating;
  constants
      Max_Classes = 16000;
  vars
      objArray : ObjectArray;
      cls      : Class;
      obj      : Object;
      total    : Integer;
  begin
      create objArray transient;
      Class.allInstances(objArray, Max_Classes, true);
      foreach obj in objArray do
          cls := obj.Class;          // Object is a superclass of Class
          if not cls.isSystemObject and cls.superschemaType = null then
              total := total + 1;
          endif;
      endforeach;
      return total;
  end;

• When accessing a method or a property of an object where the declared class of the reference to the object is a superclass of the class in which the feature was defined; for example:

  foreach form in formDictionary do
      if form.isKindOf(SpecialForm) then
          form.SpecialForm.cutEnabled := true;
      endif;
  endforeach;

  In the above example, a reference is obtained to a form from a generic form dictionary, where the membership type is Form. To access the cutEnabled property that is defined in SpecialForm (a subclass of Form), a type guard is inserted that asserts that form is indeed a SpecialForm.

  The use of the isKindOf method guarantees that the form reference in the guarded expression is of type SpecialForm, so a runtime exception will not be raised.