Using Type Guards

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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.