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ABL Reference
ABL Syntax Reference : CAST function

CAST function

Returns a new object reference to the same class instance as an existing object reference, but with a different data type. This different data type is cast from the object type of the original object reference according to another specified object type. The two object types must be related, where one is a class type and the other is a subclass of that class type or where one is an interface type and the other is a class that implements the interface of that type.
When you cast an object reference, ABL treats it as if it referenced an instance of the object type to which it is cast. The underlying class hierarchy of the object instance does not change.


CAST( object-reference, object-type-name ).
An object reference defined with the object type to be cast.
Specifies the type name of an ABL or .NET class or interface type to which the object reference is cast. This object type must be a class type in a class hierarchy that includes or implements the object-reference data type or it must be an interface type that the object-reference data type implements. Specify the object type name using the syntax described in the Type-name syntax reference entry. With an appropriate USING statement, you can also specify an unqualified class or interface name alone.


*You typically cast an object reference down a class hierarchy—that is, from a super class to a derived class within a class hierarchy, or from an interface to a class that implements that interface. However, you do not always need to explicitly cast an object reference. Because a derived class contains all the super classes in its inherited class hierarchy, ABL implicitly casts any object reference up within its class hierarchy, and because a class that implements an interface implements all of the methods specified for the interface, ABL implicitly casts any object reference from an implementing class to any interface that the class implements.
*At compile time, ABL verifies that the specified object type is within the class hierarchy of the specified object reference. At run time, the AVM checks the validity of the cast operation. Therefore, if you access a class member on the cast object reference that exists for the cast data type, but the referenced object at run time does not actually define the accessed class member, the AVM raises ERROR at run time.
*A .NET generic type can be part of a cast. For example, you can cast from a System.Object to a "System.Collections.Generic.List<SHORT>", because all .NET classes, including generic classes, derive from the .NET root class. However, note that you cannot cast from a "System.Collections.Generic.List<System.Object>" to a "System.Collections.Generic.List<System.Windows.Forms.Button>". You cannot assign a "List<Button>" reference to an object reference defined as a "List<Object>", because, even though the type parameters are compatible, the two objects as a whole are not equivalent and have no inheritance relationship. Therefore, a cast between these two objects cannot work either. For more information on .NET generic types, see the Data types reference entry.
*You can also use the DYNAMIC-CAST function to cast object references to object types determined at run time. This is especially useful in object-oriented applications that conform to the OpenEdge Reference Architecture (OERA). For more information on the OERA, see Progress Communities:
*You can use the CAST function to cast a parameter in a parameter list for a method using the following syntax:
method-name( INPUT CAST( object-reference, subclass-name ), ... ).
*You can use the CAST function to cast a temp-table field, which is defined as a Progress.Lang.Object, to use as an object of another class type. For example:
DEFINE VARIABLE rCustObj AS CLASS acme.myObjs.CustObj.

DEFINE TEMP-TABLE mytt FIELD CustObj AS Progress.Lang.Object.

rCustObj = CAST(mytt.CustObj, acme.myObjs.CustObj).
You can now use the object reference in RCustObj to invoke methods in the acme.myObjs.CustObj class.
*You can use the CAST function to cast an object reference to a subclass and invoke a method defined in that subclass using the following syntax:
CAST( object-reference, object-type-name ):method-name( parameters ).
You can also use this syntax to invoke a method on a class that implements the referenced interface from which you cast the specified class.

See also

DYNAMIC-CAST function, Type-name syntax, USING statement