Class Contracts

Class Contracts cover two constructs of the Oxygene language that enable Design by Contract-like syntax to create classes that can test themselves:

  • Pre-conditions and Post-conditions
  • Invariants

If a contract is not upheld, an assertion is generated in the same fashion as calling the assert() system function would.

Oxygene Language Only

This topic applies to the Oxygene language, only. Class Contracts are not available in C# or Swift.

Pre-Conditions and Post-Conditions

Pre- and post-conditions are used to describe conditions that are required to be true when a method is called or after a method exits, respectively. They can be used to check for the validity of input parameters, results, or for the state of the object required by the method.

The require and ensure keywords will expand the method body to list the preconditions; both sections can contain a list of Boolean statements, separated by semicolons.


method MyObject.DivideBy(aValue: Integer);
  aValue <> 0;
  MyValue := MyValue/aValue;

method MyObject.Add(aItem: ListItem);
  Count = old Count +1;

method MyObject.DoWork(aValue);
  fMyValue > 0;
  aValue > 0;
  //... do the work here
  fMyResult.Value >= 5;

The old scope prefix can be used in the ensure section for local variables, parameters and properties to refer to the original values before the execution.

The compiler will add code to save these to local variables before executing the method body. old is supported for strings and value types. When used with reference types, it will capture the old pointer, not the old state of the object being pointed to.


In contrast to pre- and post-conditions, invariants are used to define a fixed state the object must fulfill at any given time. Invariants can be marked public or private.

Public invariants will be checked at the end of every public method (after the method's "ensure" block, if present, has been checked) and if an invariant fails, an assertion is raised.

Private invariants will be checked at the end of every method call, public or private.

The idea behind this separation is that public invariants must not be met by private methods, so theoretically a public method can defer work to several private worker methods, and public invariants would only be checked after the public method finishes.


  MyClass = class;
    ... some methods or properties
  public invariants
    fField1 > 35;
    SomeProperty = 0;
    SomeBoolMethod() and not (fField2 = 5);
  private invariants
    fField > 0;

Note that both types of invariant sections have full access to all private fields of the class, the only difference is the method (and property) calls they apply to.

If a class specifies invariants, all fields must be marked as private.

Custom Messages

By default, a generic assertion method is generated. For example, suppose that Method1 failed the following requirement:

  require A > 10;

This would generate the following message: Method1 assertion failed A \> 10.

Custom messages can be generated instead by adding : 'your message' to the require, ensure or invariant statement, e.g.:

method MyObject.Add(aItem: ListItem);
  assigned(aItem) : 'List Item for MyObject cannot be nil';
  Count = old Count +1 : 'MyObject: Count logic error';


Note that not all members of a class can be used inside invariants because some elements of a class (or the entire class system of your application) can be accessed and written to directly, without the knowledge of the class.

If, for example, your invariant were to depend on a public field, other parts of your system would be able to modify this field directly, bypassing the invariant checking. Of course non-private fields are discouraged in general and you should always use private fields and – where applicable – a property with a higher visibility to make the field's value accessible.

Members that can be used in invariants are:

  • private fields
  • properties
  • methods