Contract based programming Using pre- and post-conditions, and object invariants Contract based programming1

Design by contract Idea A program is correct if given correct input the program produces correct output. Correct input → PROGRAM → correct output A program is considered a kind of “black box” Same idea applies to parts of a program Methods, functions, etc. Correct input → METHOD → correct output Precondition Specification of correct input Postcondition Specification of correct output Contract based programming2

Contracts A supplier (programmer) writes a class or method to be used by a client (another programmer) The contract specifies The public interface of the class / method Pre-conditions Post-conditions Object invariants Contract based programming3

Pre- and post-conditions Pre-condition What does the method expect? Must be true before it makes sense to call the method Methods should check this and throw appropriate exceptions C# examples Convert.ToInt32(String str) Assumes str is a string that contains number. If not it throws FormatException, or OverflowException Post-condition What does the method guarantee? After the method has executed Returned value, and/or change of object state Contract based programming4

Object invariant An invariant is a statement that is invariable true Object invariant Statement about the objects state between method invocations Example: Class Student Name != null, age >= 0 Contract based programming5

C# language support In C# there is no direct language support for programming by contract Pre- and post-conditions, can be specified as comments to the methods Invariants can be specified as comments to the class. Pre-conditions and invariants must be checked in the beginning of all modifying methods, like set methods, etc. The exception throw is often ArgumentException ArgumentNullException, a sub-class of ArgumentException ArgumentOutOfRangeException, a sub-class of ArgumentException Contract based programming6

ArgumentException Thrown if there is something wrong (according to the pre-condition) with the argument (parameter) to the method. Some properties Message The error message: Should be readable to humans ParamName Name of the parameter that caused the exception Some constructors InnerException Different from null if the exception is chained Some constructors ArgumentException() No parameter: The catcher does not know what is wrong. Don’t use … ArgumentException(String message) The catcher can use the message to write to the user, etc. ArgumentException(String message, String paramName) ArgumentException(String message, Exception innerException) Useful for exception chaining Contract based programming7

ArgumentNullException Thrown when null reference is thrown is passed to a method that does not accept it as a valid argument us/library/System.ArgumentNullException(v=vs.110).aspx us/library/System.ArgumentNullException(v=vs.110).aspx Some properties Like base class: ArgumentException Constructors ArgumentNullException() Default message, no parameter name set ArgumentNullException(String paramName) Default message with parameter name. Use this for most cases! ArgumentNullException(String message, String paramName) ArgumentNullException(String message, Exception innerException) Contract based programming8

ArgumentOutOfRangeException Thrown if the actual value of an argument (aka parameter) is outside the legal range Example: Teacher salary, legal range >= 0 Some properties Like base class ArgumentException ActualValue: The actual value of the parameter Some constructors ArgumentOutOfRangeException() Do not use! No information about the problem. ArgumentOutOfRangeException(String paramName) ArgumentOutOfRangeException(String paramName, String message) ArgumentOutOfRangeException(String paramName, Object actualValue, String message) ArgumentOutOfRangeException(String message, Exception innerException) Useful for exception chaining Contract based programming9

Some aliases Contract based programming aka. Bertrand Meier: The Eiffel programming language, 1986 Design by contract aka. Registered trademark (US) Programming by contract aka. Design-by-contract programming aka. Code contracts [Microsoft terms] Contract based programming10

References and further readings Wikipedia Design by contract Microsoft Research Code Contracts 1ce455f ce455f66970 Contract based programming11