Contracts¶

• Programming to Contracts was elaborated by Bertrand Meyer
• "Object-Oriented Software Construction" by Meyer is recommended
• [[Eifel language]]

What is Contract?¶

• Contracts have some [[semantic payload]]
• [[Interface]]s has no [[semantic payload]]s, they are not contracts

Example:

public abstract class CollectionContract<T> : IList<T>
{
    public void Add(T item)
    {
        AddCore(item);
        count++;
    }

    public int Count
    {
        get { return count; }
    }

    protected abstract void AddCore(T item);
    private int count;

    ...
}

What constitutes a method's contract?¶

• Acceptable and unacceptable input values or types, and their meanings
• [[Return value]]s or [[Return type]]s, and their meanings
• [[Error]] and [[exception]] condition values or types that can occur, and their meanings
• [[Side effect]]s
• [[Precondition]]s
• [[Postcondition]]s
• [[Invariant]]s

Example of Liskov Substitution Principle violation: - Different [[return type]]s - Different [[exception]]s - [[unused argument]]s

public interface IBankTerminal
{
    int ProcessPayment(decimal amount, string uniqueId);
}

public class BenkTerminal1 : IBankTerminal
{
    private IBankTerminal1IPaymentGateway _gateway;

    // <returns> Response Code. Always >= 0</returns>
    public int ProcessPayment(decimal amount, string uniqueId)
    {
        // doesn't require uniqueId at all
        return (int)_gateway.ProcessPayment(amount);
    }

    public class BankTerminal2 : IBankTerminal
    {
        private IBankTerminal2IPaymentGateway _gateway;
        public int ProcessPayment(decimal amount, string uniqueId)
        {
            if(string.IsNullOrWhiteSpace(uniqueId))
            {
                throw new ArgumentException("A client must provide a unique ID or BankTerminal2");
            }

            return _gateway.ProcessPayment(amount, uniqueId);
        }
    }
}

Code Contracts in [[Csharp]]¶

• You can write contracts in [[Csharp]] with a library called "[[Code Contracts]]". Harness the power of [[static code verification]] on correctness.
• [[Code Contracts]] library is not very popular though.