Add contract to interface implementation

Question

I understand that I cannot add preconditions on an interface implementation. I have to create a contract class where I define contracts on elements that are seen by the interface.

But in the following case, how can add a contract on an internal state of the implementation that is therefore not known at the interface definition level ?

[ContractClass(typeof(IFooContract))]
interface IFoo
{
  void Do(IBar bar);
}

[ContractClassFor(typeof(IFoo))]
sealed class IFooContract : IFoo
{
  void IFoo.Do(IBar bar)
  {
    Contract.Require (bar != null);

    // ERROR: unknown property
    //Contract.Require (MyState != null);
  }
}

class Foo : IFoo
{
  // The internal state that must not be null when Do(bar) is called.
  public object MyState { get; set; }

  void IFoo.Do(IBar bar)
  {
    // ERROR: cannot add precondition
    //Contract.Require (MyState != null);

    <...>
  }
}

Answer 1

You can't - that postcondition isn't appropriate for all implementations of IFoo, because it's not declared in IFoo. You can only refer to members of the interface (or other interfaces it extends).

You should be able to add it in Foo though, because you're adding a postcondition (Ensures) rather than a precondition (Requires).

You can't add an implementation-specific precondition because then a caller wouldn't be able to know whether or not they were going to violate the contract:

public void DoSomething(IFoo foo)
{
    // Is this valid or not? I have no way of telling.
    foo.Do(bar);
}

Basically, contracts aren't allowed to be "unfair" to callers - if the caller violates a precondition, that should always indicate a bug rather than something they couldn't have predicted.