Code Contracts: invariants on super class of template method pattern not working?

Question

Assume I have the following code:

public abstract class TemplateBase {
    public void TemplateMethod() {
        Operation(); }

    protected abstract void Operation(); }

public sealed class Implementation : TemplateBase {
    bool _alwaysTrue;

    public Implementation(bool alwaysTrue) {
        _alwaysTrue = alwaysTrue; }

    [ContractInvariantMethod] void ObjectInvariant() {
        Contract.Invariant(_alwaysTrue == true); }


    protected override void Operation() {
        _alwaysTrue = false; } }

[TestClass] public sealed class InvariantTest {
    [TestMethod] public void Constructor() {
        new Implementation(false); }

    [TestMethod] public void Method() {
        new Implementation(true).TemplateMethod(); } }

InvariantTest.Constructor always fails with an 'Invariant failed' exception.

How can I get InvariantTest.Method to fail based on the invariant?

I've set the runtime checking to full and even enabled 'Call-site Requires Checking', but even that does not help.

When base methods are called directly, the runtime only checks the invariant in that class and base classes thereof. No virtual invariant invocation is done. As a result, the derived invariant is not checked. In order to perform that check, you need to override TemplateMethod and just call the base method there. That will instrument the check. In general, your pattern is tricky to make completely safe, since you need the derived class to control all entry points. — Manuel Fahndrich, Apr 25 '13 at 16:49

score 0 · Answer 1 · answered Oct 16 '12 at 01:20

If you use the latest version (1.4.50910.0), you will get a warning from the static checker that the invariant is false.

But for the runtime, this is intended behaviour. From the manual:

During runtime checking, invariants are checked at the end of each public method.

There is probably a good case for protected overrides to be considered 'public' for the purpose of object invariants. You might like to start a discussion on the Code Contracts forum.

Code Contracts: invariants on super class of template method pattern not working?

1 Answers1