Question

I have a code which interacts with some object and then should call finish() method on it.

void completeTransaction(PaymentTransaction transaction) {  
    recordTransaction(transaction.getId());

    transaction.finish();
}

PaymentTransaction is some third-party class which behaviour after finish() is undefined — it may throw an exception or just fail silently.

I need to write a unit test which passes then and only then:

1. recordTransaction(transaction.getId()) called
2. transaction.finish() called
3. transaction.finish() called after recordTransaction(transaction.getId())

Test satisfying the above conditions should prohibit code like this:

void completeTransaction(PaymentTransaction transaction) {
   transaction.finish();
   recordTransaction(transaction.getId()); //oops
}

Test case for the first condition:

void testCompleteTransaction_TransactionRecorded() {
    completeTransaction(transactionMock);
    // assert that recordTransaction(transaction.getId()) 
    // called with correct argument

completeTransaction(PaymentTransaction transaction)
}
For the second one:

void testCompleteTransaction_TransactionCompleted() {
    completeTransaction(transactionMock);
    // assert that transaction.finish() called
}

I wonder how can I enforce the 3rd condition via test case.

Answer 1

You could pass in a fake PaymentTransaction that overrides finish() and getId() such that finish() throws an exception if some internal flag isn’t set when getId() is called.

public class FakePaymentTransaction {
    private bool _getIdWasCalled = false;

    public override void finish () {
        if (!_getIdWasCalled) {
            throw new Exception ("getId wasn't called first!");
        }
    }

    public override /* your return type */ getId() {
        _getIdWasCalled = true;
        // Some other logic to return your specified return type
    }
}

Now when you pass it into your SUT, you will see if the calls were made in the right order.