Question

I’m trying to demonstrate invariants in Code Contracts, and I thought I’d give an example of a sorted list of strings. It maintains an array internally, with spare space for additions etc – just like List<T>, basically. When it needs to add an item, it inserts it into the array, etc. I figured I had three invariants:

• The count must be sensible: non-negative and at most as big as the buffer size
• Everything in the unused part of the buffer should be null
• Each item in the used part of the buffer should be at least as “big” as the item before it

Now, I’ve tried to implement that in this way:

[ContractInvariantMethod]
private void ObjectInvariant()
{
    Contract.Invariant(count >= 0 && count <= buffer.Length);
    for (int i = count; i < buffer.Length; i++)
    {
        Contract.Invariant(buffer[i] == null);
    }
    for (int i = 1; i < count; i++)
    {
        Contract.Invariant(string.Compare(buffer[i], buffer[i - 1]) >= 0);
    }
}

Unfortunately, ccrewrite is messing up the loops.

The user documentation says that the method should just be a series of calls to Contract.Invariant. Do I really have to rewrite the code as something like this?

Contract.Invariant(count >= 0 && count <= buffer.Length);
Contract.Invariant(Contract.ForAll
    (count, buffer.Length, i => buffer[i] == null));
Contract.Invariant(Contract.ForAll
    (1, count, i => string.Compare(buffer[i], buffer[i - 1]) >= 0));

That’s somewhat ugly, although it does work. (It’s much better than my previous attempt, mind you.)

Are my expectations unreasonable? Are my invariants unreasonable?

(Also asked as a question in the Code Contracts forum. I’ll add any relevant answers here myself.)

Answer 1

From the (preliminary) MSDN pages it looks like the Contract.ForAll member could help you with the 2 range contracts. The documentation isn’t very explicit about its function though.

//untested
Contract.Invariant(Contract.ForAll(count, buffer.Length, i => buffer[i] == null));
Contract.Invariant(Contract.ForAll(1, count, 
    i => string.Compare(buffer[i], buffer[i - 1]) >= 0));