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.)