Now, before Greg hurls a modopt on me, I want to be clear that this isn't the same thing that Spec# is doing. But it is a very cool way to specify constraints that must always be valid when a method exists.
Here is the code:
[AttributeUsage(AttributeTargets.Class)] class EnsureAttribute(AbstractAstAttribute): expr as Expression def constructor(expr as Expression): self.expr = expr def Apply(target as Node): type as ClassDefinition = target for member in type.Members: method = member as Method continue if method is null block = method.Body method.Body = [| block: try: $block ensure: assert $expr |].Block
And the usage:
[ensure(name is not null)] class Customer: name as string def constructor(name as string): self.name = name def SetName(newName as string): name = newName
Now, any attempt to set the name to null will cause an assertion exception. This technique is quite powerful, and very easy to use. A few years ago I wrote a design by contract implementation for boo that was far more ambitious (handling inheritance, etc). I remember it being much more complicated, and while things like quasi quotation do make it easier, it is not that big a change.
I think that mostly it is the way I write code now, striving to simplicity is something that I am trying to apply recently, and I think it works.