>>I'm looking at the code contracts library included in .NET 4
>>Anyone using this regularly and/or have opinions ?
>>I'm currently working with VS Pro so, whilst I can see some benefit in the runtime checks, it seems to me that I'm missing the most useful facet (i.e. static checking).....
>
>Where is that exactly? I never heard of that before.

Basically (but oversimplified) a replacement for asserts or other parameter checking. For example you might currently have code such as (C# I'm afraid but you will get the jist):

public int ProcessInvoice(EdifactInvoice invoice)
        {
            if (invoice == null)
                throw new NullReferenceException("Invoice cannot be null");

            //Elided
            return 5;
        }

Using code contracts you can refactor this to:

public int ProcessInvoice(EdifactInvoice invoice)
        {
            Contract.Requires<NullReferenceException>(invoice != null);
            Contract.Ensures(Contract.Result<int>() > 0);
            //Elided
            return 5;
        }

The 'Contract.Requires' serves the same purpose as the original null check at runtime - but with versions higher than VS Pro will *also* catch any existing call to the method which might fail to fullfill the 'contract at compile time. The Contract.Ensures line will, in this non-realistic example, also throw an exception if the return value does not meet the condition. A neat thing with the latter is that it will do this even if there are multiple return paths.....

Here's a link to an overview: http://research.microsoft.com/en-us/projects/contracts/