.NET Code Contracts: Invariants Explained

I have recently started using .NET Code Contracts. I love the streamlined approach to implementing pre-conditions and post-conditions that previously required multi-line if statements along with the ability to check coding assumptions statically and even at runtime (release or debug builds). .NET Code Contracts are another valuable tool in a software engineers arsenal to combat the growing complexity of software systems.

The latest MSDN magazine (June, 2011) has an thorough article on invariants which are an element of .NET Code contracts. The article will go into much more detail but essentially invariants allow for the verification of an object's internal state before and after a set of operations. To create an object invariant, a class simply needs to contain a new private method with the ContractInvariantMethod attribute and then specify any conditions within the private method that should hold true before and after any operation on the class or instances of the class. Here is an example of what the object invariant method would look like for a class called News which has internal state for the Title and Body of a news article:

[ContractInvariantMethod]
private void ObjectInvariant()
{
     Contract.Invariant(!String.IsNullOrEmpty(Title));
     Contract.Invariant(!String.IsNullOrEmpty(Body));
}

What this accomplishes for programmers is that if they inadvertently try to alter the internal state on an instance of the News class to something it should not be (such as setting Title = string.empty), the .NET Code Contract static checker will warn the programmer that they are violating the object's invariant.

Thus, programmers can now design and build classes with the help of .NET Code Contracts that enforce correct API usage. This is a very powerful feature that will avoid subtle bugs from ever occurring.