What is a practical usage of Code Contracts in .NET 4.0?

Question

In order to fully understand and take advantage of the new features and enhancements provided with the coming of the new .NET Framework 4.0, I would like to get an example of

Answer 1

Have you ever seen a NullReferenceException and wished that the compiler could have warned you about it at compile time to avoid finding out the hard way - with your program crashing?

With code contracts you can write things like:

Contract.Requires(foo != null);

This isn't just a runtime check - you can set it up so that if you call this function with an argument that might be null you will get a compile error.

Here's a real world example:

Address GetAddress(Customer customer)
{
    Contract.Requires(customer != null, "customer");
    return addressBook.FindCustomer(customer);
}