C# 5.0 Programmer's Reference

212 ❘ CHAPTER 9 Error Handling
Figure 9-1: Use the Code Contracts property page to turn on code contracts.
Now whenever the program executes a public method from outside of the class’s code, it checks the
invariant after that method call returns to make sure the invariant property is still true.
For example, suppose the Order class has a Customer property and that property should never be
null throughout the lifetime of the Order object. The following code shows how you might use a
contract to enforce that condition in the Order class.
class Order
{
public List Items = new List();
public Customer Customer { get; set; }
public Order(Customer customer)
{
Customer = customer;
}
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