How do you test a business rule when it's a property accessibility?

Question

Here is the catch. I have a business object that has a field named RegisterDate. The business rule as usual says that once it is set, can't be changed.

My first thinking about implementing this field as a property and defining the setter as protected, preventing users from using (and setting) it after the object is created. But after thinking a while I got me sneaking a business rule in a property accessibility rule.

My second thought was to leave the property setter public, implement an exception when setting it, which makes the business rule pretty explicit. There will be a test case that expect the property to throw an exception every time someone tries to set it.

Well, but that's a lot of boilerplate to avoid a behavior, why not making the thing protected after all, avoiding even misuse in other parts of the code? But then I wondered, what if another developer accidentally changes the property accessibility and makes it public again and the users of the business object starts using this field violating the business rule?

What is the best approach for this situation? How would you solve this issue?

Answer 1

If you can afford to only set it only in constructor you can make the field read only.

public class SomeClass
{
  public SomeClass(DateTime regDate)
  {
     registerDate = regDate;
  }

  public DateTime RegisterDate { get { return registerDate; } }

  private readonly DateTime registerDate;
}