How many constructor arguments is too many?

Question

Let\'s say you have a class called Customer, which contains the following fields:

• UserName
• Email
• First Name
• Last Name

Answer 1

I think the "pure OOP" answer is that if operations on the class are invalid when certain members aren't initialized, then these members must be set by the constructor. There's always the case where default values can be used, but I'll assume we're not considering that case. This is a good approach when the API is fixed, because changing the single allowable constructor after the API goes public will be a nightmare for you and all users of your code.

In C#, what I understand about the design guidelines is that this isn't necessarily the only way to handle the situation. Particularly with WPF objects, you'll find that .NET classes tend to favor parameterless constructors and will throw exceptions if the data has not been initialized to a desirable state before calling the method. This is probably mainly specific to component-based design though; I can't come up with a concrete example of a .NET class that behaves in this manner. In your case, it'd definitely cause an increased burden on testing to ensure that the class is never saved to the data store unless the properties have been validated. Honestly because of this I'd prefer the "constructor sets the required properties" approach if your API is either set in stone or not public.

The one thing I am certain of is that there are probably countless methodologies that can solve this problem, and each of them introduces its own set of problems. The best thing to do is learn as many patterns as possible and pick the best one for the job. (Isn't that such a cop-out of an answer?)