Question

For a class instance to work properly, some fields should be properly initialized, what’s your strategy to initialize these field, what should be given through constructor, what should be given through property?

My confusion is that, if the constructor requires a long parameter list, it’s hard to use, if through properties, i tend to forget to set some properties.

What’s the best practice?

Answer 1

You must ask yourself, if your class needs that many things to be created, perhaps it is doing too much. It’s a sign that you should reconsider your design, or just how much “work” the constructor is doing.

It is true that you should not be able to create an instance with invalid state; thus the constructor should take all properties you need to be in a valid state.

To a degree, it depends on your model. For example, the ORM I use has the constructor take one parameter; an ID by which it can load all the other properties. It would be annoying if I had to pass them all in (that’s the job of the ORM really; to set this object up). So in that sense, you can argue you have an “invalid” object (no properties set). But you’d be wrong; what you actually have is a “blank” or “empty” object. That’s not the same as invalid.

So think carefully about what it means for your object to be “invalid”. I would consider it invalid if other methods took this object, and threw an exception because something wasn’t set. Use this logic to determine what needs to be in the constructor, and what can be set later (by some other process).