What is difference between new in a constructor and new in a member declaration?

Question

What is the difference between new in a constructor and new in a member declaration?

Example

public class PspGame {

  private List<string>name = new List<string>();
  private List<string>_value;

  public PspGame() {
    _value = new List<string>();
  }
}

What is the best way to do it and are there any performance issues?

Answer 1

They're pretty much equivalent (any differences in terms of performance and memory usage are negligible). The only real difference is that when you do:

private List<string>name = new List<string>();

...the assignment always happens no matter what constructor is used to create an instance of the object. When you do the assignment within a constructor, then it only happens when that specific constructor is used.

So if you have multiple constructors but you always want to initialize name exactly the same way, it is a bit shorter to use the first form than to explicitly initialize it in each constructor.

As a general rule, however, I prefer initializing fields in the constructor implementations, even if it does make the code more verbose in some cases.

Answer 2

Having an argument in the constructor (non-default constructor), for me, allows for better testing. You can 'inject' certain data into a member field that you wouldn't necessarily be able to do without making that member public (or at least internal) or making a second call with a 'setter' property.

Because you can have multiple constructors, you could have a second one for testing, in conjunction with a default constructor, if you really wanted.

There aren't any real performance issues other than having to make a separate call to populate data later, or having less-maintainable code with multiple calls to the class (one to create the object, the second to populate the member).

EDIT: I realized I sorta answered the wrong question. I thought he was asking about the difference between default and non-default constructors. Now I see it was about a default constructor that initializes the member in the constructor vs. in member declaration...