Initialize class fields in constructor or at declaration?

Question

I\'ve been programming in C# and Java recently and I am curious where the best place is to initialize my class fields.

Should I do it at declaration?:

Answer 1

I normally try the constructor to do nothing but getting the dependencies and initializing the related instance members with them. This will make you life easier if you want to unit test your classes.

If the value you are going to assign to an instance variable does not get influenced by any of the parameters you are going to pass to you constructor then assign it at declaration time.

Answer 2

I think there is one caveat. I once committed such an error: Inside of a derived class, I tried to "initialize at declaration" the fields inherited from an abstract base class. The result was that there existed two sets of fields, one is "base" and another is the newly declared ones, and it cost me quite some time to debug.

The lesson: to initialize inherited fields, you'd do it inside of the constructor.

Answer 3

Consider the situation where you have more than one constructor. Will the initialization be different for the different constructors? If they will be the same, then why repeat for each constructor? This is in line with kokos statement, but may not be related to parameters. Let's say, for example, you want to keep a flag which shows how the object was created. Then that flag would be initialized differently for different constructors regardless of the constructor parameters. On the other hand, if you repeat the same initialization for each constructor you leave the possibility that you (unintentionally) change the initialization parameter in some of the constructors but not in others. So, the basic concept here is that common code should have a common location and not be potentially repeated in different locations. So I would say always put it in the declaration until you have a specific situation where that no longer works for you.