Question

Is it better to initialize class member variables on declaration

private List<Thing> _things = new List<Thing>(); private int _arb = 99; 

or in the default constructor?

private List<Thing> _things; private int _arb;  public TheClass() {   _things = new List<Thing>();   _arb = 99; } 

Is it simply a matter of style or are there performance trade-offs, one way or the other?

Answer 1

In terms of performance, there is no real difference; field initializers are implemented as constructor logic. The only difference is that field initializers happen before any ‘base’/’this’ constructor.

The constructor approach can be used with auto-implemented properties (field initializers cannot) – i.e.

[DefaultValue('')] public string Foo {get;set;} public Bar() { // ctor   Foo = ''; } 

Other than that, I tend to prefer the field initializer syntax; I find it keeps things localized – i.e.

private readonly List<SomeClass> items = new List<SomeClass>(); public List<SomeClass> Items {get {return items;}} 

I don’t have to go hunting up and down to find where it is assigned…

The obvious exception is where you need to perform complex logic or deal with constructor parameters – in which case constructor-based initialization is the way to go. Likewise, if you have multiple constructors, it would be preferable for the fields to always get set the same way – so you might have ctors like:

public Bar() : this('') {} public Bar(string foo) {Foo = foo;} 

edit: as a side comment, note that in the above, if there are other fields (not shown) with field initializers, then they are only directly initialized in the constructors that call base(...) – i.e. the public Bar(string foo) ctor. The other constructor does not run field initializers, since it knows they are done by the this(...) ctor.