Is it okay to refer to "this" in the constructor?

Question

In C#, a common pattern that I use is to populate the details of a lower calculation class with a form object.

The constructor for MyForm is:

MyForm()
{
   _MyFormCalcs = new MyFormCalcs(this);
}

But I encountered an error today which makes me think that as my constructor had not finished completing, it creates a new instance of MyForm to pass into MyData. Thus it calls the constructor twice. I found that a static list in MyFormCalcs was being filled twice and was failing the second time as the keys were already present in the list.

Can I use this in the constructor to refer to this instance? What will it contain in the lower class - has the constructor been run or not.

What is a better way of passing my form into the lower class?

score 9 · Answer 1 · edited Apr 01 '15 at 18:16

9

No, that won't create a new instance of MyForm.

In general, allowing this to "escape" from a constructor is dangerous as it means it can be used before the constructor has completed, but it's not going to be creating a new instance. If you could give a short but complete example of the problem you've seen, we could help diagnose it further. In particular, it's not clear what you mean about the "static list being filled twice". Usually it's not a good idea to populate a static variable in an instance constructor.

edited Apr 01 '15 at 18:16

talles

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answered Feb 23 '11 at 07:10

Jon Skeet

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Aggreed, and also, if you can ensure it is the last statement to execute, there should not be any other problems. – Adriaan Stander Feb 23 '11 at 07:11
@astander: Well, other than that the normal guarantees of the .NET memory model won't have been applied... very unlikely to bite you, but still possible. – Jon Skeet Feb 23 '11 at 07:13
Yes, rather safe than sorry **X-)** – Adriaan Stander Feb 23 '11 at 07:16
@Jon: Sometimes it's dangerous, but sometimes there aren't too many other options available... for parent-child relationships (or container-containee relationships) where both the parent and the child need references to each other in order to constitute valid objects, there's not may ways around this. :\ – user541686 Feb 23 '11 at 07:31
@astander: You can't ensure that if your class is subclassed. – user541686 Feb 23 '11 at 07:32
@Mehrdad: Yes, it can be a pain to work round it - but it's a design smell to avoid if possible. However, it definitely won't start implicitly calling constructors as the OP fears :) – Jon Skeet Feb 23 '11 at 07:37

score 2 · Answer 2 · answered Feb 23 '11 at 07:16

In fact, it is really nice to avoid such call inside constructor, because your object is not already built (constructor hasnt finish its work) but you are already use this "unfinished" object as a parameter. Its a bad way. Good way is creating some special method:

class MyClass
{

 var obj:SomeClass;

  public MyClass()
  {
  }

  public Init()
  {
    obj = SomeClass(this);
  }

}

score 1 · Answer 3 · answered Feb 23 '11 at 09:27

Make a private property that instantiate MyFormCalcs only when it is first used, like this:

public class MyForm {

  private MyFormCalcs MyFormCalcs {
    get {
      _MyFormCalcs = _MyFormCalcs ?? new MyFormCalcs(this);   
    }
  }
}

This way, you don't have to think about when to "initialize" things.

score 0 · Answer 4 · edited May 23 '17 at 12:19

0

There is this very complete response of C# constructor order:

C# constructor execution order

edited May 23 '17 at 12:19

Community

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answered Feb 23 '11 at 07:11

xanatos

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Is it okay to refer to "this" in the constructor?

4 Answers4