Two classes with constructors such that they reference each other?

I have a feeling that this is a code smell, and I could be doing this better, and if that is the case please point it out.

class main
{
  void main()
  {
    object A
    object B

    A = new SystemA(ref B)
    B = new SystemB(ref A)
   }
}
class Syst开发者_开发百科emA
{
  SystemB B;
  public SystemA (ref B)
  {
    this.B = B;
  }
}
class SystemB
{
  SystemA A;
  public SystemA (ref A)
  {
    this.A = A;
  }
}

Basically I need to initialize two classes with references to each other.

This doesn't work and generates a null reference exception for the class fields in the two child classes when they are later accessed after initialization.

I realize I could probably set these after initialization, but this adds a touch more bloat that I would like to avoid, since I view these assignments as initialization actions.

If you gave code which actually compiled, I suspect it would just end up with SystemA storing a reference to null - because that's the value of B when you construct A.

Having two classes which need a reference to each other is definitely a code smell, but you simply can't make them both refer to each other, unless one of them constructs the other and passes in "this", e.g.

class SystemA
{
    private readonly SystemB systemB;

    public SystemA()
    {
        systemB = new SystemB(this);
    }
}

class SystemB
{
    private readonly SystemA systemA;

    public SystemB(SystemA systemA)
    {
        this.systemA = systemA;
    }
}

Now if you need access to both values afterwards, you could either put a property in one of them giving access to the other, or (and this is really nasty) use an out parameter in the constructor:

public SystemA(out systemB)
{
    systemB = new SystemB(this);
    this.systemB = systemB;
}

Then call it as:

SystemB b;
SystemA a = new SystemA(out b);

Please don't do this though.

• It's horrendously complicated
• It's generally a bad idea to let this escape a constructor anyway