Nested use of C# Object Initializers

Question

so, object initializers are all kinds of handy - especially if you're doing linq, where they're downright necessary - but I can't quite figure out this one:

public class Class1 {
   public Class2 instance;
}

public class Class2 {
   public Class1 parent;
}

using like this:

Class1 class1 = new Class1();
class1.instance = new Class2();
class1.parent = class1;

as an initializer:

Class1 class1 = new Class1() {
    instance = new Class2() {
        parent = class1
    }
};

this doesn't work, class1 is supposedly an unassigned local variable. it gets even trickier in Linq when you are doing something like

select new Class1() { ...

it doesn't even have a name to refer to it by!

how do I get around this? can I simply not make nested references using object initializers?

Daniel Brückner · Accepted Answer · 2009-04-27 22:36:58Z

7

can I simply not make nested references using object initializers?

You are right - you cannot. There would be a cycle; A requires B for initialization but B requires A before. To be precise - you can of course make nested object initializers but not with circular dependencies.

But you can - and I would suggest you should if possible - work this around as follows.

public class A
{
   public B Child
   {
      get { return this.child; }
      set
      {
         if (this.child != value)
         {
            this.child = value;
            this.child.Parent = this;
         }
      }
   }
   private B child = null;
}

public class B
{
   public A Parent
   {
      get { return this.parent; }
      set
      {
         if (this.parent != value)
         {
            this.parent = value;
            this.parent.Child = this;
         }
      }
   }
   private A parent = null;
}

Building the relation inside the property has the benifit that you cannot get an inconsitent state if you forget one of the initialization statements. It is quite obvious that this is an suboptimal solution because you need two statements to get one thing done.

b.Parent = a;
a.Child = b;

With the logic in the properties you get the thing done with only one statement.

a.Child = b;

Or the other way round.

b.Parent = a;

And finally with object initializer syntax.

A a = new A { Child = new B() };

edited Apr 27, 2009 at 22:36

answered Apr 27, 2009 at 22:08

Daniel Brückner

59.8k16 gold badges103 silver badges144 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Andy Hohorst Over a year ago

I like this answer best but it doesn't work for me, because I left out that Class2 is actually List<Class2>, to make the examples clearer. drat, it backfired.

Mike Over a year ago

Update from 2024: Unfortunately this approach does not work if #nullable is enabled.

Martin Harris · Accepted Answer · 2009-04-27 22:09:37Z

2

I don't think there is anyway to get around this, when the application is instantiating the Class1 object it needs to have created the Class2 object first so that it knows where in memory the reference is located. You can see this if you try the following:

        Class1 myClass1 = null;

        myClass1 = new Class1()
        {
            instance = new Class2
            {
                parent = myClass1
            }
        };

This will compile and run, but the parent property of Class2 will be null since that is what the value was at the time the inner line of code was run, meaning that the inner most line was the first to be executed.

answered Apr 27, 2009 at 22:09

Martin Harris

28.7k8 gold badges92 silver badges102 bronze badges

Comments

tofi9 · Accepted Answer · 2009-04-27 22:16:16Z

2

You can't do that with Object Initializers. However, you can do the trick using propery code:

class A
{
    B b;

    public B B
    {
        set
        {
            b = value;
            b.a = this;
        }
        get
        {
            return b;
        }
    }
}

class B
{
    public A a;
}

Calling it:

var a = new A  {  B = new B() };

answered Apr 27, 2009 at 22:16

tofi9

5,8634 gold badges32 silver badges51 bronze badges

Comments

JaredPar · Accepted Answer · 2009-04-27 22:07:57Z

1

This problem is not specific to object initializers, it's a general restriction in the C# language. You cannot use a local variable until it has been definitely assigned. Here is a simpler repro

Class1 Foo(Class1 c1) {
  return c1;
}

void Example() {
  Class1 c1 = Foo(c1);
}

answered Apr 27, 2009 at 22:07

JaredPar

759k152 gold badges1.3k silver badges1.5k bronze badges

Comments

Eric King · Accepted Answer · 2009-04-27 22:11:56Z

0

You can certainly use nested object initializers, I do it all the time.

However, in your particular case, your objects have a circular reference. You need to finish instantiating one before assigning it to the other. Otherwise, you are handing the compiler a classic chicken and egg problem it can't handle.

answered Apr 27, 2009 at 22:11

Eric King

11.8k5 gold badges48 silver badges53 bronze badges

2 Comments

Andy Hohorst Over a year ago

it's not a chicken and egg problem, at least not how I understand initializers: just syntactic sugar for declaring an object and setting its properties.

Andy Hohorst Over a year ago

sorry - it "shouldn't" be one according to how the object initializers are explained. The first object is created, and then its parameters are assigned values. Since I can do this in the style that the object initiailizers supposedly compile down to, I should be able to do it with object initializers.

Randolpho · Accepted Answer · 2009-04-28 13:38:24Z

0

Your example does not reflect good class design, IMO; it's inappropriately cohesive and creates a circular reference. That's what makes it impossible to instantiate them together in a single expression.

I would suggest you go back to the drawing board and refactor your classes into a parent/child relationship. I'd use constructor injection on the child class and have the child tell the parent that it's a child of it.

For example:

public class ParentClass 
{
    public List<ChildClass> Children;
    public void AddChild(ChildClass child)
    {
        Children.Add(child);
        // or something else, etc. 
    }
    // various stuff like making sure Children actually exists before AddChild is called
}

public class ChildClass 
{
    public ParentClass Parent;
    public ChildClass(ParentClass parent)
    {
        Parent = parent;
        Parent.AddChild(this);
    }
}

Then, in your calling code:

var parent = new ChildClass(new ParentClass()).Parent;

And, yes, that works in LINQ:

// qry, etc.
select new ChildClass(new ParentClass()).Parent

but then how do I make all the ChildClass have the same ParentClass instance? – Andy Hohorst

Then you must know the parent class in advance.

var parent = new ParentClass();
var child = new ChildClass(parent);

or

var parent = new ParentClass();
// qry, etc.
select new ChildClass(parent)

edited Apr 28, 2009 at 13:38

answered Apr 27, 2009 at 22:39

Randolpho

56.5k18 gold badges153 silver badges182 bronze badges

1 Comment

Andy Hohorst Over a year ago

but then how do I make all the ChildClass have the same ParentClass instance?

Collectives™ on Stack Overflow

Nested use of C# Object Initializers

6 Answers 6

2 Comments

Comments

Comments

Comments

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

2 Comments

Comments

Comments

Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related