Object initialized in other class's method isn't referenced by calling method in C#

Question

I have a class where I declare an object, but don't initialize the object. Then I pass the object to a method in another class for initialization. What I expect to happen is that the object in the calling class will now have a reference to the initialized object, but instead it is null.

Here is an example of what I mean:

class MainClass
{
    ObjectA foo;

    OtherClass.InitializeObjectA(foo);

    // why is foo null over here?
}

class OtherClass
{
    public static void InitializeObjectA(ObjectA device)
    {
        device = new ObjectA();
    }
}

My problem is that device when I try to use foo after calling InitializeObjectA() it is still pointing to null! If I change InitializeObjectA() to out ObjectA device it works. Can anyone explain why this is needed?

MSDN about passing by reference or by value: http://msdn.microsoft.com/en-us/library/s6938f28.aspx. — Vladimir, Aug 29 '13 at 19:36
A good understanding of pass by value vs pass by reference will really help you! http://msdn.microsoft.com/en-us/library/0f66670z.aspx — Craig Shearer, Aug 29 '13 at 19:37
It's because foo has not been instantiated yet and thus has no reference to pass, thus there's no way for the method to pass the new reference back. — Rake36, Aug 29 '13 at 19:39

score 4 · Accepted Answer · answered Aug 29 '13 at 19:34

4

If you want this to work, you need to pass by reference:

public static void InitializeObjectA(ref ObjectA device)
{

Or:

public static void InitializeObjectA(out ObjectA device)
{

Without that, InitializeObjectA sets the device parameter to a new ObjectA(), but that will not affect the caller, because, by default, references are passed by value.

Note that, if you're just trying to initialize, returning an instance instead of void is often a better way to handle this:

public static ObjectA InitializeObjectA()
{
     return new ObjectA();
}

This avoids the need to use ref or out passing.

answered Aug 29 '13 at 19:34

Reed Copsey

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or, simply return ObjectA instead – Justin Pihony Aug 29 '13 at 19:34
Thanks, I think my confusion is that I though all objects were passed by reference to begin with. – Aug 29 '13 at 19:40
@w1res No - classes are reference types, so they're a reference passed by value. This means you can change values inside of the class itself, but you can't reassign the object reference and have that passed back. – Reed Copsey Aug 29 '13 at 19:40
@STW Give it to a charity of your choice ;) – Reed Copsey Aug 29 '13 at 19:43
hiding my ignorance, deleting my comments :) -- but the dollar shall be donated! – STW Aug 29 '13 at 19:43

score 1 · Answer 2 · answered Aug 29 '13 at 19:33

1

A class is a reference type, you pass the address of foo as parameter (copy), within you change the copy, but the new address won't be set back to the original foo. This will only be done with the out or ref keyword.

answered Aug 29 '13 at 19:33

Jeroen van Langen

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If foo had been instantiated before it was sent, then changing something about foo would be ok. In this case, he has no reference to start and thus no reference to pass. – Rake36 Aug 29 '13 at 19:38
@Rake36, Even when foo had an instance, before the method is called, the original instance pointer is push onto the stack and after the call it's popped back. So if you change the pointer, it will be referred to the original. You cannot change the instance of the parameter without _ref_ or _out_. – Jeroen van Langen Aug 29 '13 at 19:40
if it's a value type, true, but if you were to change a property of ObjectA, that would definitely be allowed. That's the only point I was trying to make. The word 'change' is misleading here. – Rake36 Aug 29 '13 at 19:43

score 1 · Answer 3 · answered Aug 29 '13 at 19:35

1

The simplest way to create the instance of ObjectA named foo is to return the instance to assign to the variable

class MainClass
{
    ObjectA foo = OtherClass.InitializeObjectA();
}

class OtherClass
{
    public static ObjectA InitializeObjectA()
    {
        return new ObjectA();
    }
}

answered Aug 29 '13 at 19:35

Steve

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Beat me by seconds :) – Joel Aug 29 '13 at 19:36

score 1 · Answer 4 · answered Aug 29 '13 at 19:36

1

Why not just have the Initialize method return the created object?

class MainClass
{
    var foo = OtherClass.InitializeObjectA(foo);
}

class OtherClass
{
    public static ObjectA InitializeObjectA(ObjectA device)
    {
        return new ObjectA();
    }
}

answered Aug 29 '13 at 19:36

Jeff Siver

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I didn't mention this in the question but in my actual code I'm already returning a bool. – Aug 29 '13 at 19:38
If the main responsibility of the method is to create and initialize the object, why wouldn't you return it. Doing it by using a reference or Out parameter mean this method has a side effect (http://stackoverflow.com/questions/810289/when-exactly-does-a-method-have-side-effects). Side effects make debugging and maintenance more difficult and should be avoided. – Jeff Siver Aug 29 '13 at 20:22

score 1 · Answer 5 · answered Aug 29 '13 at 19:38

From what I understand, the C# compiler doesn't actually initialize variables for you like that. I'm not even able to compile this code in Visual Studio 2010, because it's an error to pass an uninitialized parameter to a function.

You want:

ObjectA foo = null;

Which is I think what you're trying to do anyway, C# just doesn't do it for you as it would in Java. This still doesn't get the behavior I think you're trying to achieve though, which is probably best accomplished by refactoring your code to something like

ObjectA foo = InitializeObjectA()

public static ObjectA InitializeObjectA(){
    return new ObjectA();
}

Alternately, you can use pass by reference, where you pass a reference (or pointer if you're familiar with it) to the function, so changes made to that reference are reflected outside the scope of your function.

Yeah. I just figured he wanted to use the InitializeObject method for whatever reason — UpQuark, Aug 30 '13 at 02:26

score 0 · Answer 6 · answered Aug 29 '13 at 19:32

That's because when you do

device = new ObjectA();

you are setting the object to a new one, with a different reference in memory.

You should do something like:

class MainClass
{
    ObjectA foo = OtherClass.InitializeObjectA(foo);
}

class OtherClass
{
    public static ObjectA InitializeObjectA(ObjectA device)
    {
        return = new ObjectA();
    }
}

or instead

class MainClass
{
    ObjectA foo = null;
    OtherClass.InitializeObjectA(out foo);
}

class OtherClass
{
    public static void InitializeObjectA(out ObjectA device)
    {
        device = new ObjectA();
    }
}

More information about why this happens can be found here.

Object initialized in other class's method isn't referenced by calling method in C#

6 Answers6