why is my passed object getting edited

Question

I pass an 'global' object as a param to a method. I then init a new instance of the object and set it equal to the original, in my head i now have 2 instances of that object.

Why when i change the second instance does the first instance change? I never used 'ref'.

namespace myNamespace
{
    public class myClass
    {
        private myObjectClass myGlobalInstance;

    public void myMethod()
    {
        doSomething(myGlobalObject);    
    }

    private String doSomthing(myObjectClass myObjectInstance)
    {
            myObjectClass newObject = myObjectInstance;
        newObject.variable1 = "boo"; //this seems to change both newObject.variable1 as required  AND myObjectInstance.variable1 and its calling classes object
    }
    }
}

myObjectClass newObject = myObjectInstance; that sets a reference to you myObjectInstance. Try the [ReferenceEquals](https://www.google.de/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=c%23+ReferenceEquals) function, it'll return true if both object are the same object in memory — Nitro.de, Feb 05 '16 at 15:30

Robert Moskal · Answer 1 · 2016-02-05T15:47:58.300

You are not initializing a new object. You are just copying a reference to the original one here:

myObjectClass newObject = myObjectInstance;

There is not necessarily a clean solution to your problem. Depending on what your class/object looks you might be able to use MemberwiseClone. But since this is a protected method, you'll have to expose it on myObjectClass something like so:

public myObjectClass ShallowCopy()
{
   return (myObjectClass) this.MemberwiseClone();
}

And then call it in myClass:

myObjectClass newObject = myObjectInstance.ShallowCopy();

This will make a shallow copy of your object which may or may not accomplish what you want. From the docs:

The MemberwiseClone method creates a shallow copy by creating a new object, and then copying the nonstatic fields of the current object to the new object. If a field is a value type, a bit-by-bit copy of the field is performed. If a field is a reference type, the reference is copied but the referred object is not; therefore, the original object and its clone refer to the same object.

If that is not sufficient, then you might have to implement a clone method to do a deep copy. How to do this is also spelled out in the above docs.

you cant use protected MemberwiseClone method everywhere. – Szer Feb 05 '16 at 15:37 — Szer, Feb 05 '16 at 15:37
you are right. He'll have to add a method to the class! – Robert Moskal Feb 05 '16 at 15:41 — Robert Moskal, Feb 05 '16 at 15:41

score 1 · Answer 2 · answered Feb 05 '16 at 15:33

1

The RemoteConfigMgr class is a reference type, meaning that each of the variables of this type are just references (addresses) to the actual object. The line:

myObjectClass newObject = myObjectInstance;

Means that newObject variable will now refer to the same object as the myObjectInstance variable. So when you modify newObject, you are also modifying myObjectInstance.

answered Feb 05 '16 at 15:33

DimitarD

117
1
5

Do you know how i would fix? – Fearghal Feb 05 '16 at 15:35

score 0 · Accepted Answer · edited May 23 '17 at 11:59

0

You are copying reference to object thats the problem. To create another object with the same properties you could use MemberwiseClone:

public class GlobalObject
{
    public GlobalObject GetCopy()
    {
        return (GlobalObject)MemberwiseClone();
    }
}

But you have to understand the difference between ShallowCopy and DeepCopy

Your code will look like this:

namespace myNamespace
{
    public class myClass
    {
        private GlobalObject myGlobalInstance;

        public void myMethod()
        {
            doSomthing(myGlobalObject);
        }

        private String doSomthing(GlobalObject myObjectInstance)
        {
            GlobalObject newObject = myObjectInstance.Copy();
            newObject.variable1 = "boo";
        }
    }
}

edited May 23 '17 at 11:59

Community

1
1

answered Feb 05 '16 at 15:37

Szer

3,426
3
16
36

Yes, and you have to understand the definition of an answer. Not one word to the question WHY... – TomTom Feb 05 '16 at 15:37
@TomTom It is a tool. Its up to you how and why use it. – Szer Feb 05 '16 at 15:38
Yeah, Ignorane demonstrated. TRY READING HIS QUESTION. He asks for an explanation, not a workatround. – TomTom Feb 05 '16 at 15:43
thx TomTom and Szer. It took me a while to understand what you were talking about but once i hit the links and read it up it was ok. This seems an extreme fix though. I have been working in Java recently and didnt hit this nor did i hit it in c# until i used a 'global' var – Fearghal Feb 05 '16 at 15:56

why is my passed object getting edited

3 Answers3