Value assignment for reference type in C#

Question

What is the proper way to implement assignment by value for a reference type? I want to perform an assignment, but not change the reference.

Here is what I'm talking about:

void Main()
{
    A a1 = new A(1);
    A a2 = new A(2);
    a1 = a2; //WRONG: Changes reference
    a1.ValueAssign(a2); //This works, but is it the best way?
}

class A
{
    int i;

    public A(int i)
    {
        this.i = i;
    }

    public void ValueAssign(A a)
    {
        this.i = a.i;
    }
}

Is there some sort of convention I should be using for this? I feel like I'm not the first person that has encountered this. Thanks.

EDIT:

Wow. I think I need to tailor my question more toward the actual problem I'm facing. I'm getting a lot of answers that do not meet the requirement of not changing the reference. Cloning is not the issue here. The problem lies in ASSIGNING the clone.

I have many classes that depend on A - they all share a reference to the same object of class A. So, whenever one classes changes A, it's reflected in the others, right? That's all fine and well until one of the classes tries to do this:

myA = new A();

In reality I'm not doing new A() but I'm actually retrieving a serialized version of A off the hard drive. But anyways, doing this causes myA to receive a NEW REFERENCE. It no longer shares the same A as the rest of the classes that depend on A. This is the problem that I am trying to address. I want all classes that have the instance of A to be affected by the line of code above.

I hope this clarifies my question. Thank you.

Answer 1

It sounds like you're talking about cloning. Some objects will support this (via ICloneable) but most won't. In many cases it doesn't make sense anyway - what does it mean to copy a FileStream object? ICloneable is generally regarded as a bad interface to use, partly because it doesn't specify the depth of the clone.

It's better to try to change your way of thinking so this isn't necessary. My guess is that you're a C++ programmer - and without wishing to cast any judgements at all: don't try to write C# as if it's C++. You'll end up with unidiomatic C# which may not work terrible well, may be inefficient, and may be unintuitive for C# developers to understand.

One option is to try to make types immutable where possible - at that point it doesn't matter whether or not there's a copy, as you wouldn't be able to change the object anyway. This is the approach that String takes, and it works very well. It's just a shame that there aren't immutable collections in the framework (yet).

In your case, instead of having the ValueAssign method, you would have WithValue which would return a new instance with just the value changed. (Admittedly that's the only value available in your case...) I realise that this sort of copying (of all but the property that's about to change) goes against what I was saying about copying being somewhat unidiomatic in C#, but it's within the class rather than an outside body deciding when to copy.

I suspect I'm not explaining this terribly well, but my general advice is to design around it rather than to try to explicitly copy all over the place.

Answer 2

I believe you should be using a struct instead of a class than, as structs work by value and not by reference.

Answer 3

For what you want to do, I think A.ValueAssign(otherA) is the best way.

Given that you want to have one reference of A around, ensuring that the reference isn't destroyed is key.

Wouldn't you also be served by using a singleton pattern here as well?

Answer 4

One approach is to use a copy constructor. e.g.,

MyClass orig = ...; MyClass copy = new MyClass(orig);

Where you copy the elements of MyClass. Depending on how many reference types the class contains this might involve recursive use of copy constructors.

Answer 5

Others have suggested cloning in their answer, but that's only part of the deal. You also want to use the results of a (possibly deep) clone to replace the contents of an existing object. That's a very C++-like requirement.

It just doesn't come up very often in C#, so there's no standard method name or operator meaning "replace the contents of this object with a copy of the contents of that object".

The reason it occurs so often in C++ is because of the need to track ownership so that cleanup can be performed. If you have a member:

std::vector<int> ints;

You have the advantage that it will be properly destroyed when the enclosing object is destroyed. But if you want to replace it with a new vector, you need swap to make that efficient. Alternatively you could have:

std::vector<int> *ints;

Now you can swap in a new one easily, but you have to remember to delete the old one first, and in the enclosing class's destructor.

In C# you don't need to worry about that. There's one right way:

List<int> ints = new List<int>();

You don't have to clean it up, and you can swap in a new one by reference. Best of both.

Edit:

If you have multiple "client" objects that need to hold a reference to an object and you want to be able to replace that object, you would make them hold a reference to an intermediate object that would act as a "wrapper".

class Replaceable<T>
{
    public T Instance { get; set; }
}

The other classes would hold a reference to the Replaceable<T>. So would the code that needs to swap in a replacement. e.g.

Replaceable<FileStream> _fileStream;

It might also be useful to declare an event, so clients could subscribe to find out when the stored instance was replaced. Reusable version here.

You could also define implicit conversion operators to remove some syntax noise.

Answer 6

We have cases where we do exactly what you are talking about. We have many objects referencing a particular instance of an object and we want to change the instance of the object so that every object referencing that existing instance see the change.

The pattern we follow is almost what you have - just the names are different:

    class A
    {
        int i;
        public A(int i)
        {
            this.i = i;
        }
        public void Copy(A source)
        {
            this.i = source.i;
        }
    }

Answer 7

In several WinForms based applications, I've needed similar functionality, in my case to allow a data entry form to work on a copy of the object, information from which is copied onto the original object only if the user elects to save the changes.

To make this work, I brought across an idea from my Delphi days - the Assign() method.

Essentially, I wrote (well, ok, generated) a method that copies across properties (and list contents, etc etc) from one instance to another. This allowed me to write code like this:

var person = PersonRespository.FindByName("Bevan");
...
var copy = new Person();
copy.Assign(person);
using (var form = new PersonDataEntryForm(copy))
{
    if (form.ShowAsModelessDialog() == MessageReturn.Save)
    {
        person.Assign(copy);
    }
}

Changes made within the dialog are private until the user chooses to save them, then the public variable (person) is updated.

An Assign() method for Person might look like this:

public void Assign(Person source)
{
    Name = source.Name;
    Gender = source.Gender;
    Spouse = source.Spouse;

    Children.Clear();
    Children.AddRange( source.Children);
}

As an aside, having an Assign() method makes a copy-constructor almost trivially easy to write:

public Person(Person original)
    : this()
{
    Assign(original);
}

Answer 8

I wish there was a "second best" answer option, because anyone who mentioned Observer deserves it. The observer pattern would work, however it is not necessary and in my opinion, is overkill.

If multiple objects need to maintain a reference to the same object ("MyClass", below) and you need to perform an assignment to the referenced object ("MyClass"), the easiest way to handle it is to create a ValueAssign function as follows:

public class MyClass
{
    private int a;
    private int b;

    void ValueAssign(MyClass mo)
    {
        this.a = mo.a;
        this.b = mo.b;
    }
}

Observer would only be necessary if other action was required by the dependent objects at the time of assignment. If you wish to only maintain the reference, this method is adequate. This example here is the same as the example that I proposed in my question, but I feel that it better emphasizes my intent.

Thank you for all your answers. I seriously considered all of them.

Answer 9

I had the exact same problem. The way I solved it was by putting the object that everything is referencing inside another object and had everything reference the outer object instead. Then you could change the inner object and everything would be able to reference the new inner object.

OuterObject.InerObject.stuff

Answer 10

If I got it right, you are talking about proper Singleton deserialization.

1. If you are using .Net native serialization then you might take a look at the MSDN ISerializable example. The example shows exactly that - how to override ISerializable.GetObjectData to return the same instance on each call.

2. If you are using Xml serialization (XmlSerializer), then you manually implement IXmlSerializable in your object's parent class, and then take care to get a single instance each time.

3. A simplest way would be to ensure this in your parent property's setter, by accessing some kind of a static cache. (I find this pretty dirty, but that's an easy way to do it).

For example:

 public class ParentClass
 {
      private ReferencedClass _reference;
      public ReferencedClass Reference
      {
          get
          { 
              return _reference;
          }
          set
          {
              // don't assign the value, but consult the
              // static dictionary to see if we already have
              // the singleton
              _reference = StaticCache.GetSingleton(value);
          }
      }
 }

And then you would have a static class with some kind of a dictionary where you could quickly retrieve the singleton instance (or create it if it doesn't exist).

Although this may work for you, I also agree with the others that this is rarely the best (or only) way to do it. There is surely a way to refactor your code so that this becomes unnecessary, but you should provide some additional info about what is the intended usage, where is this data accessed from, or simply why do classes really need to reference a single object.

[Edit]

Since you are using a static cache similar to a singleton, you should take care to implement it properly. That means several things:

1. Your cache class should have a private constructor. You don't want anyone to create a new instance explicitly - this is not an option when using singletons. So this means you should expose some public static property like Cache.Instance which will always return the reference to the same private object. Since the constructor is private, you are sure that only Cache class can create the instance during initialization (or first update). Details on implementing this pattern can be found at http://www.yoda.arachsys.com/csharp/singleton.html (which is also a great thread-safe implementation).

2. When all you object have the same instance, then you can simply notify cache to update the single private instance (e.g. call Cache.Update() from somewhere). This way you can update the only instance which everyone is using.

But it still not clear from your example how exactly you are notifying your clients that the data has been updated anyway. An event-driven mechanism would work better, as it would allow you to decouple your code - Singletons are evil.

Answer 11

It's not possible to overload the assignment operator in c# as it would be in c/c++. However even if that was an option I'd say your trying to fix the symptom not the problem. Your problem is that the assignment of a new reference is breaking code, why not just assign the read values to the original reference? and If you are affraid that others might new and assign make the object into a singleton or similar so that it can't be altered after creation but the reference will stay the same

Answer 12

What if you did this:

public class B
{
    B(int i) {A = new A(i);}
    public A A {get; set;}
}

...

void Main()
{
    B b1 = new B(1);
    A a2 = new A(2);
    b1.A = a2;
}

Throughout your program, references to A should only be accessed via an instance of B. When you reassign b.A, you're changing the reference of A, but that doesn't matter because all of your external references are still pointing to B. This has a lot of similarities with your original solution, but it allows you to change A any way you want without having to update its ValueAssign method.