ref for variables not parameters in functions

Question

Suppose I have a Person class and have the following:

Person A = new Person("Tom");
Person B = A;

Is there a way I can change it so that if I assign a new Person to B, B = new Person("Harry"), A would be referring to the same instance too? I know you can do that in a ref parameter assignment in a function.

Answer 1

UPDATE: The feature described here was added to C# 7.

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The feature you want is called "ref locals" and it is not supported in C#.

The CLR does support generating code that contains ref locals, and a few years ago I wrote an experimental version of C# that had the feature you want, just to see if it would work. You could do something like:

Person a = whatever;
ref Person b = ref a;

and then as you say, changes to "b" would change the contents of "a". The two variables become aliases for the same storage location.

Though it was a nice little feature and worked well, we decided to not take it for C#. It's possible that it could still happen in a hypothetical future version of the language, but I would not get all excited about it in expectation; it will probably not happen.

(Remember, all of Eric's musings about hypothetical future versions of any Microsoft product are For Entertainment Purposes Only.)

Answer 2

No it isn't possible in safe code(beyond ref function parameters). In unsafe code you might be able to use pointers to achieve that, but I don't think that's a good idea.

A contains the reference to "Tom" as a value. You'd need a reference to A to change where it points.

If we consider similar code in a language which explicitly uses pointers instead of implicitly treating instances of classes as references:

Person* A=new Person("Tom");
Person* B=A;
B=new Person("Harry");//Changes only B
Person** C=&A;
(*C)=new Person("John");//Now A changes

So you need a pointer to a pointer in order to change A. If you transfer this to C# you'd need a reference to a local variable. And those are only available in the form of ref function parameters. This avoids some lifetime issues since a reference to a local variable can't safely outlive that local variable.

If these are not private variables you could make B a property that modifies A in the setter.

Person B{get{return A;} set{A=value;}}

Answer 3

There is no direct way to do that.

You can either take a source-code approach like:

A = B = new Person("Harry")

Or use a generic type. Something like:

var A = new ReferenceHolder<Person> (new Person("Tom")); 
var B = A;

In C# you could also use a pointer (so basically a ref type) but that approach is not statically verifyable and not suggested.