Is it possible to make an noncopyable type in C#

Question

Is it somehow possible in C# to create a class that can not be copied around but rather can only be passed around by reference?

The equivalent in C++ would be to delete the copy-constructor and the copy-assignment operator.

I don't think you could copy an object unless it implemented the `ICloneable` interface — mikeyq6, Jan 27 '16 at 13:38
So what happens if I write a = b assuming that a and b are classes. Will it only copy the reference or will it copy the values of the members from b to a? — Knitschi, Jan 27 '16 at 13:39
@Knitschi Then they would point to the same object, copy the reference if you will. — Chris Wohlert, Jan 27 '16 at 13:39
Try it out... as @mikeyq6 already said: classes are passed by reference. — MakePeaceGreatAgain, Jan 27 '16 at 13:39
a = b : will have different effects depending on the underlying type of b. Eg: a value type vs a reference type. If b was a string literal - then a will become a string that has the same value as b. If b was a TcpClient, then a will become a pointer to the same TcpClient that b was. — Baaleos, Jan 27 '16 at 13:41
@Baaleos A string is an reference type, not a value type, you won't get two strings with `a=b`. — Lasse V. Karlsen, Jan 27 '16 at 14:00
very true : Wont create a new object in memory, unless the 'new' keyword is invoked. — Baaleos, Jan 27 '16 at 14:03

score 6 · Accepted Answer · edited Jan 27 '16 at 14:02

6

It is default behavior. If you have class:

class foo {
    public string bar { get; set; }
}

and somewhere you do this:

foo f1 = new foo();
foo f2 = f1;

Both f1 and f2 will reference same instance. If you, for example, set f1.bar = "bar", value read from f2.bar will be "bar".

edited Jan 27 '16 at 14:02

Lasse V. Karlsen

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answered Jan 27 '16 at 13:39

Shadowed

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So I can be sure that the destructor of an object of class type is only called once no matter what I do with it? – Knitschi Jan 27 '16 at 13:45
1

@Knitschi - C# objects don't have destructors. The nearest thing is finalisers, which most classes don't define either. – Lee Jan 27 '16 at 13:48
I vote this as the answer, but want to point out that you can copy objects using serialization, e.g. using Json.Net: `foo f2 = JsonConvert.DeserializeObject(JsonConvert.SerializeObject(f1));` That's clearly outside the scope of the question; I mention it just for the sake of curiosity. – Matthew Watson Jan 27 '16 at 13:50
Ok my comment was stupid. Each object can only be destroyed once. – Knitschi Jan 27 '16 at 13:52
Destructor is called only by Garbage Collector when object is destroyed by GC. But, if you have inherited class and both it and base class have destructor, they will both be called. Check https://msdn.microsoft.com/en-us/library/66x5fx1b.aspx for details. – Shadowed Jan 27 '16 at 13:53

score 1 · Answer 2 · answered Jan 27 '16 at 13:40

You could create an internal constructor. That way, the constructor can only be called from the assembly in which it is defined.

You can also create more advanced logic through the use of Reflection and StackTrace (for instance) and in detail control who is allowed to create instances of the class, and when. Similar logic can be applied to properties, where you can control who is allowed to change property values.

score 1 · Answer 3 · answered Jan 27 '16 at 13:50

There is no such thing as a destructor in C#, only Finalizers. However the GC is smart enough to know when no more rerfences exist for a given object and than calls this finalizer. Thus when two variables reference the same object this object lives as long as those two references exist. If both (!!) are out of scope GC collects the object for deleting.

Is it possible to make an noncopyable type in C#

3 Answers3