Is there a particular reason why a generic ICloneable<T>
does not exist?
It would be much more comfortable, if I would not need to cast it everytime I clone something.
Is there a particular reason why a generic ICloneable<T>
does not exist?
It would be much more comfortable, if I would not need to cast it everytime I clone something.
In addition to Andrey's reply (which I agree with, +1) - when ICloneable
is done, you can also choose explicit implementation to make the public Clone()
return a typed object:
public Foo Clone() { /* your code */ }
object ICloneable.Clone() {return Clone();}
Of course there is a second issue with a generic ICloneable<T>
- inheritance.
If I have:
public class Foo {}
public class Bar : Foo {}
And I implemented ICloneable<T>
, then do I implement ICloneable<Foo>
? ICloneable<Bar>
? You quickly start implementing a lot of identical interfaces...
Compare to a cast... and is it really so bad?
ICloneable is considered a bad API now, since it does not specify whether the result is a deep or a shallow copy. I think this is why they do not improve this interface.
You can probably do a typed cloning extension method, but I think it would require a different name since extension methods have less priority than original ones.
List<T>
had a clone method, I would expect it to yield a List<T>
whose items have the same identities as those in the original list, but I would expect that any internal data structures would be duplicated as needed to ensure that nothing done to one list will affect the identities of items stored in the other. Where's the ambiguity? A bigger problem with cloning comes with a variation of the "diamond problem": if CloneableFoo
inherits from [not publicly cloneable] Foo
, should CloneableDerivedFoo
derive from...
identity
of the list itself, for example (in case of list of lists)? However, ignoring that, your expectation is not the only possible idea people may have when calling or implementing Clone
. What if library authors implementing some other list do not follow your expectation? The API should be trivially unambiguous, not arguably unambiguous.
Commented
Aug 11, 2012 at 1:11
Clone
on all its parts, it will not work predictably -- depending on whether that part was implemented by you or that person who likes deep cloning. your point about the patterns is valid but having IMHO in API is not clear enough -- it either should be called ShallowCopy
to stress the point, or not provided at all.
Commented
Aug 14, 2012 at 0:36
I need to ask, what exactly would you do with the interface other than implement it? Interfaces are typically only useful when you cast to it (ie does this class support 'IBar'), or have parameters or setters that take it (ie i take an 'IBar'). With ICloneable - we went through the entire Framework and failed to find a single usage anywhere that was something other than an implementation of it. We've also failed to find any usage in the 'real world' that also does something other than implement it (in the ~60,000 apps that we have access to).
Now if you would just like to enforce a pattern that you want your 'cloneable' objects to implement, that's a completely fine usage - and go ahead. You can also decide on exactly what "cloning" means to you (ie deep or shallow). However, in that case, there's no need for us (the BCL) to define it. We only define abstractions in the BCL when there is a need to exchange instances typed as that abstraction between unrelated libraries.
David Kean (BCL Team)
ICloneable<out T>
could be quite useful if it inherited from ISelf<out T>
, with a single method Self
of type T
. One doesn't often need "something that's cloneable", but one may very well need a T
that's cloneable. If a cloneable object implements ISelf<itsOwnType>
, a routine that needs a T
that's cloneable can accept a parameter of type ICloneable<T>
, even if not all of the cloneable derivatives of T
share a common ancestor.
ICloneable<T>
could be useful for that, though a broader framework for maintaining parallel mutable and immutable classes might be more helpful. In other words, code which needs to see what some type of Foo
contains but is neither going to mutate it nor expect that it won't ever change could use an IReadableFoo
, while...
Foo
could use an ImmutableFoo
while code that to wants to manipulate it could use a MutableFoo
. Code given any type of IReadableFoo
should be able to get either a mutable or immutable version. Such a framework would be nice, but unfortunately I can't find any nice way to set things up in a generic fashion. If there were a consistent way to make a read-only wrapper for a class, such a thing could be used in combination with ICloneable<T>
to make an immutable copy of a class which holds T
'.
List<T>
, such that the cloned List<T>
is a new collection holding pointers to all of the same objects in the original collection, there are two easy ways of doing that without ICloneable<T>
. The first is the Enumerable.ToList()
extension method: List<foo> clone = original.ToList();
The second is the List<T>
constructor that takes an IEnumerable<T>
: List<foo> clone = new List<foo>(original);
I suspect the extension method is probably just calling the constructor, but both of these will do what you're requesting. ;)
I think the question "why" is needless. There is a lot of interfaces/classes/etc... which is very usefull, but is not part of .NET Frameworku base library.
But, mainly you can do it yourself.
public interface ICloneable<T> : ICloneable {
new T Clone();
}
public abstract class CloneableBase<T> : ICloneable<T> where T : CloneableBase<T> {
public abstract T Clone();
object ICloneable.Clone() => return this.Clone();
}
public abstract class CloneableExBase<T> : CloneableBase<T> where T : CloneableExBase<T> {
protected abstract T CreateClone();
protected abstract void FillClone(T clone);
public override T Clone() {
T clone = this.CreateClone();
if (clone is null ) {
throw new NullReferenceException( "Clone was not created." );
}
this.FillClone(clone);
return clone
}
}
public abstract class PersonBase<T> : CloneableExBase<T> where T : PersonBase<T> {
public string Name { get; set; }
protected override void FillClone( T clone ) {
clone.Name = this.Name;
}
}
public sealed class Person : PersonBase<Person> {
protected override Person CreateClone() => return new Person();
}
public abstract class EmployeeBase<T> : PersonBase<T> where T : EmployeeBase<T> {
public string Department { get; set; }
protected override void FillClone(T clone) {
base.FillClone(clone);
clone.Department = this.Department;
}
}
public sealed class Employee : EmployeeBase<Employee> {
protected override Employee CreateClone() => return new Employee();
}
It's pretty easy to write the interface yourself if you need it:
public interface ICloneable<T> : ICloneable
where T : ICloneable<T>
{
new T Clone();
}
Having read recently the article Why Copying an Object is a terrible thing to do?, I think this question needs additional clafirication. Other answers here provide good advices, but still the answer isn't complete - why no ICloneable<T>
?
Usage
So, you have a class that implements it. While previously you had a method that wanted ICloneable
, it now has to be generic to accept ICloneable<T>
. You would need to edit it.
Then, you could have got a method that checks if an object is ICloneable
. What now? You can't do is ICloneable<>
and as you don't know the type of the object at compile-type, you can't make the method generic. First real problem.
So you need to have both ICloneable<T>
and ICloneable
, the former implementing the latter. Thus an implementer would need to implement both methods - object Clone()
and T Clone()
. No, thanks, we already have enough fun with IEnumerable
.
As already pointed out, there is also the complexity of inheritance. While covariance may seem to solve this problem, a derived type needs to implement ICloneable<T>
of its own type, but there is already a method with the same signature (= parameters, basically) - the Clone()
of the base class. Making your new clone method interface explicit is pointless, you will lose the advantage you sought when creating ICloneable<T>
. So add the new
keyword. But don't forget that you would also need to override the base class' Clone()
(the implementation has to remain uniform for all derived classes, i.e. to return the same object from every clone method, so the base clone method has to be virtual
)! But, unfortunately, you can't both override
and new
methods with the same signature. Choosing the first keyword, you'd lose the goal you wanted to have when adding ICloneable<T>
. Chossing the second one, you'd break the interface itself, making methods that should do the same return different objects.
Point
You want ICloneable<T>
for comfort, but comfort is not what interfaces are designed for, their meaning is (in general OOP) to unify the behavior of objects (although in C#, it is limited to unifying the outer behavior, e.g. the methods and properties, not their workings).
If the first reason hasn't convinced you yet, you could object that ICloneable<T>
could also work restrictively, to limit the type returned from the clone method. However, nasty programmer can implement ICloneable<T>
where T is not the type that is implementing it. So, to achieve your restriction, you can add a nice constraint to the generic parameter:
public interface ICloneable<T> : ICloneable where T : ICloneable<T>
Certainly more restrictive that the one without where
, you still can't restrict that T is the type that is implementing the interface (you can derive from ICloneable<T>
of different type that implements it).
You see, even this purpose couldn't be achieved (the original ICloneable
also fails at this, no interface can truly limit the behavior of the implementing class).
As you can see, this proves making the generic interface is both hard to fully implement and also really unneeded and useless.
But back to the question, what you really seek is to have comfort when cloning an object. There are two ways to do it:
public class Base : ICloneable
{
public Base Clone()
{
return this.CloneImpl() as Base;
}
object ICloneable.Clone()
{
return this.CloneImpl();
}
protected virtual object CloneImpl()
{
return new Base();
}
}
public class Derived : Base
{
public new Derived Clone()
{
return this.CloneImpl() as Derived;
}
protected override object CloneImpl()
{
return new Derived();
}
}
This solution provides both comfort and intended behavior to users, but it's also too long to implement. If we didn't want to have the "comfortable" method returning the current type, it is much more easy to have just public virtual object Clone()
.
So let's see the "ultimate" solution - what in C# is really intented to give us comfort?
public class Base : ICloneable
{
public virtual object Clone()
{
return new Base();
}
}
public class Derived : Base
{
public override object Clone()
{
return new Derived();
}
}
public static T Copy<T>(this T obj) where T : class, ICloneable
{
return obj.Clone() as T;
}
It's named Copy not to collide with the current Clone methods (compiler prefers the type's own declared methods over extension ones). The class
constraint is there for speed (doesn't require null check etc.).
I hope this clarifies the reason why not to make ICloneable<T>
. However, it is recommended not to implement ICloneable
at all.
ICloneable
is for value types, where it could circumvent the boxing of the Clone method, and it implies you have the value unboxed. And as structures can be cloned (shallowly) automatically, there is no need to implement it (unless you make it specific that it means deep copy).
A big problem is that they could not restrict T to be the same class. Fore example what would prevent you from doing this:
interface IClonable<T>
{
T Clone();
}
class Dog : IClonable<JackRabbit>
{
//not what you would expect, but possible
JackRabbit Clone()
{
return new JackRabbit();
}
}
They need a parameter restriction like:
interfact IClonable<T> where T : implementing_type
class A : ICloneable { public object Clone() { return 1; } /* I can return whatever I want */ }
Commented
Jul 27, 2012 at 20:23
ICloneable<T>
could constrain T
to match its own type, that wouldn't force an implementation of Clone()
to return anything remotely resembling the object upon which it was cloned. Further, I would suggest that if one is using interface covariance, it may be best to have classes which implement ICloneable
be sealed, have the interface ICloneable<out T>
include a Self
property which is expected to return itself, and...
ICloneable<BaseType>
or ICloneable<ICloneable<BaseType>>
. The BaseType
in question should have a protected
method for cloning, which would be called by the type which implements ICloneable
. This design would allow for the possibility that one might wish to have a Container
, a CloneableContainer
, a FancyContainer
, and a CloneableFancyContainer
, the latter being usable in code which requires a cloneable derivative of Container
or which requires a FancyContainer
(but doesn't care if it's cloneable).
FancyList
type which could be cloned sensibly, but a derivative might automatically persist its state in a disk file (specified in the constructor). The derived type couldn't be cloned, because its state would be attached to that of a mutable singleton (the file), but that shouldn't preclude use of the derived type in places which need most features of a FancyList
but wouldn't need to clone it.
It's a very good question... You could make your own, though:
interface ICloneable<T> : ICloneable
{
new T Clone ( );
}
Andrey says it's considered a bad API, but i have not heard anything about this interface becoming deprecated. And that would break tons of interfaces... The Clone method should perform a shallow copy. If the object also provides deep copy, an overloaded Clone ( bool deep ) can be used.
EDIT: Pattern i use for "cloning" an object, is passing a prototype in the constructor.
class C
{
public C ( C prototype )
{
...
}
}
This removes any potential redundant code implementation situations. BTW, talking about the limitations of ICloneable, isn't it really up to the object itself to decide whether a shallow clone or deep clone, or even a partly shallow/partly deep clone, should be performed? Should we really care, as long as the object works as intended? In some occasions, a good Clone implementation might very well include both shallow and deep cloning.