I have the following:
public class InstanceList : List<Instance> {}
I would like to make this cloneable. Following the example here: Why no ICloneable<T>?
I tried the following:
public interface ICloneable<T> : ICloneable Where T : ICloneable<T>
{ new T Clone(); }
public class InstanceList : List<Instance>, ICloneable<List<Instance>> {}
But I get a compiler error. The error message states that
List<Instance>
must be convertible to
ICloneable<List<Instance>>
in order to use parameter T in the generic interface
ICloneable<T>.
What am I missing here?
You can't do this, because you can't define List<T> yourself. You would only be able to do this if you could declare your own List<T> because of the way you've constrained ICloneable<T>. Since List<T> truly doesn't implement ICloneable<T>, you're going to have to have the type of T be InstanceList instead, which you do have control over.
Here's how you would implement it:
public class InstanceList : List<Instance>, ICloneable<InstanceList>
{
public InstanceList Clone()
{
// Implement cloning guts here.
}
object ICloneable.Clone()
{
return ((ICloneable<InstanceList>) this).Clone();
}
}
public class Instance
{
}
public interface ICloneable<T> : ICloneable where T : ICloneable<T>
{
new T Clone();
}
Of course, there is another alternative you could do. You could widen your generics a little bit, to create a CloneableList<T> type:
public class CloneableList<T> : List<T>, ICloneable<CloneableList<T>>
{
public CloneableList<T> Clone()
{
throw new InvalidOperationException();
}
object ICloneable.Clone()
{
return ((ICloneable<CloneableList<T>>) this).Clone();
}
}
public interface ICloneable<T> : ICloneable where T : ICloneable<T>
{
new T Clone();
}
And if you really want to get fancy, create something that restricts T to ICloneable. Then you could implement ICloneable on the Instance class, and anything else you want to include in an ICloneable<T> list, thus treating every CloneableList<T> in the exact same way, avoiding a different implementation of ICloneable<T> for each and every cloneable list you want to create.
public class CloneableList<T> : List<T>, ICloneable<CloneableList<T>> where T : ICloneable
{
public CloneableList<T> Clone()
{
var result = new CloneableList<T>();
result.AddRange(this.Select(item => (T) item.Clone()));
return result;
}
object ICloneable.Clone()
{
return ((ICloneable<CloneableList<T>>) this).Clone();
}
}
public interface ICloneable<T> : ICloneable where T : ICloneable<T>
{
new T Clone();
}
The problem is your generic constraint where T : IClonable<T>. Because you're "instantiating" your interface as ICloneable<List<Instance>>, List<Instance> is your T, and so the generic constraint translates to where List<Instance> : IClonable<List<Instance>>. List<Instance> does not fulfill that constraint.
Perhaps you're trying to do something like this:
public interface ICloneableList<T> : ICloneable where T : ICloneable
{
}
To add to the other good answers already there - when you clone, you expect to get an identical copy back, right? So instead of:
public class InstanceList : List<Instance>, ICloneable<List<Instance>> {}
Shouldn't it actually be:
public class InstanceList : List<Instance>, ICloneable<InstanceList> {}
That way you will also get no compiler errors.
I don't think you can really do what you want. While it is useful not to require the type argument of ICloneable<T> to implement ICloneable<T>, I don't think the List<T> class can be very well extended to support cloning since it does not provide any means of detaching or duplicating the array which holds all the data items, does not allow a subclass access to that array, and does not allow a subclass to override enough virtual methods to render the array irrelevant. Although clone methods should start by using MemberwiseClone (to ensure that the cloned object is the same type as the original), there would be no guaranteed way to force the newly-cloned list to create a new array to hold its objects without disturbing the old one.
The closest thing I can suggest to doing what you want would be to define an ICloneableList<T> which inherits from IList<T> and ICloneable<IList<T>> and define an CloneableList class which implementats that by wrapping a list. Cloning a CloneableList should create a new List<T> with items copied from the old one, which can be done by using the appropriate constructor for the new List.
Related
I'm reposting a question I've just asked, but want to re-ask this question in a more concise way as I think I was causing some confusion.
I have a base class: RoomObject.
I have two subclasses: Bed and Table, which inherit from RoomObject.
I have a variable currentObject, which is of type RoomObject, but will actually hold either an instance of Bed or Table (RoomObject is never instantiated itself).
How can I clone my currentObject, without knowing its full type?
i.e. if currentObject is a Bed, I want to clone the Bed using
currentObject = new Bed(currentObject);
and if currentObject is a Table, I want to use
currentObject = new Table(currentObject);
I could use reflection, by calling Activator.CreateInstance(currentObject.GetType()), and then copy across any attributes I need, but this seems messy.
You should use the pattern known as the virtual constructor, or a cloning method.
Add a virtual method to RoomObject that returns a copy of the current object:
abstract RoomObject Clone();
Now implement this method in Bed to return new Bed(...), and in the Table to return new Table(...). Pass whatever parameters necessary to the constructors of Bed and Table to copy what's in the current object.
.NET has an interface ICloneable that is commonly used to implement this pattern. A small disadvantage of that approach is that Clone must return object, not RoomObject, so if you need RoomObject, you'd need to cast it.
This is one of the best things about reflection: The ability to create an object without your client code knowing what type it is. Sometimes it can get messy, or even slow down the code at times, but--if used correctly--will make your code a lot more manageable.
For example, take a look at the Factory Pattern, and how one can implement it with Reflection and here as well
I think one solution would be implement ICloneable interface for all your objects. Here's some sample code:
class RoomObject : ICloneable
{
public abstract object Clone();
}
class Bed : ICloneable
{
public override object Clone()
{
return new Bed();
}
}
class Table : ICloneable
{
public override object Clone()
{
return new Table();
}
}
class Program
{
public static void Main(String[] args)
{
RoomObject ro = /* from some other places*/
RoomObject newOne = ro.Clone() as RoomObject; /* here's what you what */
}
}
Instead of that, implement the ICloneable interface that's out-of-the-box on .NET Framework as others said in their answers.
Since ICloneable.Clone() method returns object, what about a custom ICloneable<T> that also implements ICloneable?
public interface ICloneable<T> : ICloneable
where T : class
{
T TypedClone();
}
public class MyCloneableObject : ICloneable<MyCloneableObject>
{
public string Some { get; set; }
public object Clone()
{
MyCloneableObject clone = new MyCloneableObject { Some = this.Some };
}
public MyCloneableObject TypedClone()
{
return (MyCloneableObject)Clone();
}
}
Later, in your code...
MyCloneableObject some = new MyCloneableObject();
if(some is ICloneable<MyCloneableObject>)
{
MyCloneableObject myClone = some.TypedClone();
// .. or the standard `Clone()`:
myClone = (MyCloneableObject)some.Clone();
}
Implementing both built-in and custom interface is a good idea, as your cloneable will operate with other libraries that may accept ICloneable implementations.
Finally, rather than using reflection, this case should be solved in design-time. I would argue that doing with reflection should be done if you can't modify the library containing the ICloneable wannabe.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Casting an object to two interfaces at the same time, to call a generic method
I'm fairly sure you can't do this so I'm wondering if there's a workaround, but I need/want to cast an object to represent multiple interfaces for use with generic constraints. For example:
public void Foo<T>(T t) where T : IInterfaceA, IInterfaceB
{
}
If I have an object I want to say something like var t = (IInterfaceA | IInterfaceB)someObj; so I can pass t into this method.
Is there a nifty way of doing this? I'm using C# 3.5 so no dynamic available, but if it's possible with dynamic please post it anyway.
EDIT
Despite the answer below, I would say the better solution is the one that most other answers point to. (This assumes that you can redefine the multiple classes that implement both interfaces.)
Create an interface that inherits from both InterfaceA and InterfaceB, then, for all classes that implement interfaces A and B, replace those interfaces with the new one. Before:
class SomeClass : IInterfaceA, IInterfaceB { }
class AnotherClass : IInterfaceA, IInterfaceB { }
class AdditionalClass : IInterfaceA, IInterfaceB { }
After:
interface IInterfaceC : IInterfaceA, IInterfaceB { }
class SomeClass : IInterfaceC { }
class AnotherClass : IInterfaceC { }
class AdditionalClass : IInterfaceC { }
The implementation of Foo is then fairly trivial. And, again, since you don't know at compile time what type you have on hand, you may be able just to declare it as
public void Foo(IInterfaceC someObj) { }
END EDIT
You can do it using reflection, though some will say that this isn't particularly "nifty":
public class FooClass
{
public void Foo<T> (T t) where T : IInterfaceA, IInterfaceB
{
//... do your thing here
}
private static void Example(object someObj)
{
var type = someObj.GetType();
if(typeof(IInterfaceA).IsAssignableFrom(type) && typeof(IInterfaceB).IsAssignableFrom(type))
{
var genericMethod = typeof(FooClass).GetMethod("Foo");
var constructedMethod = genericMethod.MakeGenericMethod(type);
var instance = new FooClass();
var result = constructedMethod.Invoke(instance, new [] { someObj });
Assert.IsNull(result);
}
}
}
you could also do this, which could allow you to make Foo non-generic. It's also fairly ugly, so I would hide this ugliness by making it private:
private void PrivateFoo(IInterfaceA objA, IInterfaceB objB)
{
if (!ReferenceEquals(objA, objB))
throw new ArgumentException("objA and objB must refer to the same object");
//... do your thing here
}
public void Foo(object someObj)
{
PrivateFoo((IInterfaceA)someObj, (IInterfaceB)someObj);
}
public void Foo<T>(T t) where T : IInterfaceA, IInterfaceB{}
{
// T already implements IInterfaceA and IInterfaceB, just call the methods.
t.MethodFromA();
t.MethodFromB();
}
T t = (T)someObj;
This will cast the object to T, not the two interfaces... so it's up to you to make sure that'll work.
IInterfaceA tA = (IInterfaceA)someObj;
IInterfaceB tB = (IInterfaceB)someObj;
Two references to the one instance.
No, there is no way.
The only thing that comes close is to create another interface the inherits those two interfaces. But then all your classes need to implement that third interface instead of the two others, so in most circumstances, this is not practical.
Just cast it to that interface you need at that moment.
UPDATE:
The only way I can see is to create a container class that implements those interfaces:
class Container : IInterfaceA, IInterfaceB
{
private object _obj;
public Container(object obj)
{
// Check that the object really implements those two interfaces.
_obj = obj;
}
void IInterfaceA.Method1()
{
((IInterfaceA)_obj).Method1();
}
// And so on for all methods of the interfaces.
}
One possible way to do this is have inheritance of interfaces. Move the common functionality, to the parent interface, which should be used in above scenarios.
You may be able to restructure your inheritance to achieve your goal.
public class ArrayList : IList, ICollection, IEnumerable, ICloneable
public interface IList : ICollection, IEnumerable
Note that ArrayList inerits from IList, which inherits from two interfaces already included in Arraylist.
I'm writing two APIs that I will use with many of my projects. Some projects my use one of the APIs, some the other, but the majority of my projects will use both. I'm trying to design them as if they're completely separate, but I'm struggling on one thing.
namespace FirstApi {
public abstract class MyBaseClass {
//constructor, some methods and properties
public IEnumerable<T> Search<T>() where T : MyBaseClass, new() {
//search logic here. must use generics as I create new instances of T here
}
}
}
namespace SecondApi {
public interface IMyInterface {
//some property and method signatures
IEnumerable<T> Search<T>() where T : IMyInterface, new();
}
}
namespace MyProject {
public class MyDerivedClass : MyBaseClass, IMyInterface {
}
}
Both APIs require this search method. The second API has some functionality in other classes that calls IMyInterface.Search<T>(), and I would like those classes that inherit MyBaseClass to use the Search<T> function defined in MyBaseClass.
Compilation error: The constraints for type parameter 'T' of method 'MyBaseClass.Search()' must match the constraints for type parameter 'T' of interface method 'IMyInterface.Search()'. Consider using an explicit interface implementation instead.
Note: When Search is called, T will always be the derived class of whichever abstract class or interface has been inherited. This was the only way I could find of achieving this in C# 2.0 (C# abstract class return derived type enumerator), and it's just caused more problems!
Is there a type-safe way that I can achieve this, without using objects and casting?
Solution:
Based on the accepted answer by Andras Zoltan, I created this class in my project, and will have to re-create this class for each project that uses both APIs.
public abstract class ApiAdapter<TAdapter> : MyBaseClass, IMyInterface where TAdapter: MyBaseClass, IJsonObject, new()
{
IEnumerable<T> IJsonObject.Search<T>()
{
foreach (TAdapter row in base.Search<TAdapter>())
yield return (T)(IMyInterface)row;
}
}
I then inherit this class like so.
public class Client : ApiAdapter<Client> {
//everything else can go here
}
You can explicitly implement the interfaces Search method, e.g.
public class MyDerivedClass : BasicTestApp.FirstApi.MyBaseClass, BasicTestApp.SecondApi.IMyInterface
{
IEnumerable<T> SecondApi.IMyInterface.Search<T>()
{
// do implementation
}
}
However, I think you are asking for the MyBaseClass Search method to be called when the part of the code that handles your object as IMyInterface calls the Search<T> method. I cannot see a way because you have two T types with different constraints that cannot be related.
If you did where T : BasicTestApp.FirstApi.MyBaseClass, IMyInterface, new(); in both definitions of the Search method then you would not have a problem but this would tie both your APIs together
Here is a possible implementation of your explicitly implemented interface method. It doesn't avoid the cast but at least keeps it neat.
IEnumerable<T> SecondApi.IMyInterface.Search<T>()
{
var results = base.Search<MyDerivedClass>();
return results.Cast<T>();
}
I started my answer with exposition on why it's not working for you, but I think that's well understood now so I'll leave it out.
I've upvoted #IndigoDelta's answer but it highlights something I don't like about the overall design here - I have a sneaking suspicion you should actually be using a generic interface and generic class; not generic methods because it doesn't make any sense that:
Note: When Search is called, T will always be the derived class of whichever abstract class or interface has been inherited.
I'm throwing this solution into the mix; which I think is better because it means that each derived type doesn't need to reimplement the IMyInterface.Search method, and it goes some way to actually enforcing this rule you mention. It's a generic type dedicated to join the two APIs together, meaning the derived types don't need to do anything:
namespace MyProject
{
using FirstApi;
using SecondApi;
public class SecondAPIAdapter<T2> : MyBaseClass, IMyInterface
where T2 : SecondAPIAdapter<T2>, new()
{
#region IMyInterface Members
IEnumerable<T> IMyInterface.Search<T>()
{
return Search<T2>().Cast<T>();
}
#endregion
}
//now you simply derive from the APIAdapter class - passing
//in your derived type as the generic parameter.
public class MyDerivedClass : SecondAPIAdapter<MyDerivedClass>
{ }
}
i think you can do explicit implementation of interface and when you will access methor thru IMyInterface.Search - compiler will run the right method.
You need to use an explicit implementation.
public class MyDerivedClass : MyBaseClass, IMyInterface
{
// The base class implementation of Search inherited
IEnumerable<T> IMyInterface.Search<T>()
{
// The interface implementation
throw new NotImplementedException();
// this would not work because base does not implement IMyInterface
return base.Search<T>();
}
}
Since the implementations are different this makes sense. If they are not different then either the base class should implement the interface and you should use covariance (.Net 4.0 only) to combine your contraints or, perhaps you don't need the interface at all.
I hope I'm not confused, could you not change your definitions, such that:
public interface IMyInterface<in T>
{
//some property and method signatures
IEnumerable<U> Search<U>() where U : T, new();
}
Providing a generic argument of T which can use to enforce that the implementation provides a search function constraint to types of T:
public abstract class MyBaseClass : IMyInterface<MyBaseClass>
{
public virtual IEnumerable<T> Search<T>() where T : MyBaseClass, new()
{
}
}
That way, your derived types are simply:
public class MyDerivedClass : MyBaseClass
{
}
Which you can then do searches as:
var derived = new MyDerivedClass();
IMyInterface<MyDerivedClass> iface = impl;
var results = iface.Search<MyDerivedClass>();
Basically my setting is this:
public abstract class BaseObject{
public abstract BaseObject Clone();
}
public class DerivedObject : BaseObject{
public DerivedObject Clone()
{
//Clone logic
}
}
The above code doesn't compile because it isn't possible to change the return type when overriding a method.
Is it possible to achieve that every derived type's Clone method returns an argument of it's own type (maybe through generics)?
Well, C# doesn't allow covariant return types as you've found... but you can use generics:
public abstract class BaseObject<T> where T : BaseObject<T>
{
public abstract T Clone();
}
public class DerivedObject : BaseObject<DerivedObject>
{
public override DerivedObject Clone()
{
// ...
}
}
This solution can be a pain in various ways - not least because it's hard to understand - but it can work reasonably well in many situations.
EDIT: The reason I've included the constraint on T is so that BaseObject can call "its own" methods on instances of T, which is usually very handy. If you don't need this though, you can lose the constraint.
You can do something like this. Instead of returning default(T), return something based on the cloning logic.
public class MyBase<T>
{
public T Clone()
{
return default(T);
}
}
public class MyDerived : MyBase<MyDerived>
{
}
By the way, for object cloning I like to serialize the current object to memory using the binary serializer, then deserialize that memory back into a new instance.
I have the following interface:
internal interface IRelativeTo<T> where T : IObject
{
T getRelativeTo();
void setRelativeTo(T relativeTo);
}
and a bunch of classes that (should) implement it, such as:
public class AdminRateShift : IObject, IRelativeTo<AdminRateShift>
{
AdminRateShift getRelativeTo();
void setRelativeTo(AdminRateShift shift);
}
I realise that these three are not the same:
IRelativeTo<>
IRelativeTo<AdminRateShift>
IRelativeTo<IObject>
but nonetheless, I need a way to work with all the different classes like AdminRateShift (and FXRateShift, DetRateShift) that should all implement IRelativeTo. Let's say I have a function which returns AdminRateShift as an Object:
IRelativeTo<IObject> = getObjectThatImplementsRelativeTo(); // returns Object
By programming against the interface, I can do what I need to, but I can't actually cast the Object to IRelativeTo so I can use it.
It's a trivial example, but I hope it will clarify what I am trying to do.
If I understand the question, then the most common approach would be to declare a non-generic base-interface, i.e.
internal interface IRelativeTo
{
object getRelativeTo(); // or maybe something else non-generic
void setRelativeTo(object relativeTo);
}
internal interface IRelativeTo<T> : IRelativeTo
where T : IObject
{
new T getRelativeTo();
new void setRelativeTo(T relativeTo);
}
Another option is for you to code largely in generics... i.e. you have methods like
void DoSomething<T>() where T : IObject
{
IRelativeTo<IObject> foo = // etc
}
If the IRelativeTo<T> is an argument to DoSomething(), then usually you don't need to specify the generic type argument yourself - the compiler will infer it - i.e.
DoSomething(foo);
rather than
DoSomething<SomeType>(foo);
There are benefits to both approaches.
unfortunately inheritance doesn't work with generics. If your function expects IRelativeTo, you can make the function generic as well:
void MyFunction<T>(IRelativeTo<T> sth) where T : IObject
{}
If I remember correctly, when you use the function above you don't even need to specify the type, the compiler should figure it out based on the argument you supply.
If you want to keep a reference to one of these IRelativeTo objects inside a class or method (and you don't care what T is that), you need to make this class/method generic again.
I agree, it is a bit of pain.
If all you care about is that IRelativeTo deals with IObjects then you don't need to make it generic:
interface IRelativeTo
{
IObject getRelativeTo();
void setRelativeTo(IObject relativeTo)
}
The implementing classes may still be generic, however:
abstract class RelativeTo<T> : IRelativeTo where T : IObject
{
public virtual T getRelativeTo() {return default(T);}
public virtual void setRelativeTo(T relativeTo) {}
IObject IRelativeTo.getRelativeTo() {return this.getRelativeTo(); }
void IRelativeTo.setRelativeTo(IObject relativeTo)
{ this.setRelativeTo((T) relativeTo);
}
}
class AdminRateShift : RelativeTo<AdminRateShift>, IObject {}
Then you can do this:
IRelativeTo irt = new AdminRateShift();
IObject o = irt.getRelativeTo();
irt.setRelativeTo(o);