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.
Related
Explanation:
I have an abstract class like so:
public abstract class Serializer<T> where T : new()
{
T obj { get; set; }
public string ToXML()
{
// return string XML
}
}
And another class that inherits this abstract class:
public class Account : Serializer<Account>
{
// Code
// I don't want to have to implement the methods of
// the inherited class/interface.
}
I want to access it like such:
Account account = new Account();
Console.WriteLine(account.ToXML());
Question:
Can I do this and pass the account to the property obj so the ToXML can perform its task of converting the object to a string?
Serializer s = new Serializer();
s.ToXML(account);
I'd prefer to have each object inherit the Serialize class and all its methods, and just be able to know that without editing anything but adding the inheritance of the class that I can now access these methods.
On another note, I feel like inheriting a class violates the is-a and can-do principles between choosing an interface or a class, but I don't want to override all the methods, when I already have the code written to do it generically in a class (i.e., I don't want to implement the interface). Is there a way to inherit the methods of an interface like a class (no implementing/overriding).
Try to return this:
T obj { get { return (T)this; } }
But, this means that the child class has to provide itself as a type parameter, it's the curiously recurring "template" pattern... To be sure though, you don't necessarily need to know the type of the object at compile time to serialize it to XML (if you use the usual serializers), so accessing this within the serializer method would be OK, without the need for that type parameter and that property.
I'd personally prefer a more hands off approach to an abstract base class (using the XmlSerializer):
public interface MXmlSerializable { }
public static class XmlSerializable {
public static string ToXml(this MXmlSerializable self) {
if (self == null) throw new ArgumentNullException();
var serializer = new XmlSerializer(self.GetType());
using (var writer = new StringWriter()) {
serializer.Serialize(writer, self);
return writer.GetStringBuilder().ToString();
}
}
}
M stands for mixin. It's effectively a marker interface with an extension method. Use it like this:
public class Account : MXmlSerializable {
...
}
...
Account account = new Account();
...
string accountXml = account.ToXml();
Your construct hardly makes sense. There's no clear point for the Serializer<T> instance to point to itself. You can easily use this and cast it to T. Also, unless your ToXML method implements some really generic XML serialization algorithm (like processing the current instance via reflection), you should make it virtual and place the specific implementations in Serializer<T>'s ancestors.
Also, I would also object your approach to inheritance. If your Account class is in fact a single purpose account serializer, then name it so (AccountSerializer). If not, and Account represents an actual account, then yes, from an independent reader's point of view you are mixing two primary concepts: a business object (Account) and some technical mechanism (Serializer<T>).
If you have a general-purpose serialization algorith, why don't you just have a separate, non-abstract Serializer<T> class, accepting T instances in ToXML()? You will end up with better separation of concerns.
Is there a way to inherit the methods of an interface like a class (no implementing/overriding).
No. Interfaces are interfaces, not classes than embed particular code.
What about just doing it like this;
public string ToXML()
{
convert `this` to xml directly
// return string XML
}
Why do you need the property to return a reference to the class itself.
With the above structure you can do the following, without even needing the T property
Account account = new Account();
Console.WriteLine(account.ToXML());
If your code in the Abstract class will not change just to serialize the object, this approach doesn't make sense, Serialization you do not need to know the object type (Especially if your using xml which means building a easy to access string).
If you want the Serialization only available to certain objects make an interface i.e
public interface ISerialize
{
}
public class Account : ISerialize
{
}
and then create an Extension method
public static class ExtenstionMethods
{
public static string ToXml(this ISerialize obj)
{
// code to build xml string
}
}
This way you can do what you want to do because Account is of the Interface and the ExtensionMethods will only work on that Interface, thus you only need the code in the ExtensionMethods class and then include the namespace wherever you want to use the "ToXml()" etc
Account account = new Account();
Console.WriteLine(account.ToXML());
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.
I have class called Employee with 3 property called ID,Name,Dept. I need to implement the Copy and Clone method? When I am using Copy or Clone method I need to avoid Casting? how will I do that?.
example: same as DataTable which is having DataTable.Copy() and DataTable.Clone().
You need to implement IClonable interface and provide implementation for the clone method. Don't implement this if you want to avoid casting.
A simple deep cloning method could be to serialize the object to memory and then deserialize it. All the custom data types used in your class need to be serializable using the [Serializable] attribute. For clone you can use something like
public MyClass Clone()
{
MemoryStream ms = new MemoryStream();
BinaryFormatter bf = new BinaryFormatter();
bf.Serialize(ms, this);
ms.Position = 0;
object obj = bf.Deserialize(ms);
ms.Close();
return obj as MyClass;
}
If your class only has value types, then you can use a copy constructor or just assign the values to a new object in the Clone method.
Do you have to use the ICloneable interface or is it enough if you just have two methods called Clone and Copy that is defined in a generic interface?
public class YourClass : ICloneable<YourClass>
{
// Constructor logic should be here
public YourClass Copy() { return this; }
public YourClass Clone() { return new YourClass(ID, Name, Dept); }
}
interface IClonable<T>
{
T Copy();
T Clone();
}
Or have I misunderstood something?
What I am trying to say is that you don't have to make it more complex than it is? If you need your objects to conform to somekind of you can write it yourself if the one specified in the .Net framework is to complex for the situation. You should also define the difference with Clone and Copy, that is, what do they mean to you? I know there are several sites specifying that Clone is a deep copy and Copy is a shallow copy.
Do you mean, how to implement ICloneable.Clone() and have it return the type of the class itself.
public class MyType : ICloneable
{
public MyType Clone() //called directly on MyType, returns MyType
{
return new MyType(/* class-dependant stuff goes here */);
}
object ICloneable.Clone() // called through ICloneable interface, returns object
{
return Clone();
}
}
Check this Object Cloning Using IL in C# http://whizzodev.blogspot.com/2008/03/object-cloning-using-il-in-c.html
I often see copy constructors suggested as an alternative to a cloning method, but except with sealed classes the behaviors are very different. If I have a type Car, which simply supports properties VIN, BodyColor and BodyStyle, and a derivative type FancyCar, which also supports InteriorFabric and SoundSystem, then code which accepts an object of type Car and use the Car copy constructor to duplicate it will end up with a Car. If a FancyCar is passed to such code, the resulting "duplicate" will be a new Car, which has a VIN, BodyColor, and BodyStyle that match the original car, but which will not have any InteriorFabric or SoundSystem. By contrast, the code were to accept a Car and use a cloning method on it, passing a FancyCar to the code would cause a FancyCar to be produced.
Unless one wants to use Reflection, any cloning method must at its base involve a call to base.MemberwiseClone. Since MemberwiseClone is not a virtual method, I would suggest defining a protected virtual cloning method; you may also want to prevent any child classes from calling MemberwiseClone by defining a dummy nested class of protected scope with the same name (so if a descendent class tries to call base.MemberwiseClone, it wouldn't be interpreted as a nonsensical reference to the dummy class).
Here an example:
namespace XXX
{
[Serializable]
public class ItemChecklist : ICloneable
{
// [...here properties, attributes, etc....]
object ICloneable.Clone()
{
return this.Clone();
}
public ItemChecklist Clone()
{
return (ItemChecklist)this.MemberwiseClone();
}
}
}
i.e If you use this function, you will have in "itemAdd" a entire copy of the object "itemTemp" with all its values.
ItemChecklist itemAdd = itemTemp.Clone();
I've been dealing a lot lately with abstract classes that use generics. This is all good and fine because I get a lot of utility out of these classes but now it's making for some rather ugly code down the line. For example:
abstract class ClassBase<T>
{
T Property { get; set; }
}
class MyClass : ClassBase<string>
{
OtherClass PropertyDetail { get; set; }
}
This implementation isn't all that crazy, except when I want to reference the abstract class from a helper class and then I have to make a list of generics just to make reference to the implemented class, like this below.
class Helper
{
void HelpMe<C, T>(object Value)
where C : ClassBase<T>, new()
{
DoWork();
}
}
This is just a tame example, because I have some method calls where the list of where clauses end up being 5 or 6 lines long to handle all of the generic data. What I'd really like to do is
class Helper
{
void HelpMe<C>(object Value)
where C : ClassBase, new()
{
DoWork();
}
}
but it obviously won't compile. I want to reference ClassBase without having to pass it a whole array of generic classes to get the function to work, but I don't want to reference the higher level classes because there are a dozen of those. Am I the victim of my own cleverness or is there an avenue that I haven't considered yet?
I suppose that your HelpMe method would be used for initializing the concrete ClassBase<T> type (a guess based on the constraints). To keep the code fully generic (if you need both T and C somewhere in the method), you probably need to keep both of the type parameters.
However, you could add a non-generic base class and then write something like this:
abstract class ClassBase {
object UntypedProperty { get; set; }
}
abstract class ClassBase<T> : ClassBase {
T Property { get; set; }
public override object UntypedProperty {
get { return Property; }
set { Property = (T)value; }
}
}
Then you could be to write the helper method like this:
void HelpMe<C>(object Value) where C : ClassBase, new() {
var n = new C();
c.UntypedProperty = Value;
}
Depending on your specific scenario, something along these lines might work and make the code a little bit simpler. However, you need to modify the base class to make this possible.
Generics do have a tendency to propogate through the code, and they're seldomly used as "mixin" classes for that reason.
Thomas mentioned the one possibility of introducing an equivalent non-generic API. I would prefer revisiting the design and making these base classes non-generic if possible while maintaining full type safety. Whether or not this is actually possible is determined by your requirements.
There is one other possibility short of a re-design (or API duplication): dynamic. If you're willing to lose IntelliSense in your helper methods (and are willing to pay a very, very small runtime performance penalty), you can use dynamic in your helper method:
class Helper
{
void HelpMe<C>(object Value)
// where C : ClassBase<T>, new() // no longer needed, but should be documented
{
dynamic cObj = Activator.CreateInstance<C>(); // instead of "new C()"
cObj.PropertyDetail = ...;
cObj.Property = ...;
...
}
}
without having to pass it a whole array of generic classes to get the function to work
A small change might ease these calls. Move repeatedly specified types to the generic declaration of the class.
//before
Helper x = new Helper();
x.HelpMe<MyClass, string>(x);
x.HelpMe<MyClass, string>(y);
//after
Helper x = new Helper<string>();
x.HelpMe<MyClass>(x);
x.HelpMe<MyClass>(y);
//the change
class Helper<T>
{
void HelpMe<C>(object Value)
where C : ClassBase<T>, new()
{
DoWork();
}
}
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.