I want create interface with CloneInstance method which return Generic class of that instance. For Example:
public interface ICloneableExtended<T> where T : this
{
T CloneInstance();
}
public class Car : ICloneableExtended
{
...
...
public Car CloneInstance()
{ .. }
}
Foo()
{
Car car ...;
var clonedCar = car.CloneInstance();
}
In definition of class Car, I need use only ICloneableExtended, not ICloneableExtended<T>. Is there some way how to do this?
You can accept a generic T parameter for the concrete class that will implement ICloneableExtended:
interface ICloneableExtended<T> {
Clone();
}
class Car : ICloneableExtended<Car> {
public Car Clone() {
throw new NotImplementedException();
}
}
You may consider to make T parameter covariant (if you wish to keep ICloneableExtended<Car> with many concrete classes - which will implement ICloneableExtended<T>):
interface ICloneableExtended<out T> {
Clone();
}
Note that you may not need a generic-less interface, you already have ICloneable (with all its drawbacks and misuses):
interface ICloneableExtended<out T> : ICloneable {
Clone();
}
For binary serializable types you may even implement a basic and reusable (but pretty inefficient) base class:
interface ICloneableExtended<T> : ICloneable {
T Clone();
}
abstract class Cloneable<T> : ICloneableExtended<T> {
public virtual T Clone() {
using (var ms = new MemoryStream()) {
var formatter = new BinaryFormatter();
formatter.Serialize(ms, this);
ms.Seek(0, SeekOrigin.Begin);
return (T)formatter.Deserialize(ms);
}
}
object ICloneable.Clone() {
return Clone();
}
}
sealed class Car : Cloneable<Car> { }
With this approach each concrete class must implement ICloneableExtended<T> but you can't overload Clone() method differentiating only with return value then you'd better to implement ICloneableExtended<T> explicitly. A less confusing way (both for who implements this interface and who will use it) is to provide an extension method:
static class Extensions {
public static T Clone<T>(this object obj) {
var cloneable = obj as ICloneable;
if (cloneable != null)
return (T)cloneable.Clone();
using (var ms = new MemoryStream()) {
return (T)...
}
}
}
(here I'm using ICloneable for clarity but if you don't want to use it because of its worldwide random usage then just pick your own equivalent non-generic interface).
Related
I want a base class method to return the type of the derived class.
according to Can a Base Class Method return the type of the derived class?
base class
public class BaseClass<T>
{
}
extension
public static class ExtensionMethods
{
public static U Project<U, T>(this U node)
where U : BaseClass<T>
{
// do something
return node;
}
}
child class
public class ChildClass: BaseClass<string>
{
}
usage
var child= new ChildClass();
var projected = child.Project(); // error: can't infer type T
var projected = child.Project<ChildClass, string>(); // ok
Question:
One solution is How to return a derived class using only code in the base class?, but class inherit from child class won't work.
How can I use method without specifying T?
The answer is that you need to provide all the piping to clone objects manually and let the compiler decide the correct overload.
public interface ICSGNode<T> where T:ICSGNode<T>
{
T Clone();
void Invert();
}
public class NodeList<T> : Collection<T>
where T : ICSGNode<T>
{
public NodeList(params T[] items) : base(items) { }
public static implicit operator NodeList<T>(T[] array) { return new NodeList<T>(array); }
public T[] Clone() { return this.Select((n) => n.Clone()).ToArray(); }
}
public class PolygonNode : ICSGNode<PolygonNode>, ICloneable
{
public PolygonNode()
{
// create a unique object
}
public PolygonNode(PolygonNode other)
{
// create a copy
}
public void Invert()
{
throw new NotImplementedException();
}
public PolygonNode Clone() { return new PolygonNode(this); }
object ICloneable.Clone() { return Clone(); }
}
public class PolygonList : NodeList<PolygonNode>, ICloneable
{
public PolygonList(params PolygonNode[] items) : base(items) { }
public static implicit operator PolygonList(PolygonNode[] array) { return new PolygonList(array); }
public new PolygonList Clone() { return new PolygonList(base.Clone()); }
object ICloneable.Clone() { return Clone(); }
}
class Program
{
static void Main(string[] args)
{
var list = new PolygonList
{
new PolygonNode(),
new PolygonNode(),
new PolygonNode(),
};
var clone = list.Clone();
// clone is of `PolygonList` type
}
}
Tricks Used:
Apply the ICloneable interface to the derived classes only.
Overwrite the default behavior of returning object with a strongly typed Clone() method.
Implement copy constructors that copy objects properties within the constructor only.
Lastly avoid returning a concrete or generic collection type from any base Clone() method. Return an array and let the derived class assembly the strongly typed clone from the array of items.
I have a generic interface (MyInterface<T>), which is implemented by the class ChildA in the example below:
public interface MyInterface<T>
{
MyObj<T> GetObj(); // Irrelevant
}
class ChildA : MyInterface<ChildA>
{
// Irrelevant:
MyObj<ChildA> GetObj() {
return new MyObj<ChildA>();
}
}
This works, but I need to make sure that <T> always has the type of the implementing class, so in this case T should always be of type ChildA, because it is implemented by ChildA.
Another correct implementation could be this, for example:
class ChildB : MyInterface<ChildB> { ... }
But currently, this incorrect implementation is also possible, while it should not be:
class ChildA : MyInterface<ChildB> { ... }
Is there a way to enforce this?
You cannot enforce a generic type argument to be constrained to the implementing type.
The available type constraints are the following:
where T : struct
where T : class
where T : new()
where T : <base class name>
where T : <interface name>
where T : U
There is nothing like where T : self in C#. Actually, it wouldn't even make sense, because such a thing cannot be meaningfully enforced. Besides, it wouldn't fit at all into the covariance/contravariance concepts and would be weird to inherit from, in general.
The closest thing you can do is this:
public interface IMyInterface<T> where T : IMyInterface<T>
{
MyObj<T> GetObj();
}
Why it wouldn't make sense
Let's say you could do this:
public interface IMyInterface<T> where T : self // this syntax does not exist in C#
{
MyObj<T> GetObj();
}
Now all implementing types would have to use themselves as the type argument. But you could still do this:
public class ChildC<T> : IMyInterface<T> where T : self
{
/* ... */
}
Which would go around your restriction.
Is there a way to enforce this?
Well, not with generic constraints. You can do that with reflection though i'd vote against it :
public abstract class BaseChild<T> : MyInterface<T>
{
protected BaseChild()
{
if (typeof(T) != this.GetType())
{
throw new InvalidOperationException(string.Format(
"Type {0} is not supported as valid type parameter for type {1}",
typeof(T).Name, this.GetType().Name));
}
}
}
Example :
class ChildA : BaseChild<int> { }
// Bang! throws
var instance = new ChildA();
.
class ChildB : BaseChild<ChildB> { }
// Ok here
var instance = new ChildB();
You cannot do this but you can create your own control comparing the generic type of the interface and the type of your class. See the example:
class ChildA : MyInterface<ChildB>
{
public ChildA()
{
this.ValidateGenericType();
}
public MyObj<ChildB> GetObj()
{
return new MyObj<ChildB>();
}
protected void ValidateGenericType()
{
//throws an Exception because ChildB is different of ChilA
if (this.GetType().Name != this.GetType().GetInterfaces()[0].GetGenericArguments()[0].Name)
{
throw new Exception("The generic type must be of type ChildA.");
}
}
}
It seems that you should use extension methods instead of enforcing some interface for this purpose
public interface ISomeInterface {}
public class Child: ISomeInterface {}
public class OtherChild : ISomeInterface { }
public static class MyInterfaceExtensions
{
public static MyObj<T> GetMyObj<T>(this T child) where T : ISomeInterface
{
return new MyObj<T>();
}
}
public static class Test
{
public static void RunTest()
{
var child = new Child();
var otherChild = new OtherChild();
MyObj<Child> myObj = child.GetMyObj();
MyObj<OtherChild> myOtherObj = otherChild.GetMyObj();
}
}
As I've learned that it is not advised to implement ICloneable (due to the fact that it does not differentiate between Deep Copy or Shallow Copy), I'm trying to determine whether I should implement it as an abstract or an interface.
I feel that my implementation would stay largely the same, e.g. a binary deep copy and a MemberwiseClone shallow copy, so to that end I felt that an abstract method would be ideal. However, my understanding is also that C# does not do Multiple Inheritance, thus if I ever need to use another abstract class, then I no longer can.
In that case, I feel that implementing a custom ICloneable (e.g. ICustomCloneable) would be the better option, but if the implementation is effectively the same across many classes, I feel like I'm not adequately taking advantage of code reuse.
That being said, is it valid to use an interface to keep the abstract inheritance clear for more important things in my cloneable classes? Or is there another way to do this?
Alternatively, is it valid (read: not smelly) for an abstract to implement another abstract? This would be my guess as to getting around the single-inheritance that prevents me from implementing the CloneCapable class as well as another abstract, but it sounds like it might be questionable. e.g.:
public abstract class CloneCapable
{
public object ShallowCopy()
{
// implementation
}
public object DeepCopy()
{
// implementation
}
}
public abstract class ClassA : CloneCapable {}
// abstract-abstract since I can't do ClassB : ClassA, CloneCapable
public abstract class ClassB : ClassA {}
I would definitely make this an interface. The reason being is that, interfaces are supposed to be very general, and that's why we can implement multiple interfaces. If there is some boiler-plate code you want to write, there's nothing stopping you taking advantage of both interfaces and abstract classes.
public interface ICustomCloneable<T>
{
T ShallowCopy();
T DeepCopy();
}
public abstract class CustomCloneable<T> ICustomCloneable<T> where T : class
{
public T ShallowCopy() { return ShallowCopy(this); }
public T DeepCopy() { return DeepCopy(this); }
// static helpers
public static object ShallowCopy(T obj) { /* boilerplate implementation */ }
public static object DeepCopy(T obj) { /* boilerplate implementation */ }
}
public class ClassA : CustomCloneable<ClassA> { /* Use boilerplate functionality */ }
public class ClassB : SomeOtherClass, ICustomCloneable<ClassB>
{
// implement ICustomCloneable using static helpers
public ClassB ShallowCopy() { return CustomCloneable<ClassB>.ShallowCopy(this); }
public ClassB DeepCopy() { return CustomCloneable<ClassB>.DeepCopy(this); }
}
I've used generics here, but there's no reason you need to... It might even be desirable not to. This approach allows you to write boilerplate code, but not be tied down by it:
public class ClassC : ICustomCloneable<ClassC>
{
public ClassC ShallowCopy() { /* Do special cloning for ClassC */ }
public ClassC DeepCopy() { /* Do special cloning for ClassC */ }
}
I was thinking that creating an interface was the way to go, but then I found this question and the first answer. That is a fine way to do the cloning, but I thought that that might go very well with a Custom Extension Method so I wrote the fallowing code, based on the code in the first post and on the MS help page:
Some classes to play with:
[Serializable]
public abstract class Base
{
public abstract int m1();
}
[Serializable]
public class derived : Base
{
public int a = 42;
public override int m1()
{
throw new NotImplementedException();
}
}
A class with an extension method based on code samples from both linkes
//Extension methods must be defined in a static class
public static class StringExtension
{
// This is the extension method.
// The first parameter takes the "this" modifier
// and specifies the type for which the method is defined.
public static T MyCloneExtension<T>(this T t)
{
// Code in this function was copied from https://stackoverflow.com/questions/78536/deep-cloning-objects-in-c-sharp
if (!typeof(T).IsSerializable)
{
throw new ArgumentException("The type must be serializable.", "source");
}
// Don't serialize a null object, simply return the default for that object
if (Object.ReferenceEquals(t, null))
{
return default(T);
}
IFormatter formatter = new BinaryFormatter();
Stream stream = new MemoryStream();
using (stream)
{
formatter.Serialize(stream, t);
stream.Seek(0, SeekOrigin.Begin);
return (T)formatter.Deserialize(stream);
}
}
}
and finally a call to clone an object
derived d = new derived();
derived d2 = d.MyCloneExtension<derived>();
Is short:
How come that adding a derived type to collection passes but when trying to add a generic of the derived type it fails?
The "short" code:
//a generic repository
public class EfRepository<T> : IRepository<T> where T: BaseCatalogModel{...}
public CatalogRepository(IRepository<Product> productRepository, IRepository<Category> categoryRepository)
{
//This passes
Dictionary<int, BaseCatalogModel> dic1 = new Dictionary<int, BaseCatalogModel>();
dic1.Add(1, new Product());
dic1.Add(2, new Category());
dic1.Add(3, new BaseCatalogModel());
//This not.
//The error: cannot convert from 'YoYo.Core.Data.Repositories.EfRepository<YoYo.Commerce.Common.Domain.Catalog.Product>'
//to 'YoYo.Core.Data.Repositories.EfRepository<YoYo.Commerce.Common.Domain.Catalog.BaseCatalogModel>'
Dictionary<int, EfRepository<BaseCatalogModel>> dic2 = new Dictionary<int, EfRepository<BaseCatalogModel>>();
dic2.Add(1, new EfRepository<Product>());
dic2.Add(2, new EfRepository<Category>());
}
The long deal:
Working on an on-line store, I would like to hold in a catalog repository a collection of all repositories relevant to managing the catalog.
The idea is to manage the entire catalog from one repository.
The repositories collection is of type Dictionary)
I fail to add any BaseCatalogModel derived type repository to the collection.
I will be happy to get any assistance on the above or suggestions for better implementations.
public class BaseCatalogModel
{
public int Id { get; set; }
...
}
public class Category:BaseCatalogModel
{
...
}
public class Product : BaseCatalogModel
{
...
}
public class CatalogRepository : ICatalogRepository
{
private readonly Dictionary<Type, IRepository<BaseEntity>> _repositoriesCollection= new Dictionary<Type, IRepository<BaseEntity>>();
public CatalogRepository(IRepository<Product> productRepository, IRepository<Category> categoryRepository)
{
_repositoriesCollection.Add(typeof(Category), categoryRepository); //==> this fails
_repositoriesCollection.Add(typeof(Product), productRepository); //==> this fails
}
public T GetCatalogItem<T>(int id) where T : BaseCatalogModel
{
//returns a catalog item using type and id
}
public IEnumerable<T> GetCatalogItem<T>() where T : BaseCatalogModel
{
//returns the entire collection of catalog item
}
}
So this is a fairly common problem with generics.
imagine 2 classes class Base and class A. class A descends from Base.
public class Base { }
public class A : Base { }
Now consider List<T>. You can make classes from List<T> to hold Base or A:
List<Base> x = new List<Base>();
and
List<A> y = new List<A>();
It is a common misconception that the class of y must be a descendant of the class of x, but this cannot be true because x has a methods like Add(Base item) and y has a methods like Add(A item) and it is not possible for the compiler to guarantee that on y the interface will be compatible with the interface of x. this is because if you treat an instance of List<A> as an instance of List<Base> there is nothing to stop Add being called with an instance of Base or another subclass of Base.
Now there are some parts of the interface that can be guarantee as compatible. They are any parts that return an instance of class A, since A can always take the place of Base.
If your interface only output the generic and you are using .net 4 there is an easy solution. The out generic modifier:
public class Example
{
private readonly Dictionary<Type, IRepository<Base>> _repositoriesCollection =
new Dictionary<Type, IRepository<Base>>();
public void DoSomething()
{
_repositoriesCollection.Add(typeof(A), new Repository<A>());
}
}
interface IRepository<out T> where T : Base
{
T MakeSomeItem(string info);
//void AddSomeItem(string info, T itemToAdd); <- this will not
// work because T
// is out - so can't
// go in...
IEnumerable<T> MakeSomeListOfItems(); // This is OK because
// IEnumerable<T> is declared as IEnumerable<out T> in the fx
//List<T> Something(); <- not ok because List<T> is not List<out T>
}
public class Repository<T> : IRepository<T> where T : Base
{
public T MakeSomeItem(string info)
{
throw new NotImplementedException();
}
public IEnumerable<T> MakeSomeListOfItems()
{
throw new NotImplementedException();
}
}
public class Base { }
public class A : Base { }
This solution will not work for 2 cases; when you need to pass an item into you interface and when you are not using .net 4.
There are numerous different solutions for both of those case also.
1) I need to pass an item into the interface too and I am using .net 4 - just pass it as Base, if you need to maintain type safety wrap it with a generic method somewhere else.
interface IRepository<out T> where T : Base
{
T MakeSomeItem(string info);
void AddSomeItem(string info, Base itemToAdd);
}
public class Repository<T> : IRepository<T> where T : Base
{
public T MakeSomeItem(string info){ throw new NotImplementedException(); }
public void AddSomeItem(string info, Base itemToAdd)
{
T castedItem = (T) itemToAdd; //fails here at
//run time if not
// correct type
AddSomeItem(info, itemToAdd);
}
public void AddSomeItem(string info, T itemToAdd)
{
/// do it for real...
}
}
2) If you are not working with .net 4 then there are other things that you can do too, force the repository to implement the Base version of your interface:
interface IRepository<T> where T : Base
{
T MakeSomeItem(string info);
void AddSomeItem(string info, T itemToAdd)
}
public class Repository<T> : IRepository<Base>, IRepository<T> where T : Base
{
public T MakeSomeItem(string info) { throw new NotImplementedException(); }
public void AddSomeItem(string info, Base itemToAdd)
{
T castedItem = (T) itemToAdd; //fails here at
//run time if not
// correct type
AddSomeItem(info, itemToAdd);
}
public void AddSomeItem(string info, T itemToAdd)
{
/// do it for real...
}
Base IRepository<Base>.MakeSomeItem(string info)
{
return MakeSomeItem(info);
}
}
There is still yet more that you can do if you want to keep your input strongly typed - but I think my answer is long enough for now.
I have an example where I want an abstract class interface to return something like this
abstract class AnimalProcessor {
public abstract IList<Animal> ProcessResults();
}
Then the concrete examples
class GiraffeProcessor : AnimalProcessor {
public override IList<Animal> ProcessResults() {
return new List<Giraffe>();
}
}
class LionProcessor : AnimalProcessor {
public override IList<Animal> ProcessResults() {
return new List<Lion>();
}
}
The problem is that the concrete classes need to have the same signature to override the ProcessResults() method so they need to return an IList<Animal>, however the ACTUAL data I want to return is an IList<Lion>, IList<Giraffe> etc, but then the calling code has to do
GiraffeProcessor processor = new GiraffeProcessor();
IList<Animal> results = processor.GetResults();
Which does not give me an Ilist which is what I want.
Problems
1) Above code does not compile. The giraffeProcessor has to return a concrete List<Animal>, you can populate it with Giraffe objects but the object type you construct to return has to be List<Animal>. Not ideal.
2) When you return the results, you can only get an IList<Animal>, not IList<Giraffe>. I have tried casting explicitly to IList<Giraffe> with
IList<Giraffe> results = (IList<Giraffe>) processor.GetResults();
which gives a runtime error, presumably because the object returned is NOT an IList<Giraffe>, it is an IList<Animal> which CONTAINS Giraffe objects.
Can anyone suggest what I am doing wrong here with my design as Im a bit stumped as to the best way to accomplish this.
How about:
abstract class AnimalProcessor<T> where T : Animal {
public abstract IList<T> ProcessResults();
}
class GiraffeProcessor : AnimalProcessor<Giraffe> {
public override IList<Giraffe> ProcessResults() {
return new List<Giraffe>();
}
}
class LionProcessor : AnimalProcessor<Lion> {
public override IList<Lion> ProcessResults() {
return new List<Lion>();
}
}
You could resolve this by declaring AnimalProcessor with a generic type constraint, e.g.
public abstract class AnimalProcessor<T> where T : Animal
{
public abstract IList<T> ProcessResults();
}
If that doesnt work, you could use the LINQ Cast operator, for example:
public class GiraffeProcessor : AnimalProcessor
{
public override IList<Animal> ProcessResults()
{
return new List<Giraffe>().Cast<Animal>();
}
}
Or, store the list internally as Animal but add Giraffe's to it, e.g.
public class GiraffeProcessor : AnimalProcessor
{
private List<Giraffe> _innerList = new List<Giraffe>();
public override IList<Animal> ProcessResults()
{
return new List<Animal>(innerList ); }
}
Best regards,
If you are using C# 4.0, you can ask yourself whether the processor should return IEnumerable<T> rather than IList<T>. If the answer is "yes", then you can profit from covariance:
abstract class AnimalProcessor {
public abstract IEnumerable<Animal> ProcessResults();
}
class GiraffeProcessor : AnimalProcessor {
public override IEnumerable<Animal> ProcessResults() {
return new List<Giraffe>();
}
}
class LionProcessor : AnimalProcessor {
public override IEnumerable<Animal> ProcessResults() {
return new List<Lion>();
}
}
You have a couple of advantages here. First, you could implement these as iterator blocks:
class GiraffeProcessor : AnimalProcessor {
public override IEnumerable<Animal> ProcessResults() {
yield break;
}
}
Second, and less trivially, you allow the client code to decide what kind of collection to dump the animals into -- if any. For example, consider that the consumer might want a LinkedList<Animal>:
var animals = new LinkedList<Animal>(animalProcessor.ProcessResults());
Or consider that the client might need only to iterate the sequence:
foreach (var animal in animalProcessor.ProcessResults())
{ /*... do something ...*/ }
In either case, if you were using a ToList() call in ProcessResults, you'd be creating a list for nothing. If the consumer really wants a List<Animal>, that can be accomplished very easily:
var animals = new List<Animal>(animalProcessor.ProcessResults());
Finally, you can also benefit from the generic approach, even if you change the interface type of the method's return value:
abstract class AnimalProcessor<T> where T : Animal {
public abstract IEnumerable<T> ProcessResults();
}
class GiraffeProcessor : AnimalProcessor<Giraffe> {
public override IEnumerable<Giraffe> ProcessResults() {
yield break;
}
}
class LionProcessor : AnimalProcessor<Lion> {
public override IEnumerable<Lion> ProcessResults() {
return Enumerable.Empty<Lion>();
}
}