I have a
public class A<T> where T : IBase
{
//Does something
}
I need a second class that behaves like a collection of class A
public class B<A<T>> : IEnumerable<A<T>> where T : IBase
{
}
The problem is that I do not want to create classes like
public class B<A<MyCustomObjectP>> : IEnumerable<A<MyCustomObjectP>>
{
}
public class C<A<MyCustomObjectQ>> : IEnumerable<A<MyCustomObjectQ>>
{
}
and so on.. I would like to let the CustomObject be a generic type parameter that implements IBase.
I found that even doing this is illegal:
public class B<T, U> : IEnumerable<T> where T : A<U> where U : IBase
{
}
How could I achieve this type of behaviour, if this is illegal? Is there a better design pattern of sorts that might help?
The IBase constraint is defined on A<T>, so it must be defined again on all generic classes, that want to use A<U> (using U to distinguish from T in A<T> class definition, but it can be called anything). You should be able to do simply:
public class B<T> : IEnumerable<A<T>> where T : IBase { ... }
You wrote that you need a second class that behaves like a collection of class A.
Since you have other classes (like B) inheriting from IBase as well, which you want to add, you can make the collection a collection of IBase.
Hence the solution would look like this (note that I have used List but you can easily replace that by IEnumerable - but then you have to implement methods like .Add yourself):
void Main()
{
var items = new CollectionOf<IBase>(); // create list of IBase elements
items.Add(new A() { myProperty = "Hello" }); // create object of A and add it to list
items.Add(new B() { myProperty = "World" }); // create object of B and add it to list
foreach(var item in items)
{
Console.WriteLine(item.myProperty);
}
}
// this is the collection class you asked for
public class CollectionOf<U>: List<U>
where U: IBase
{
// collection class enumerating A
// note you could have used IEnumerable instead of List
}
public class A: IBase
{
// class A that implements IBase
public string myProperty { get; set; }
}
public class B: IBase
{
// class B that implements IBase too
public string myProperty { get; set; }
}
public interface IBase {
// some inteface
string myProperty { get; set; }
}
Related
I have the following interface
public interface IInterface
{
int GetId();
}
ClassA is a generic class but T should implement IInterface
public class ClassA<T> where T : IInterface {
}
ClassB implements IInterface
public class ClassB : IInterface {
public int GetId() {
return 1;
}
}
ClassC is where my problem is, namely adding items to the list.
public class ClassC {
List<ClassA<IInterface>> list = new List<ClassA<IInterface>>();
public void Add<T>(ClassA<T> item) where T : IInterface {
list.Add(item);
}
}
The code above gives the following error which makes sense
cannot convert from 'Program.ClassA<T>' to'Program.ClassA<Program.IInterface>'
The core of the problem is in the Add() method where my goal is to be able to add any type of ClassA. I am not sure how to design this in order to achieve the following
ClassC classC = new ClassC();
ClassA<ClassB> classA = new ClassA<ClassB>();
classC.Add(classA);
Any ideas?
What about using Covariant Generic Type Argument.
public interface IInterface
{
int GetId();
}
// Contra-variant and covariant generic type argument can be used only in interfaces and delegates
public interface IClassA<out T> where T : IInterface
{
}
public class ClassA<T> : IClassA<IInterface> where T : IInterface { }
public class ClassB : IInterface
{
public int GetId()
{
return 1;
}
}
public class ClassC
{
List<IClassA<IInterface>> list = new List<IClassA<IInterface>>();
public void Add(IClassA<IInterface> item)
{
list.Add(item);
}
}
public class Test
{
public static void Run()
{
ClassC classC = new ClassC();
ClassA<ClassB> classA = new ClassA<ClassB>();
classC.Add(classA);
}
}
Depending on the specifics of your design, you may be able to split ClassA in two:
public abstract class ClassA {
//Stuff that only works in terms of IInterface,
//maybe with some abstract methods, required constructors, etc
}
public class ClassA<T> : ClassA where T : IInterface {
//Stuff specific to T
}
You then store ClassAs in your list and cast them to their known concrete type as/when you know what that is and need to work in those terms (e.g. to be able to call ClassA<T> specific functionality)
In My class I have
class MyClass : IMyInterface
{
//I want MyClass only to be able to accept object of type List<SomethingElse>
public List<ISomething> ListOfSomethings {get; set;}
}
interface IMyInterface{
List<ISomething> ListOfSomethings {get; set;}
}
class SomethingElse : ISomething{
}
class SomethingMore : Isomething{
}
Basically I want to know if it is possible to constrain the type that the list uses in MyClass, so if someone tried coding it to the wrong type (i.e. a List of SomethingMore) it would throw an exception.
EDIT: If this is not possible to do, is there an alternative solution that would work?
You can constrain the T (type) of the list items (and any other T) by using the where restriction:
for more details see Constraints on Type Parameters
Interfaces:
interface ISomething { }
Allows to only use Ts that implement the interface ISomething.
interface IMyInterface<T> where T : ISomething
{
List<T> ListOfSomethings { get; set; }
}
Classes:
class SomethingElse : ISomething { }
class SomethingMore : ISomething { }
class MyClass1 : IMyInterface<SomethingElse>
{
public List<SomethingElse> ListOfSomethings { get; set; }
}
class MyClass2 : IMyInterface<SomethingMore>
{
public List<SomethingMore> ListOfSomethings { get; set; }
}
You can restrict the T whereever it suits you. Here for example on the class itself.
This allows only SomethingElse
class MyClass3<T> : IMyInterface<T> where T : SomethingElse
{
public List<T> ListOfSomethings { get; set; }
}
an example with a Dictionary:
var dic = new Dictionary<string, IMyInterface<ISomething>>();
dic.Add("MyClass1", (IMyInterface<ISomething>)new MyClass1());
dic.Add("MyClass2", (IMyInterface<ISomething>)new MyClass2());
if you won't cast it everytime then the only solution that I can currently think of is to create your custom dictionary and encapsulate the casting:
class MyDictionary : Dictionary<string, IMyInterface<ISomething>>
{
public void Add(string key, MyClass1 value)
{
base.Add(key, (IMyInterface<ISomething>)value);
}
public void Add(string key, MyClass2 value)
{
base.Add(key, (IMyInterface<ISomething>)value);
}
}
var dic2 = new MyDictionary();
dic2.Add("MyClass1", new MyClass1());
dic2.Add("MyClass2", new MyClass2());
//I want MyClass only to be able to accept object of type List<SomethingElse>
Then you can't define it as List<ISomething>, but rather use a different interface or even a concrete type. If you define it as List<ISomething>, than it automatically accepts anything, that implements ISomething interface. There's no way around that.
C# is a typesafe by language design, so there is no way the consumer of your list can inject into it a type which is not SomethingElse or SomethingMore.
If you need to constrain to some of types derived from common interface, like SomethingElse, I would go
Hide a property that exposes a List itself
private List<ISomething> ListOfSomethings {get; set;}
Add a public member function, say FromList
public void FromList(List<SomethingElse> somethings)
{
ListOfSomethings = somethings;
}
This function becomes the only way to assign a list to a class, and considering that it accepts only lists of certain type, you are created desired limitation.
You can do this using explicit interface implementation:
class MyClass : IMyInterface
{
List<ISomething> IMyInterface.ListOfSomethings
{
get { return this.ListOfSomethings.Cast<ISomething>().ToList(); }
set { this.ListOfSomethings = value.Cast<SomethingMore>().ToList(); }
}
List<SomethingMore> ListOfSomethings { get; set; }
}
Please note that it is not recommended to make such a restriction, because this violates the Liskov substitution principle: The user of your class might be using the IMyInterface interface and has no idea that its type is restricted.
Another problem is exposing a List<T> like this in an interface: The caller can either call list methods like Add or Remove or set the entire List instance. This is probably not what you want. If you want to expose a readonly collection, use a getter with an array or enumerable type.
I have the following Classes / Interfaces:
// Model
public class A : IA { }
// ModelLogic
public class B : IB<A> { }
// Model Interface
public interface IA { }
// ModelLogic Interface
public interface IB<T> where T : IA { }
I try to create a new instance using the following code:
IB<IA> foo = new B();
I am getting the following error:
Cannot implicitly convert type 'B' to 'IB<IA>'. An explicit conversion exists (are you missing a cast?)
Can someone please explain why this is not possible?
OK, let's replace A with Fish, IA with IAnimal, B with Aquarium, and IB<T> with IContainer<T>. And we'll add a member to IContainer<T>, and a second implementation of IAnimal:
// Model
public class Fish : IAnimal { }
public class Tiger : IAnimal { }
// ModelLogic
public class Aquarium : IContainer<Fish>
{
public Fish Contents { get; set; }
}
// Model Interface
public interface IAnimal { }
// ModelLogic Interface
public interface IContainer<T> where T : IAnimal
{
T Contents { get; set; }
}
IContainer<IAnimal> foo = new Aquarium(); // Why is this illegal?
foo.Contents = new Tiger(); // Because this is legal!
You can put a Tiger into foo -- foo is typed as a container that can contain any animal. But you can only put a Fish into an Aquarium. Since the operations you can legally perform on an Aquarium are different than the operations you can perform on an IContainer<IAnimal>, the types are not compatible.
The feature you want is called generic interface covariance and it is supported by C# 4, but you have to prove to the compiler that you will never put a tiger into your fish tank. What you want to do is:
// Model
public class A : IA { }
// ModelLogic
public class B : IB<A> { }
// Model Interface
public interface IA { }
// ModelLogic Interface
public interface IB<out T> where T : IA { }
Notice the covariance annotation on IB. This out means that T can only be used as an output, not as an input. If T is only an output then there is no way for someone to put a tiger into that fish tank because there is no "put into" property or method possible.
I wrote a number of blog articles while we were adding that feature to C#; if you are interested in the design considerations that went into the feature, see:
http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/
To fix your code, just change
public interface IB<T> where T : IA { }
to
public interface IB<out T> where T : IA { }
It's not easy to see when you have empty interfaces. Consider you have one method M in interface IB:
public interface IB<T> where T : IA
{
void M(T t);
}
And here is implementation of B:
public class B : IB<A>
{
public void M(A t)
{
// only object of type A accepted
}
}
Then you have object C, which also implements IA:
public class C : IA { }
So, if your code would be possible, then you could call:
IB<IA> foo = new B();
foo.M(new C());
Problem is that class B accepts only objects of type A. Error!
I am receiving the following error:
ClassName.PropertyName cannot implement IClassType.PropertyName
because it does not have the matching return type of IBasePropertyType
Now, for the code:
public class ClassName : IClassType
{
public IChildPropertyType PropertyName { get; set; }
}
public interface IClassType
{
public IBasePropertyType PropertyName { get; set; }
}
public interface IBasePropertyType
{
// some methods
}
public interface IChildPropertyType : IBasePropertyType
{
// some methods
}
Is there a way to do what I am attempting? I know that the issue is with co/contravariance, but I can't seem to figure out how to do this.
In order to implement the given interface, you must have the same return type. However, there are a couple of potential work-arounds to make life easier:
make your interface generic
implement the interface explicitly.
If you make IClassType generic, like so:
public interface IClassType<T> where T : IBasePropertyType
{
public T PropertyName { get; set; }
}
... then you can implement this interface using various property types:
public class ClassName : IClassType<IChildPropertyType>
{
public IChildPropertyType PropertyName { get; set; }
}
Another option would be to leave your interface non-generic, but to have a generic base type that explicitly implements the interface:
public class ClassBase<T> : IClassType
where T : IChildPropertyType
{
IBasePropertyType IClassType.PropertyName {
get {return PropertyName;}
set {PropertyName = (IChildPropertyType)value;}
}
T PropertyName {get;set;}
}
Note that this last option is not quite ideal because you must dynamically cast the property to the given child type: while you can guarantee that every IChildProperty type is an IBasePropertyType, you cannot guarantee that every IBasePropertyType is an IChildPropertyType. However, if you can eliminate the setter from the original interface, or if you can take other steps to guarantee that the setter will never be called with the wrong type in your code, then this could work.
You are correct that this has to do with covariance; specifically it has to do with virtual method return type covariance, which is not a kind of covariance that the C# language supports.
UPDATE: This answer is over ten years old. C# may soon implement return type covariance. Please see https://github.com/dotnet/csharplang/issues/49 for details.
Note that even if it did, the system you describe is not type safe. Suppose we have:
interface IAnimal {}
interface IGiraffe : IAnimal {}
interface ITiger: IAnimal {}
class Tiger : ITiger {}
interface IHaveAnAnimal { IAnimal Animal { get; set; } }
class C : IHaveAnAnimal
{
public IGiraffe Animal { get; set; }
}
...
IHaveAnAnimal x = new C();
x.Animal = new Tiger(); // Uh oh. We just put a Tiger into a property of type IGiraffe.
Even if the covariance were legal at all, this kind of covariance would not be legal; you'd have to have no setter for the covariance to be legal.
Suppose then you did have no setter:
interface IAnimal {}
interface IGiraffe : IAnimal {}
interface ITiger: IAnimal {}
class Tiger : ITiger {}
interface IHaveAnAnimal { IAnimal Animal { get; } }
class C : IHaveAnAnimal
{
public IGiraffe Animal { get; }
}
Unfortunately this is still not legal. But you can do this:
class C : IHaveAnAnimal
{
IAnimal IHaveAnAnimal.Animal { get { return this.Animal; } }
public IGiraffe Animal { get; }
}
Now when C is used as a C, Animal returns a giraffe, and when used an an IHaveAnAnimal, it returns an IAnimal.
I am trying to do the following in C# 4.0:
I have a Base Class and 2 derived classes
public class Base {}
public class DerivedClass1 : Base {}
public class DerivedClass2 : Base {}
I want to do something like this, but it doesn't work.
How to I tell a Generic List to accept a Base Class and the derived classes of the base class.
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2(list);
}
}
public class Class_2
{
public Class_2(List<Base> list)
{
}
}
In Java I can do something like this
public class Class_2
{
public Class_2(List<? extends Base> list)
{
}
}
Does something like that exists in C#
I hope my question is clear, its just about the generics of the List.
Thanks a lot in Advance :)
General case:
function Foo<T>(List<T> list) where T : Base {
...
}
plus for interfaces and delegates, C# allows co/contravariance.
For e.g. IEnumerable<T> and IList<T>, your code will therefore work! Note that if this was allowed for List<T>s, you could insert a Derived1 into a list of Derived2s by using the common base class, which would break type safety. So simply stick to the above, readonly interfaces.
I think you mean either:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2<DerivedClass2>(list);
}
}
public class Class_2<T> where T : Base
{
public Class_2(List<T> list)
{
}
}
Or, if you want the constructor to be generic, and not the class:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
Class_2.Create(list);
}
}
public class Class_2
{
public static Class_2 Create<T>(List<T> list) where T : Base
{
// Stuff
return new Class_2(/*Stuff*/);
}
public Class_2()
{
}
}