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C# - Multiple generic types in one list
(3 answers)
Closed 1 year ago.
I have the following class and interface
abstract class MyAbstractClass<T> where T : MyInterface
{
}
interface MyInterface {
}
I would like to have the following class:
class ContainerClass
{
private List<MyAbstractClass<T>> actions = new List<MyAbstractClass<T>>();
}
Where T is any class that extends MyInterface.
The important bit is here that is do not want to add a generic parameter to the ContainerClass and I need the exact implementation class, not the interface, so I can't just say:
private List<MyAbstractClass<MyInterface>> actions = new List<MyAbstractClass<MyInterface>>();
Long story short, I would like the following Java code in C#:
public class ContainerClass {
private List<MyAbstractClass<? extends MyInterface>> list = new ArrayList<>();
}
abstract class MyAbstractClass<T extends MyInterface> {
}
interface MyInterface {
}
Is possible in C#?
EDIT:
To clarify the question, let's see the below code piece:
abstract class GameAction<T> where T : IGameEntity
{
protected T Subject;
public GameAction(T subject)
{
Subject = subject;
}
public abstract bool Run();
}
class ActionRunner {
private List<GameAction<T>> actions = new List<GameAction<T>>();
public void RunAll() {
foreach (GameAction<T> action in actions) {
action.Run();
}
}
}
My goal here is to have different implementation classes of GameAction, each should be parameterized with different implementations of IGameEntity. IGameEntity can be considered now as marker interface (it has some methods, but they don't matter here), I just use it to enforce the type of Subject and to avoid repeated casting in the implementation classes. The member Subject can be anything, it can do anything in the Run method of the child classes.
What you ask for is not possible in C# because unfortunately only interfaces are allowed to have variant type parameters. As you said, in Java you would be able to declare a covariant variable, but in .net, due to various design decisions, you cant.
The only choice you have is to create a new interface
public interface MyAbstractClassWhenInReadOnlyList<out T>
{
T Method1();
void Method2(out T result);
...
}
and have your MyAbstractClass implement that interface.
There are some other choices, but they boil down to this, just in various guises. There is no way to access or use an object with a type variable without knowing the exact type that was used to construct the object. You must have a class without a type variable to use it (This includes reflection) or have an interface with the appropriate variance.
Please be aware that this has noting to do with Java's wild-card syntax nor the use-site vs declarations site variance syntax. These are both just notations used by the two programming languages to express an idea to the type checker.
Ultimately it comes down to static variables.
If a method accesses the List< int >.count variable vs the List< String > variable then it better be the correct value. In Java, static variables are associated with the "raw class", eg the type "List< ? extends Object >. In C# there are two variables, one associated with each set of type parameters.
Because Interfaces are unable to have static variables, there is no way that a static variable could be access incorrectly.
Related
Is it possible to declare a generic collection to hold only objects implementing a generic Interface with any <T>?
My question burns down to: If I want to/have to store objects implementing a generic interface, is there a better way to express that fact than using a non generic collection or (generic of <Object>).
Example:
// An example Generic Interface
interface ISyncInterface<T>
{
Task DoSync();
IEnumerable<T> NewItems { get; }
}
// a manager-class that registers different classes implementing
// the generic interface.
// The code works - can it be done better?
class Manager
{
private List<Object> _services = new List<Object>(); // <- works but is basically non generic
// however the RegisterService() ensures that only correct types can be added.
// would like to have something like below to indicate the Interface-Type
// however: this would only allow _services2.Add to hold types of ISyncInterface<Object>
// - ISyncInterface<ServiceA_DTO> would fail.
private List<ISyncInterface<Object>> _services2 = new List<ISyncInterface<Object>>();
void RegisterService<T, U>(T service)
where T : ISyncInterface<U>
{
_services.Add(service); // <- works e.g. for SyncServiceA
// _services2.Add(service); // <- FAILS for SyncServiceA - no conversion
// _services2.Add((ISyncInterface<Object>) service); // <- FAILS also - no explicit cast
}
}
// SETUP - The classes used above. Just to clarify.
class ServiceA_DTO { }
class ServiceB_DTO { }
class SyncServiceA : ISyncInterface<ServiceA_DTO>
{
public Task DoSync() {}
public IEnumerable<ServiceA_DTO> NewItems { get; }
}
class SyncServiceB : ISyncInterface<ServiceB_DTO>
{
public Task DoSync() {}
public IEnumerable<ServiceB_DTO> NewItems { get; }
}
Is this possible at all? Any advice is highly appreciated!
Update: New, more verbose code to clarify the problem.
Below there was a suggestion to base the generic interface on an non generic one. But as a consequence all implementing classes of the generic interface would have to implement the non generic methods, properties etc. - or is there a way around it?
Thanks for your input!
Is it possible to declare a generic collection to hold only objects implementing a generic interface instantiated with any T?
Short answer: no.
Longer answer: no, because that is not useful.
Let's consider a simple generic interface:
interface I<T> { T Get(); }
And a bunch of objects that implement it:
class Lion : I<Lion>
{
public Lion Get() => this;
}
class TaxPolicyFactory : I<TaxPolicy>
{
public TaxPolicy Get() => new TaxPolicy();
}
class Door: I<Doorknob>
{
public Doorknob Get() => this.doorknob;
...
}
OK, now suppose you have a List<I<ANYTHING>> like you want:
var list = new List<I<???>> { new TaxPolicyFactory(), new Lion(), new Door() };
You've got a list with a tax policy factory, a lion and a door in it. Those types have nothing in common with each other; there's no operation you can perform on each of those objects. Even if you could call Get on each of them, then you'd have a sequence with a tax policy, a lion and a doorknob in it, and what are you going to do with that?
Nothing, that's what. The constraint "implements interface I<T> for any T" is simply not a useful constraint in C#, so there is no way to express it.
It sounds like you have an "XY" problem. That is a problem where you have a bad solution in mind, and now you are asking questions about your bad solution. Ask us a question about the real problem you have, not the bad idea you've got for its solution. What's the real problem?
UPDATE: With the new information in the question it is now much more clear. The feature you want is called generic interface covariance, which was my favourite feature for C# 4.
If you update your interface definition to
interface ISyncInterface<out T> { ... }
then you can use an ISyncInterface<String> in a context where an ISyncInterface<Object> is expected. For example, you could put an ISyncInterface<Giraffe> into a List<ISyncInterface<Animal>> or whatever.
However you are required to ensure that your interface definition only uses T in a covariantly valid position. Your interface is valid as stated, but if for example you ever want to add a method void M(T t); to your interface, it will no longer be covariantly valid. The "out" is a mnemonic telling you that T can only be used as output of methods. Since IEnumerable<T> is also covariantly valid, it's fine; there are no inputs of T in an IEnumerable<T>.
Also, variance only works with generic interfaces and delegates, and the varying types must be reference types. You can't put an ISyncInterface<int> into a List<ISyncInterface<Object>> because int is not a reference type.
There are many posts on SO about covariance and contravariance; you should also read the Microsoft documentation. It can be a confusing feature. If you're interested in the historical details of how we designed and implemented the feature, see my blog.
Perhaps you can try something like this:
public interface MyInterface
{//methods common to all types
void FirstMethod();
}
public interface MyInterface<T> : MyInterface
{//methods specific to a type
void FirstMethod(T parameter);
}
public class MyClassThatHandlesAllInterfaces
{
private List<MyInterface> _allInterfacesT; //first interface in the chain
public void AddInterface<T>(MyInterface<T> ifToAdd)
{
_allInterfacesT.Add(ifToAdd); // <- this is what I'd like to do
}
}
I use this pattern quite often. Because I do not know all the details about your scenario it might not be suitable for you.
But it might help other people searching google.
Context: .NET 4.0, C#
I'm creating a set of interfaces and a set of clases that implement them to provide some service. The clients use the concrete clases but call methods that are declared using the interfaces as parameter types.
A simplified example is this one:
namespace TestGenerics
{
// Interface, of fields
interface IField
{
}
// Interface: Forms (contains fields)
interface IForm<T> where T : IField
{
}
// CONCRETE CLASES
class Field : IField
{
}
class Form <T> : IForm<T> where T : IField
{
}
// TEST PROGRAM
class Program
{
// THIS IS THE SIGNATURE OF THE METHOD I WANT TO CALL
// parameters are causing the error.
public static void TestMethod(IForm<IField> form)
{
int i = 1;
i = i * 5;
}
static void Main(string[] args)
{
Form<Field> b = new Form<Field>();
Program.TestMethod(b);
}
}
}
The code makes sense to me, but I get the compiler error:
Argument 1:
cannot convert from 'TestGenerics.Form<TestGenerics.Field>' to
'TestGenerics.IForm<TestGenerics.IField>' TestGenerics
I'm not sure what I'm doing wrong, I've read lots of pages on the internet but none solved my problem.
Is there a solution that would not modify that much the architecture of what I'm trying to build:
Edit:I designed the interfaces in a way such that they should be independent of concrete clases that implement them. The concrete clases could be loaded from a dll, but most of the application Works with the interfaces. In some cases I need to use concrete clases, specially when using clases that need to be serialized.
Thanks in advance.
Alejandro
The problem is that Form<Field> implements IForm<Field> but not IForm<IField>. You cannot use an inherited class (or interface) as a generic parameter unless it is marked as covariant with the out identifier. However, marking your interface as covariant will restrict the usage significantly (basically making in an "output-only" interface like IEnumerable) so it may not work for you.
One way to get it to work is to make TestMethod generic as well:
public static void TestMethod<T>(IForm<T> form) where T:IField
{
int i = 1;
i = i * 5;
}
You can use Covariance, like so:
interface IForm<out T> where T : IField
{
}
More about Covariance and Contravariance here.
Others have pointed out the reasoning behind the error message, but let's examine the design of your sample code for a moment. Perhaps you're using a generic where none is needed.
You've already said you're using methods declared in the IField interface, so there may be no need to make your IForm class generic - simply have it store references to IField, instead of the generic argument 'T' (which is already guaranteed to be an IField anyway).
For instance, use:
public interface IForm
{
IEnumerable<IField> Fields { get; set; }
}
instead of
public interface IForm<T> where T : IField
{
IEnumerable<T> Fields { get; set; }
}
The following code shows what I would like to do; that is, I would like to constrain anObject, so that it can be used as a parameter to various methods with use IInterfaceOne or IInterfaceTwo, where neither inherits from the other.
public interface IInterfaceOne { }
public interface IInterfaceTwo { }
public class Implementation : IInterfaceOne, IInterfaceTwo
{
}
public interface IInterfaceOneAndTwo : IInterfaceOne, IInterfaceTwo { }
public class UsingImplementation
{
IInterfaceOneAndTwo anObject = (IInterfaceOneAndTwo)(new Implementation()); //fails because Implementation doesnt acctually implement IInterfaceOneAndTwo
}
This example fails however as IInterfaceOneAndTwo is an interface in its own right, and Implementation does not implement it.
I know if I used generics I could constrain them, but I am wondering, if there is a way to do this without generics?
Is there a way to say anObject shall implement IInterfaceOne and IInterfaceTwo, without using IInterfaceOneAndTwo?
Not the way you have it currently. Only generic constraints have that ability.
You could rewrite it to use generics:
public class UsingImplementation<T>
where T : IInterface1, IInterface2, new()
{
T anObject = new T();
void SomeMethod() {
anObject.MethodFromInterface1();
}
}
You can also have generic methods, not only generic classes
public void DoSomething<T>(T value)
where T : IInterface1, IInterface2
{
value.DoInterface1Things();
value.DoInterface2Things();
}
Or
public void DoSomething<T>()
where T : IInterface1, IInterface2, new()
{
T anObject = new T();
}
You can't do that in C# without generics but there is an alternative workaround to solve the problem without generics that was not mentioned here and might fit for you. This style is often used together with the IoC principle. You could inject the same object twice. Let me change your sample quite a bit...
public interface IInterfaceOne { void Hello(); }
public interface IInterfaceTwo { void World(); }
public class Implementation : IInterfaceOne, IInterfaceTwo
{
public void Hello() { };
public void World() { };
}
public class UsingImplementation
{
private readonly IInterfaceOne one;
private readonly IInterfaceTwo two;
public UsingImplentation(IInterfaceOne one, IInterfaceTwo two)
{
this.one = one;
this.two = two;
}
// do the stuff you want to do with an IInterfaceOne using field one
public DoSomeThingWithOne() { one.Hello(); }
// do the stuff you want to do with an IInterfaceTwo using field two
public DoSomeThingWithTwo() { two.World(); }
}
Then you could wire up the things this way:
var oneAndTwo = new Implementation();
var a = new UsingImplementation(oneAndTwo, oneAndTwo);
// operates on the first param (which is the same as the second)
a.DoSomeThingWithOne();
// operates on the second param (which is the same as the first)
a.DoSomeThingWithTwo();
Have a look for IoC principle (Inversion of Control) and Dependency Injection and you'll find more solutions similiar to this one.
This way you don't need to create an extra Interface combining InterfaceOne and InterfaceTwo, two.
"Incoming" generic class parameters and generic method parameters can combine types, but there is no facility for variables or fields to represent "composite" types. Further, in order to pass an object to a parameter of a generic type which combines multiple constraints, the object must be cast to a type which in fact implements all of those constraints. This can be difficult.
For example, suppose class Foo and Bar both implement Intf1 and Intf2. One wishes to write a function AddToList<T>(thing as T) where T:Intf1,Intf2. Such a function will perfectly happily accept objects of type Foo or Bar. Suppose, however, one wishes to use such a function to add all objects to the same list (which might be a mix of Foo, Bar, and any number of other types that also happen to implement Intf1 and Intf2) and then later pass those objects to a function whose parameter is likewise constrained to implement both Intf1 and Intf2. One could cast to Foo any object which happened to be a Foo, and cast to Bar any object which happened to be a Bar, but if other types are written which also handle Intf1 and Intf2, it would be difficult to deal with them.
It is possible to solve the problem, somewhat awkwardly, without using Reflection or other such tricks. Define an interface IActUpon<Base1, Base2> with a method ActUpon<thingType>ActUpon(thingType thing) where thingType: Base1, Base2. Implementations of such a method will be able to pass parameter thing to other methods requiring generic method parameter constrained to Base1 and Base2. The biggest difficulties with such an approach are that one must write separate code for each possible number of constraints, and that in many places where one would have used a lambda expression one will instead have to write an implementation of IActUpon....
If this is desirable then there has to be a logical connection between IInterfaceOne and IInterfaceTwo and the implementing class should implement the combined interface:
class Implementation : IInterfaceOneAndTwo { ... }
If this is not possible, because it's not (all) your code then you may have to rethink the UsingImplementation. It simply doesn't fit the available surface.
I want to write the equivalent Java code of a C# code.
My C# code is as follows:
public abstract class A<T> where T : A<T>, new()
{
public static void Process()
{
Process(new T());
}
public static void Process(T t)
{
// Do Something...
}
}
public class B : A<B>
{
}
public class C : A<C>
{
}
Java equivalent of my code looks like this.
public abstract class A<T extends A<T>>
{
public static <T extends A<T>> void process()
{
process(new T()); // Error: Cannot instantiate the type T
}
public static <T extends A<T>> void process(T t)
{
// Do Something...
}
public class B extends A<B>
{
}
public class C extends A<C>
{
}
}
Here the "new()" syntax in class declaration forces derived classes to write a default constructer which makes possible to call "new T()" from base class. In other words when i am wrting the base class i am sure that the derived class will have a default constructer, so that i can instantiate a derived class object from base class.
My problem in Java is, I cannot instantiate a derived class object from super class. I get "Cannot instantiate the type T" error for "new T()" call. Is there any C# similar way in Java or should I use something like prototype pattern and cloning?
Java doesn't support reified generics, so there is no equivalent to "new T();". The way I work around this is to use reflection against a type token. The type token indicates what the generic type is.
public abstract class A<T> {
private Class<T> typeToken;
// constructor
public A() {
typeToken = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];
}
}
Then use reflection to instantiate the class. It's ugly, but it gets the job done.
You can find some explanation of the difference between generics in C# and Java from this li nk - comparing java and C# generics.
Java generics are a completely compile-time construct. You cannot do anything with generic type parameters that rely in any way on runtime information. This includes:
Creating instances of generic type
parameters.
Creating arrays of generic type
parameters.
Quering the runtime class of a
generic type parameter.
Using instanceof with generic type
parameters.
You can bypass this restriction with java.lang.reflect namepsace. For example see this stackoverflow question: Genercs and Class.forName()
Also, beware of this if you are using generics.
T[] someArray = new T[];
This is one reason to prefer ArrayList to arrays. The reason for the problem lies with reifiability and type erasure.
Just use the bog standard Abstract Factory pattern. You then get the additional benefits that you are not tying down to a specific type, the implementation type need not have a specific constructor, the instance can have some parameterisation, instances can be cached, etc., etc.
For the love of god, don't use reflection.
In addition to the other comments, I would suggest not using generics. They are not needed--they get stripped out at compile time anyway--and if you do not know them well you will try to make them do things they cannot.
Once you have your class working properly, then add them back in. Your IDE will, at that point, give you a lot of useful and intelligible advice, and the generics will warn you when you use objects of the wrong class.
It does look possible to me that this class will not need generics at all when finished. (I don't know what else this class may do, and I do not understand the use of the static methods--they will never have access to an individual instance's type information.)
Actually this is not a problem in Java. The idiom is passing the class
public static <T extends A<T>> T process(Class<T> clazz)
{
T o = clazz.newInstance();
process( o );
return o;
}
X x = process(X.class); // not too verbose
I added a return value to illustrate the general case.
excuse me I havent dealt much with generic in c#
according to this question ,how is it possible to make a generc collection
that implement two interfaces
i was looking for a direct way like this:of course it makes error and totally is wrong.
interface IEmployee {void DisplayInfo();}
interface ISalary {void CalculateSalary();}
class Nurse : IEmployee, ISalary
{
//some Implementation
}
class Doctor : IEmployee, ISalary
{
//some Implementation
}
class EntryPoint
{
static void Main(string[] args)
{
System.Collections.Generic .List<T> employees where T: ISalary,IEmployee
=new System.Collections.Generic .List<T>();
}
Nurse oNurse = new Nurse();
Doctor oDoctor = new Doctor();
employees.Add(oNurse);
employees.Add(oDoctor);
}
after some Reading i found that maybe i must define a generic class like this at first:
public class HospitalEmployee<T> where T : IEmployee, ISalary
{
}
and unfortunately it dosnt work ,Now I am confused and dont know what must to do exactly,please help,thank u
You can do it like this:
interface IEmployee { void DisplayInfo(); }
interface ISalaried { void CalculateSalary(); }
interface ISalariedEmployee : IEmployee, ISalaried {}
class Doctor : ISalariedEmployee { whatever }
class Nurse : ISalariedEmployee { whatever }
...
var list = new List<ISalariedEmployee>() { new Nurse(), new Doctor() };
Does that help?
Essentially the feature you really want does not exist. There is a way to say "this generic type parameter must be constructed with a type argument that implements these two interfaces" but there is, oddly enough, not a way to say "this local variable must be initialized with a reference to an object that implements these two interfaces". It is simply a shortcoming of the C# type system that you can represent that in type parameters but not in locals. What you want is:
var list = new List<IEmployee + ISalary>();
And now you can only put things into the list that implement both interfaces. But there is no such feature in C#, unfortunately. Sorry!
It is not clear what are you trying to do: create your own generic container or use List<T> to store different objects.
But as far as I understood you need something like this:
List<IEmployee> employees = new List<IEmployee>();
Nurse oNurse = new Nurse();
Doctor oDoctor = new Doctor();
employees.Add(oNurse);
employees.Add(oDoctor);
UPDATE
Just create an interface which inherits all interfaces want to use like:
interface IEmployeeWithSalery: IEmployee, ISalery {}
List<IEmployeeWithSalery> employees = new List<IEmployeeWithSalery>()
This sounds a lot like my question Storing an object that implements multiple interfaces and derives from a certain base (.net) which I asked a few weeks ago. I offer a possible workaround there which may be more work than defining and using a few "combined" interface types, but has the advantage that one can define an object to work with any particular combination of interfaces which are suitably defined without having to define a new "combined" interface type for that combination.