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)
Related
I want to have a generic class which could call a method based on its generic type which is defined by a derived class. For that I implemented a base interface and a generic interface which has the base interface as generic and also derives from the base interface.
In the generic interface I want a method based on the type T of the base interface.
After that I wanted to implement a class based on the generic interface which should be able to call the generic method. This is the example code:
public interface BaseInterface
{ }
public interface GenericInterface<T> : BaseInterface where T : BaseInterface
{
void Foo(T t);
}
public class C<T> : GenericInterface<T> where T : BaseInterface
{
public C()
{
// None of these works
Foo(this);
Foo((T)this);
Foo((BaseInterface)this);
}
public void Foo(T t) { }
}
Is there a way to achieve my desired behavior here?
The error message here is:
cannot convert from 'C<T>' to 'T'
which in my eyes should be possible because C derives from BaseInterface which is T
While both C<T> and T need to derive from BaseInterface, that does not mean that then need to be the same. I might for example declare another type B : BaseInterface, and C<B>. So we would get the method Foo(B t) , it would obviously not be possible to call the method with this as the parameter, since C<B> is not B.
If you just need a method that needs a BaseInterface parameter, just declare it as Foo(BaseInterface t) instead. That way you could call it with this without any problem.
It looks like you're looking for something like the Curiously Recurring Template Pattern for C#. In a simplified form it looks like this:
class Base<T>
{
public void Foo(T t) { }
}
class C : Base<C>
{
C()
{
Foo(this);
}
}
In your case T is a type that derives from BaseInterface. Although C<T> also derives from BaseInterface it doesn't mean that you can bind a C<T> to T t. It's like trying to bind a string to a List, just because both implement IEnumerable. They are still different types.
For your full example it could look like this
public interface BaseInterface
{ }
public interface GenericInterface<T> : BaseInterface where T : BaseInterface
{
void Foo(T t);
}
public class C<T> : GenericInterface<C<T>> where T : BaseInterface
{
public C()
{
Foo(this);
}
public void Foo(C<T> t) { }
}
Here's what you need for the Curiously Recurring Template in C#.
public interface BaseInterface { }
public interface GenericInterface<T> : BaseInterface where T : GenericInterface<T>
{
void Foo(T t);
}
public abstract class C<T> : GenericInterface<T> where T : C<T>
{
public abstract void Foo(T t);
}
Now you can go ahead and implement a real class:
public class D : C<D>
{
public D()
{
Foo(this);
Foo((D)this);
}
public override void Foo(D t) { }
}
That works fine.
However, calling Foo((BaseInterface)this); will never work in this code. It just doesn't make sense.
I Tried this, maybe can help you
public class C<T> : GenericInterface<T> where T : BaseInterface
{
public C()
{
T t = default(T);
BaseInterface bi;
bi = t; // here can cast , bacuse T : BaseInterface , not BaseInterface : T
t = bi;//here cast error ,
}
public void Foo(T t) { }
}
It's same like this case:
public class B { }
public class A : B
{
public void CastTest()
{
A a = null;
B b = null;
b = a;
a = b;//here error
}
}
My project is structured the following way:
// Abstract class
public abstract class Job
{
public abstract JobResult Run();
}
public abstract class JobResult { }
// Concrete implementer
public class Job1 : Job
{
public override Job1Result Run() { }
}
public class Job1Result : JobResult { }
Each concrete job inherits from Job and implements the method Run which returns a concrete class of JobResult.
However when I do this I get the compiler error:
Job1.Run()': return type must be JobResult to match overridden member
Job.Run()
Is it really not possible to return an inheriting object of the return type when overriding an abstract method?
This is the whole concept of inheritance. Returning parent classes is considered a feature here. Nothing stops you however from returning a Job1Result in Job1
public JobResult Run()
{
return new Job1Result();
}
Then the caller of Job1.Run() will have to know the correct return type and cast it to access Job1Result methods which are specific to that class
You could make Job generic:
public abstract class Job<TResult> where TResult : JobResult
{
public abstract TResult Run();
}
public class Job1 : Job<Job1Result>
{
public override Job1Result Run()
{
//
}
}
Here is an example, I hope it can help you.
public interface IEvent
{
Type GetEventType();
}
public abstract class AEvent<A>: IEvent where A: struct
{
public Type GetEventType()
{
return typeof (A); // return sub struct type
}
}
I have 2 classes (classA and classB) that both inherits from same class (classC). Now I need create new generic class (classAA) that inherits List of A or B. Is it possible to call methods of classC in classAA.
public class classC
{
//...
}
public class classA : classC
{
//...
}
public class classB : classC
{
//...
}
public class classAA<T> : List<T>
{
//...
}
It's generally not a good idea to inherit from List<T>. 95% of time you should prefer composition over inheritance. See for example: Why not inherit from List<T>?
Most probably you are looking for something like:
public class classAA<T>
where T : ClassC
{
public List<T> MyList { get; set; }
public void Foo()
{
foreach (var item in this.MyList)
{
item.MethodOfClassC();
}
}
}
I believe you want something like this. Assumming:
class classC
{
public void DrawObject() { }
}
you can put constraint on T that it will be classC or its derivatives:
class classAA<T> : List<T> where T : classC
{
public void SomeMethod()
{
foreach (var item in this.Cast<classC>())
{
// here you can call item.DrawObject()
}
}
}
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; }
}
What is relation between MyClass object and <T> in
public class MyClass<T> where T : myGeneric
{
}
To Add to my question after DBM's reply
How is the implemention above is better than
IGenericInterface
{
int Bar();
}
public abstract GenericClass : IGenericInterface
{
public virtual int Bar()
{
return 1;
}
}
and then using the abstraft class to implement
public class MyClass : GenericClass
{
IGenericInterface GenericObject ;
}
You may want to read up on C# generics. The code you posted is a class called MyClass that can take anything that derives from the class myGeneric. The T is a placeholder for the type.
You could, for example, do
public class myGeneric
{
public int Bar()
{
return 1;
}
}
public class MyClass<T> where T : myGeneric
{
public void DoSomething(T foo)
{
int x = foo.Bar();
}
}
So in the example above, x would be 1.
"MyClass" is the class name, "<T>" is the generic parameter list.
Does not require you to cast MyClass to IGenericInterface
At compile time, all occurrences of T will be replaced with the actual class type that was used in the call, rather than the class being casted at runtime to the IGenericInterface.