Context: this is for an engine, which is full of generic classes so that future users can extend certain parts of the behavior without concerning themselves with the inner workings of the engine itself.
Right now, I need a class C:
public class C
{
public C Copy(params) { ... }
}
But also an extension class D:
public class D : C
{
public D Copy(params) { ... }
}
Basically I have some generic classes, but I need a copy method (actually it's a "make a copy at a new location" method, but that's immaterial) which returns the right type.
public class SampleClass<T>
where T : C
{
public void Stuff()
{
...
T copy = favoriteThing.Copy(params);
...
}
}
and etc. I could just cast it, and trust future implementers to get it right, but I'd rather make it all explicit with contracts. Are there any elegant ways to do this?
One way might be to define an ICopyable<T> interface that's implemented by C and D:
public interface ICopyable<T>
{
T Copy(params);
}
public class C : ICopyable<C> { ... }
public class D : C, ICopyable<D> { ... }
Now if SampleClass<T> constrains T to ICopyable<T>, it can call Copy and get back an object of type T without any casts:
public class SampleClass<T>
where T : C, ICopyable<T> // added constraint
{
public void Stuff()
{
T favoriteThing = ...
...
ICopyable<T> copyable = favoriteThing;
T copy = copyable.Copy(params); // no cast needed
...
}
}
Related
I have a set up approximately like this:
public class A { ... }
public class A<T> : A { ... }
public class B<T> : A<T> { ... }
public class C : B<SomeType> { ... }
I am iterating over a list of type A, and I want to check if something is of type B, and if it is, convert it to... B... I can see how it already poses a problem of how am I gonna get the type... but if I cannot do that, what are my other alternatives?
EDIT:
To clarify, B contains a method I'd like to use that A does not contain as it simply has no need for it... I want to check which of the objects on that list are of type B so that I can run this method on them
EDIT 2:
I forgot to mention the A in the middle earlier... sorry for the confusion
I want to check which of the objects on that list are of type B
Then introduce a type B into your type hierarchy. Currently you don't have a type B, you've got B<SomeType> which is a different type. so
public class A { ... }
public abstract class B : A { ... }
public class B<T> : B { ... }
public class C : B<SomeType> { ... }
Or declare the method you want on an interface
public class A { ... }
public class A<T> : A { ... }
public interface IB
public class B<T> : A<T>, IB { ... }
public class C : B<SomeType> { ... }
Then for any A you can check if it is IB. This is probably the most natural solution since .NET doesn't have multiple inheritance.
I'm trying to do the following:
public class A
{
}
public class B
{
}
Somewhere along the project I want to have this:
public class C
{
public T func<T>(T obj) [where T can be either of class A or class B]
{
obj.x = 100;
return obj;
}
}
I've been trying:
public T func<T>(T obj) where T: A, B
but this gives me:
The type class constraint 'B' must come before any other constraint.
Can someone explain me how to make func accept only class A or class B?
Exactly as it's described in the question, this job is better handled by overload resolution:
public class C
{
public A func(A obj)
{
obj.x = 100;
return obj;
}
public B func(B obj)
{
obj.x = 100;
return obj;
}
}
But I understand that A and B may be placeholders for any number of types, and it could get tedious to account for them all. In that case, you'll need a common interface that's supported by each of your classes:
interface IBase
{
int x;
}
public class C
{
public IBase func(IBase obj)
{
obj.x = 100;
return obj;
}
}
Note that at this point we still have no need of generics. Additionally, you may need to support a number of types that won't all fit together under a common interface. In this case, still build the interface and put as many types with that interface as possible. If needed, build another interface for a few more types ... and so on... and then between interfaces and specific types you can handle things with overload resolution.
You need some kind of common base for both the classes, either have them implement the same interface as the below code or have them inherit from same class. You can not have a generic constrained to 2 types.
public interface IFooBar
{
void DoThis();
}
public class Foo : IFooBar
{
public void DoThis()
{
//Do something
}
}
public class Bar : IFooBar
{
public void DoThis()
{
//Do something
}
}
public class C
{
public T func<T>(T obj) where T : IFooBar
{
obj.DoThis();
return obj;
}
}
The generics classes are just like any other class, you can't (and shouldn't) have multiple inheritance of classes, you can inherit one class and multiple interfaces.
in your case you should apply an interface on both classes and restrict the generics on that interface.
you can see some documentation in:
Constraints on Type Parameters (C# Programming Guide)
interface IMarkerInterface{} // there is a such pattern called marker
// interface . No need to have methods if there
// is no need for, A and B can just implement it
public class A: IMarkerInterface
{
}
public class B: IMarkerInterface
{
}
public class C
{
public T func<T>(T obj).Where T:IMarkerInterface
{
obj.x = 100;
return obj;
}
}
public T func<T>(T obj) where T: A, B this means T should extend both A and B , but multiple inheritance is not valid in C# ,so it won't work.
You could do one of the following though :
you could make A and B have a common parent via an interface or an abstract class , but that would be code modification.
since both A and B have a default no-arg constructor you could use where T: new().
Also, you can not do obj.x = 100; as there is no way to guarantee thatT will have a instance variable x.
I have a Function in C# and it have to return the type of the Class. Also in subclasses which extends the class.
Like:
public class A
{
public typeof(this) Method()
{
//Code
}
}
public class B : A {
public override typeof(this) Method() {
//Code
}
}
So the Method in class A should have the return type A. And the Method in class B should have the return tpye B.
Is there a way to do it?
No, this isn't possible. What you're asking for is called a covariant return type, but C# doesn't support this. The closest you can get is either this:
public class A
{
public virtual A Method()
{
//Code returning an A
}
}
public class B : A
{
public override A Method()
{
//Code returning a B
}
}
Which is legal because every B is also an A, or you can use generics instead of inheritance:
public class Foo<T>
{
public virtual T Method()
{
//Code
}
}
And then you can have Foo<A> and Foo<B> -- however, Foo cannot depend on any specifics of T. You can combine this with inheritance, which will sort of achieve what you want:
public class A : Foo<A>
{
// And now A has a Method that returns A
}
public class B : Foo<B>
{
// And now B has a Method that returns B
}
But the problem with this approach is that you will have a hard time actually implementing Method in a meaningful way, because in Foo you cannot use anything specific to the type. To make this explicit, you could make Method abstract:
public abstract class Foo<T>
{
public abstract T Method();
}
public class A : Foo<A>
{
public override A Method()
{
// Code
}
}
public class B : Foo<B>
{
public override B Method()
{
// Code
}
}
I'm having a hard time imagining a scenario where you can actually make use of this, but at least it meets the requirements.
Last but not least, you are not required to use inheritance -- does B really need to derive from A or could you inherit from some common base that does not use Method?
Depending on what your method is trying to do, it might be possible to achieve what you want by using extension methods.
public class A { }
public class B : A { }
public static class AExtension {
public static T Method<T>(this T target) where T: A {
// do your thing here.
return target; // or, return a new instance of type T.
}
}
You can then call Method() and let C# infer the generic argument:
var a = new A();
a = a.Method(); // C# will infer T as A.
var b = new B();
b = b.Method(); // C# will infer T as B.
The downside to this approach is, of course, that you cannot access non-public members of your classes in Method(), unless you use reflection.
There is a way to do this, actually.
class A {
public A Method () { ... return this; }
}
class B : A {
new public B Method () => (B)base.Method();
// or { base.Method(); return this; }
}
Make sure you only use this if you know that the base returns this.
I have two Interfaces A,B (residing in different components). Both declare a method with the same signature ( MyMethod ). The two interfaces are inherited by a third Interface (C).
The method which is declared in the first two interfaces (A,B) is intended to return always the same values (for A and B) and thus, I don’t want to implement the interfaces explicitly when deriving from C.
I achieve this with declaring the method also in the third interface, while using the new-keyword.
public interface A {
MyType MyMethod();
}
public interface B {
MyType MyMethod();
}
public interface C : A,B{
new MyType MyMethod();
}
public class ImplementingClass : C{
public MyType MyMethod(){
// do somethin
// return something
}
}
Are there problems to be expected with this, or is this bad style?
Update
Sorry, my initial question has not shown the full story. The Problem arises, when I try to call MyMethod on a interface-reference of C. The Compiler will not compile this.
C aReferenceToC=new CImplementingClass();
aReferenceToC.MyMethod(); // <<< Here the compiler will throw an exception
Full example
C myCImplementationAsAnInterfaceReference = new MyCImplementation();
myCImplementationAsAnInterfaceReference.MyMethod(); // This does not compile without declaring MyMethod in C with the new-Keyword
MyCImplementation myCImplementationReference= new MyCImplementation();
myCImplementationReference.MyMethod(); // This however will always compile and run
public interface A {
int MyMethod();
}
public interface B {
int MyMethod();
}
public interface C : A, B {
}
public class MyCImplementation : C {
public int MyMethod() {
return 1;
}
}
Doing what you do, does not in any way stop people from giving different implementations of A.MyMethod, B.MyMethod, and C.MyMethod.
class TestABC : C
{
MyType C.MyMethod()
{
// 1
return null;
}
MyType A.MyMethod()
{
// 2
return null;
}
MyType B.MyMethod()
{
// 3
return null;
}
}
The new keyword does not in anyway remove the "hidden" method. It just tells the compiler to tolerate the fact that the type has now two identical methods with the same signature, one inherited from a base type, and one declared by the current type.
Edit: OK, given the development of your question, here's what I think your problem really was (this wasn't clear to me initially):
You had this design:
public interface A {
MyType MyMethod();
}
public interface B {
MyType MyMethod();
}
public interface C : A,B{
}
Your problem was that this code didn't compile:
C myInstance = CreateAnInstanceOfSomeClassImplementingC();
myInstance.MyMethod(); // does not compile, ambiguous
Your question is if it's an OK solution to get rid of the compiler error error CS0121: The call is ambiguous between the following methods or properties: [...] by introducing the new member in C.
To me it seems ugly. But i cannot come up with another solution (when you informed me in the comments that C could not be a class). There's no way for an interface to dictate that two methods inherited must unite.
Whether using new keyword or not does not really change anything here. the behavior is just the same.
Testing testing = new Testing();
testing.MyMethod(); // calls Testing.MyMethod
AA testingA = new Testing();
testingA.MyMethod(); // calls AA.MyMethod
public interface A
{
int MyMethod();
}
public class AA : A
{
public int MyMethod()
{
return 11;
}
}
public interface B
{
int MyMethod();
}
public interface C : A, B
{
int MyMethod();
}
public class Testing : AA,C
{
public int MyMethod()
{
return 10;
}
}
I have case where am given a collection of objects that all derive from the same base class. If I iterate over the collection and check each item's type, I can see that the object is of a derived type and then handle it accordingly. What I would like to know is if there is an easier way of performing the check for the derived type besides what I am already doing. Code repetition typically isn't required, so my current methodology seems a bit off to me.
class A {}
class B : A {}
class C : A {}
class D : C {}
class Foo
{
public List<A> Collection { get; set; }
}
class Bar
{
void Iterate()
{
Foo f = new Foo();
foreach(A item in f.Collection)
{
DoSomething(a);
}
}
void DoSomething(A a)
{
...
B b = a as B;
if(b != null)
{
DoSomething(b);
return;
}
C c = a as C;
if(c != null)
{
DoSomething(c);
return;
}
D d = a as D;
if(d != null)
{
DoSomething(d);
return;
}
};
void DoSomething(B a){};
void DoSomething(C a){};
void DoSomething(D a){};
}
I am working with a web service where every web service must have the same result type.
class WebServiceResult
{
public bool Success { get; set; }
public List<Message> Messages { get; set; }
}
class Message
{
public MessageType Severity { get; set; } // Info, Warning, Error
public string Value { get; set; } //
}
class InvalidAuthorization: Message
{
// Severity = MessageType.Error
// Value = "Incorrect username." or "Incorrect password", etc.
}
class InvalidParameter: Message
{
// ...
}
class ParameterRequired: InvalidParameter
{
// Severity = MessageType.Error
// Value = "Parameter required.", etc.
public string ParameterName { get; set; } //
}
class CreatePerson: Message
{
// Severity = MessageType.Info
// Value = null
public int PersonIdentifier { get; set; } // The id of the newly created person
}
The goal is that we can return as many different types of messages back to the client as we want. Instead of getting a single message per web service call, the callee can know about all of their mistakes/successes in a single trip and to eliminate string parsing specific information from the message.
I originally though about using generics, but since the web service could have varying message types, the collection was broadened to use the base message class.
It may be possible to move DoSomething to A and have each subclass provide their own implementation:
public abstract class A
{
abstract void DoSomething();
}
void Iterate()
{
Foo f = new Foo();
foreach(A item in f.Collection)
{
item.DoSomething();
}
}
An idea would be to use a generic constraint on your base class or an interface.
public class MyClass<T> where T : BaseClass, IInterface
{
public void executeCode<T>(T param) {};
}
So MyClass<T> takes only a certain type, executeCode will have an idea what methods are exposed and what operations can be performed on the data of the object passed.
This avoids the need to cast because you are specifying a contract that must be followed.
typeof(ParentClass).IsAssignableFrom(typeof(ChildClass));
Return true is the cast is possible.
Also possible this way:
typeof(ParentClass).IsAssignableFrom(myObject.GetType());
But in your example, you actually call a methods for each object type. So you would need the cast anyway, unless you don't mind refactoring to not have a collection of overload.
Something like this if you want to keep the overload:
foreach(A item in f.Collection)
{
Type itemType = item.GetType();
if (typeof(B).IsAssignableFrom(itemType)
DoSomethingB(item);
else if (typeof(C).IsAssignableFrom(itemType)
DoSomethingC(item);
//...
}
EDIT: I like more Lee's answer. Adding virtual/override function to the class type would be a better design and way easier to handle, unless the DoSomething really has nothing to do being in the classes.
Lee is right. Just let the item decide, what to do. It knows it's type best and therefore knows what to do. You might even give some standard implementation, if it is the same as in A, by not making it abstract, but virtual. Be aware though, that the compiler wont ask for an implementation then.
public class A
{
public virtual DoSomething(){"What A needs doning!"}
}
public class B : A
{
public override DoSomething() {"What B needs doing!"}
}
Another way would be the use of Interfaces.
public interface IAinterface
{
void DoSomething();
}
public class A : IAinterface
{
void DoSomething(){...}
}
public class B : IAinterface
{
void DoSomething(){...}
}
This would be more like Lees suggestion, although interfaces and abstract base classes work a bit different in the background.
I usually prefer the upper one, because I usually tend to give the base class some standard behavior and only implement derived classes, when there is something different.