Let's say I have an interface that many many distinct classes implement:
public interface IHaveObjects
{
object firstObject();
}
(Note: I can't make it an abstract base class as implementors of IHaveObjects may already have a base class.)
Now I want to add a new method to the interface, so that one implementer of the interface can have special behaviour for it. Ideally I would do something like this:
public interface IHaveObjects
{
object firstObject();
object firstObjectOrFallback()
{
return firstObject();
}
}
then go to that one implementor of the interface and give it the override:
public class ObjectHaverPlus : IHaveObjects
{
public override object IHaveObjects.firstObjectOrFallback()
{
return firstObject() ?? getDefault();
}
}
However it is forbidden in C# to provide a method body in an interface, and I would like to avoid going to every single implementer of IHaveObjects to drop in a definition of firstObjectOrFallback(). (Imagine if there are hundreds or thousands)
Is there a way to do this without lots of copy paste?
How about introducing a second interface which inherits from IHaveObjects.
Than you only have to change these classes, which need the new interface with the new method.
This looks like:
interface I1
{
void Method1();
}
interface I2 : I1
{
void Method2();
}
That's the problem with interfaces - they don't have any default implementation so any changes to them are breaking changes - i.e. code needs to be modified to work with new version of interface.
Since your implementations already have base classes on their own - you cannot turn it into abstract class, nor does C# have multiple class inheritance.
What you can do is to think - is it really a method on interface? Or could it be implemented as an extension method on interface (didn't try that but I suppose it will work just fine)?
If it is a method on interface and it should stay there - you may think of breaking this interface into two parts, second inheriting from the first (IHaveObjectsAndSupportDefault : IHaveObjects) and use this interface where default value is truly needed (like some other answers indicate).
I may have misunderstood your question, but why not use a second interface, something like:
public interface IHaveObjectsEnhanced
{
object FirstObjectOrFallback();
}
Then you could implement the first and second interface:
public class ObjectHaverPlus : IHaveObjects, IHaveObjectsEnhanced
{
public object FirstObject()
{
}
public object FirstObjectOrFallback()
{
return FirstObject() ?? GetDefault();
}
}
Related
I have a base class, say named B. I have two derived classes, D1 and D2.
Looks like:
public abstract class B
{
public abstract void DoSomething();
}
public class D1 : B
{
public override void DoSomething() { ... }
}
public class D2 : B
{
public override void DoSomething() { ... }
}
Now, I created a new interface IDeepCopyable<T> which has one method Clone():
public interface IDeepCopyable<T>
{
T Clone();
}
I want to force each subclass too implement this interface with the base class (i.e. IDeepCopyable<B>.
If I try to leave B's declaration as-is but just inherit from ('implement' is a more accurate term?) IDeepCopyable<T>, such as:
public abstract class B : IDeepCopyable<B>
Then to implement it in the derived classes (either implicitly or explicitly), the compiler gives me an error message "B does not implement interface member IDeepCopyable<B>.Clone()".
I can, of course, create an abstract method whose name is identical to the interface's method's name, but I find that ugly. Why to redeclare the method?
Can I, in any way, leave that as wanted?
I see that VS2019 has an option "Implement interface abstractly" so I think that the answer is no, but A) I want to be sure. B) If so, is there a design concept behind this behavior, or is it just a bug in C# design?
Thanks in advance.
I can, of course, create an abstract method whose name is identical to the interface's method's name, but I find that ugly.
That's the way to go. Your base class implements the interface. Hence it has to fulfil it. It can do this by implementing the method concretely (which is not desired here) or the base class can force its inheritors to do so by declaring the method abstractly.
You say that's ugly, well I think it's just explicit.
Anyway, there's no way around this ;-)
I'll start by saying that I am not a professional developer but have a ton of code that is used by various companies, mainly written with .net c# and vb.
With that said, I've never felt the need to get into extending existing classes and interfaces, and I'm struggling a bit now that I do want to do this, here's an example:
I have added a COM reference to my project of another application (can't edit this).
This reference has an interface that I want to extend, for example, _CellObject, I want to add some methods to it. In the past I'd build my own class to handle this, which works, but I think the more appropriate way would be to extend it.
So I build another interface, inherit from _CellObject and add my new methods.
Then I build a class that implements that interface, and this is where I realize I'm doing something wrong, all the methods from original interface must be added, but I don't want to do that. It's like I'm missing a "partial" somewhere or maybe this isn't possible?
Can someone push me in the right direction here?
It's true. If you extend an interface by inheriting from it, then when you implement a class that inherits from your new interface you will need to implement all the methods from your interface and the one it inherits from. In a sense, that's kinda the point of having a new interface inherit from an existing one. Otherwise you could just make a new interface and not inherit from an existing one.
COM objects don't support implementation inheritance or polymorphism, so there won't be any protected members for you to override. I don't typically see developers try to extend COM objects. If you need to add related functionality, you can wrap it (composition over inheritance) or you can write extension methods.
Your fourth point makes me assume you have some default-implementation of the existing interface that you want to re-use within the new one. As others already mentioned you can extend the existing interface by inheriting. However you also need to implement the "old" methods within a class implementing the new interface.
If you want to re-use the default-implementaion within your new class, you can just provide it to its constructor. So you have this code:
interface IBase
{
void DoSomething();
}
interface IDerived : IBase
{
void SoSomethingMore();
}
class MyBase : IBase
{
public void DoSomething() { ... }
}
class MyDerived : IDerived
{
private readonly MyBase _m;
public MyDerived(MyBase m) { this._m = m; }
// now you only need to forward the call for the existing interface to the injected base-class
public void DoSomething() => this._m.DoSomething();
public void DoSomethingMore() => ...
}
One option (excluding the issues you have with COM) is to create a new interface and class which include the functionality of the older versions. Now your new class can inherit from the original class, keeping it's functionality while extending it with a new interface.
public interface _CellObject
{
void DoSomething();
}
public interface _CellObject2 : _CellObject
{
void DoSomethingElse();
}
public class CellObject : _CellObject
{
public void DoSomething()
{
}
}
public class CellObject2 : CellObject, _CellObject2
{
public void DoSomethingElse()
{
DoSomething();
}
}
Simply you extend another Interface from your existing Interface, this is called Interface Segregation.
public interface IContract
{
void DoSomething();
}
public interface IContractChanged:IContract
{
void DoSomethingMore();
}
And now you can implement the new contract IContractChanged to meet your needs.
There are actaully two way to extend interface without modifying it. First create antoher interface that inherit from that interface. Imagine you have Interface A and you want to extend it.
interface A
{
void SomeMethodA();
}
interface B :A
{
void SomeMethodB();
}
Second you can directly implement that interface.
class C : A
{
public void SomeMethodA()
{
//your actual implementation
}
}
Been reading all day on interfaces and abstract classes trying to get a grasp on them to better understand the amazon library I'm working with. I have this code:
using MWSClientCsRuntime;
namespace MarketplaceWebServiceOrders.Model
{
public interface IMWSResponse : IMwsObject
{
ResponseHeaderMetadata ResponseHeaderMetadata { get; set; }
}
and
namespace MWSClientCsRuntime
{
public interface IMwsObject
{
void ReadFragmentFrom(IMwsReader r);
string ToXML();
string ToXMLFragment();
void WriteFragmentTo(IMwsWriter w);
void WriteTo(IMwsWriter w);
}
}
My first questions is I thought Interfaces cannot contain fields, however they can contain properties usch as ResponseHeaderMetadata?
Second, in my main program I have this line of code:
IMWSResponse response = null;
with response being later used to store the information that amazon sends back after a method call is invoked. But what is the meaning behind setting a variable of an interface type to null?
Also, a interface can implement another interface? It isn't only classes that can implement interfaces, but interfaces themselves as well?
Pproperties can be present in interfaces since properties are actually methods - the use of T GetSomeValue() alongside void SetSomeValue(T value) became so common in other languages, that C# implements these as properties.
The meaning behind setting an interface member to null is the same as setting anyother property to null - since a property's set accessor is a method, it's like calling any other method on the interface. What null means where is up to the implementation.
Interfaces do not implement each other, since and interface cannot contain code and therefore is not implementing; Interface inheritance allows one to require one interface in another. A big example is IEnumerable<T>, which is so closely tied to IEnumerable that it inherits, thus meaning any class implementing IEnumerable<T> must also implement IEnumerable.
An interface is like a contractual agreement. By inheriting an interface from a class, you are saying, "I agree to implement all of the methods defined in this interface". So if you have an interface like this:
public interface IWorker {
void DoWork();
}
and you use that interface like this:
public class Employee : IWorker
{
// you are forced to implement this method
void DoWork {}
}
public class Contractor: IWorker
{
// you are forced to implement this method
void DoWork {}
}
By "inheriting" interfaces by other interfaces, you are simply agreeing to implement any methods in the other interfaces, like so (from MSDN):
interface IBase
{
void F();
}
interface IDerived: IBase
{
void G();
}
class C: IDerived
{
void IBase.F() {...}
void IDerived.G() {...}
}
class D: C, IDerived
{
public void F() {...}
public void G() {...}
}
You do not have to set a variable of an interface type to null, though you have the power to do so. The great thing about interfaces is that you are able to set a variable of the type of interface, to anything that "inherits" that interface.
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; }
}
Suppose an Interface I has two methods. For example Method1() and Method2().
A class A Implements an Interface I.
Is it possible for class A to implement only Method1() and ignore Method2()?
I know as per rule class A has to write implementation of both methods. I am asking if there any way to violate this rule?
You can avoid implementing it (a valid scenario) but not ignore it altogether (a questionable scenario).
public interface IFoo
{
void A();
void B();
}
// This abstract class doesn't know what to do with B(), so it puts
// the onus on subclasses to perform the implementation.
public abstract class Bar : IFoo
{
public void A() { }
public abstract void B();
}
No, there's no such concept in C# of optional interface members. If A implements I, then it must provide some implementation for all of I's members, even if the implementation does nothing or only throws an exception.
public class A : I
{
public void Method1()
{
// Do nothing.
}
public void Method2()
{
throw new NotImplementedException();
}
}
From a design perspective, why would you want to do this anyway in a statically typed language? Furthermore, why not just have two interfaces?
public interface I1 { void Method1(); }
public interface I2 { void Method2(); }
With your interfaces coded like this, you can have classes that implement one interface or the other, or both, or neither. To me, this makes more sense anyway.
UPDATE 2018-06-13
The C# lang Git Hub has a proposal in progress for default interface methods. In short, the interface developer would be able to provide an implementation for a method or methods in the interface itself, and the developer using the interface on their class or struct would not have to implement those methods explicitly. Not exactly what the OP was asking about, but potentially useful.
You must implement all methods of the interfaces your class inherits from. There is no way around that. But you can use explicit interface implementation to hide the method.
That way a user doesn't see the method on a variable that has the class as type, but when he casts to the interface he can call the method.
class A : I
{
void I.Method2()
{
throw new NotSupportedException();
}
}
then
A a;
a.Method2(); //doesn't compile
I i = a;
i.Method2(); //works
If the class A is only an abstract base class, you can also use an abstract method to implement the interface, leaving the concrete implementation to the derived classes.
No, there's not.
But you can code :
public void Method2(){
throw new NotImplementedException();
}
That will inform the application that this method cannot be called from this instance.
Yes if I was a class, but No if it's an interface.