I am trying to figure out a way to force all of my Interfaces to include properties of the same name/type.
For example: I have two Interfaces; IGetAlarms and IGetDiagnostics. Each of the Interfaces will contain properties that are specific to the Interface itself, however I want to force the two Interfaces (and all other Interfaces that may be added later) to include properties of the same name. So, the result may look something like the this:
interface IGetAlarms
{
string GetAlarms();
DateTime LastRuntime { get; set; }
}
interface IGetDiagnostics
{
string GetDiagnostics();
DateTime LastRuntime { get; set; }
}
Notice that both Interfaces include a DateTime property named LastRuntime.
I would like to know if there is some way I can force other Interfaces that will be added later to include the DateTime LastRuntime property. I have naively attempted to have all my Interfaces implement another Interface (IService) - which includes the LastRuntime property. However, that doesn't solve my problem as that simply forces the class to implement the property - not all the Interfaces.
Thanks.
An interface can inherit from other interfaces.
interface IDerived : IBase
{
string Foo { get; set; }
}
interface IBase
{
DateTime LastRunDate { get; set; }
}
Any class deriving from IDerived will have to implement the methods/properties of IBase as well.
class Derived : IDerived
{
#region IDerived Members
public string Foo { get; set; }
#endregion
#region IBase Members
public DateTime LastRunDate {get;set;}
#endregion
}
If I understand your question correctly, you want to force a class to implement a number of different interfaces, the list of interfaces will grow with time but will have some properties in common.
The common property part you have solved with your IService interface. Something like this, I presume
interface IService
{
DateTime LastRuntime { get; set; }
}
interface IGetAlarms : IService
{
string GetAlarms();
}
interface IGetDiagnostics : IService
{
string GetDiagnostics();
}
The growing list of interfaces that a class will have to implement you can also solve in a similar fashion. Create a "composite" interface which inherits from all the interfaces you wish your class to implement
interface IComposite : IGetAlarms, IGetDiagnostics {}
class MyClass : IComposite
{
...
}
When you let the IComposite interface inherit a new interface, the class will have implement the new interface too.
EDIT
In response to your clarification; in that case you should not share the specification of the LastRuntime property, but declare it in each individual interface. In the implementing class you can use Explicit interface member implementation
class MyClass : IComposite
{
DateTime IGetAlarms.LastRuntime { get; set; }
DateTime IGetDiagnostics.LastRuntime { get; set; }
...
}
However, AFAIK it is not possible to force the implementing class to explicitly implement each individual interface
It really depends on exactly what you need the interface for. You can use generics to enforce the implementation of a specified pattern, but you can't enforce the implementation of each individually if they all have identical signatures.
public interface IA<T> where T: class
{
void DoIt(T ignore = null);
}
public interface IB : IA<IB>
{
}
public interface IC : IA<IC>
{
}
That would force the following class to implement each separately:
public class D : IB, IC
{
public void DoIt(IB ignore = null) { }
public void DoIt(IC ignore = null) { }
}
It's the "T ignore" parameter that forces each one to be implemented separately, and since it has a default value, you can just ignore that parameter unless calling it using reflection.
But obviously this doesn't work with properties, so they would have to be implemented using getter/setter methods.
Related
I want to have a generic interface which has a property that is the used as the Id property of the derived classes.
I wrote the interface like below:
interface IEntity<T>
{
T Id { get; set; }
}
and the derived classes can use it like below:
public class Employee : IEntity<int>
{
// don't need to define Id property
// public int Id { get; set; }
}
public class Document : IEntity<string> { ... }
Unfortunately, the compiler nags this error:
'Employee' does not implement interface member 'IEntity.Id'
What did i do wrong? thanks.
Edit:
While the accepted answer solves the problem, the #dbc comments helped me to achieve my goal, if i changed the interface IEntity to abstract class IEntity it doesn't need to implement the Id property in the derived class.
You are confusing inheritance and interface implementation.
When an interface inherits another interface, the members are inherited, and you do not repeat them:
interface IEntity<T>
{
T Id { get; set; } // necessary code for 'get' and 'set' not present (yet)
}
interface IEmployee : IEntity<int>
{
// don't need to repeat Id property
// it is inherited
}
Similarly when a class inherits another class:
class Entity<T>
{
public T Id { get; set; } // automatic code for 'get' and 'set' exists here
}
class Employee : Entity<int>
{
// don't need to repeat Id property
// it is inherited
}
The base class may be made abstract if you want to make sure only derived classes are instantiated.
But when a class (or a struct) implements and interface, every member of the interface must be implemented in some way. Usually by a public member of that class or struct. (That public member may be inherited from a base class!) Or, occasionally, by an explicit interface implementation.
An interface has no bodies, just signatures. For example the get and set accessors of the Id property must have some body to make sense. When you write T Id { get; set; } in an interface, there are no bodies of the accessors. But when you write T Id { get; set; } in a class (or struct), and there is no abstract or extern modifier, the semicolons have another meaning; then the compiler while auto-generate the necessary accessor bodies (and also auto-generate a field which is used by the accessors).
As in interface, ALL methods must be implemented!
Properties are nothing more than just methods, which have to be implemented when defined in interface: int Id { get; set; }.
You need to implement interface for every class - just like mentioned earlier:
interface IEntity<T>
{
T Id { get; set; }
}
public class Employee : IEntity<int>
{
public int Id { get; set; }
}
interface IAlpha
{
IBeta BetaProperty { get; set; }
}
interface IBeta
{
}
class Alpha : IAlpha
{
public Beta BetaProperty { get; set; } // error here
}
class Beta : IBeta
{
}
'InterfaceTest.Alpha' does not implement interface member 'InterfaceTest.IAlpha.BetaProperty'. 'InterfaceTest.Alpha.BetaProperty' cannot implement 'InterfaceTest.IAlpha.BetaProperty' because it does not have the matching return type of 'InterfaceTest.IBeta'.
My question is why is a property implementation restricted to the very same type. Why can't I use the derived type instead?
You have to implement the exact same interface. For example, this should be valid:
IAlpha alpha = new Alpha();
alpha.BetaProperty = new SomeOtherIBetaImplementation();
... but that wouldn't work with your code which always expects it to be a Beta, would it?
You can use generics for this though:
interface IAlpha<TBeta> where TBeta : IBeta
{
TBeta BetaProperty { get; set; }
}
...
public class Alpha : IAlpha<Beta>
Of course, that may be overkill - you may be better just using a property of type IBeta in Alpha, exactly as per the interface. It depends on the context.
An interface declares a set of methods the class will have, so anyone using that interface knows what to expect.
So, if you're implementing that interface, you must implement the exact interface, so all the other users get what they expected.
I am currently implementing the Quartz timer to allow scheduling of some data files. I have a abstract DataOutput class and then implementations to cover the different types of output (http, file, etc). I have implemented the interface on both specialisations but I am having compilation errors when I try and declare this on the abstract base in order to create objects of type DataOutput so I can deal with these at runtime.
Is this possible?
You'll need to at least abstractly implement the interface:
public interface IExample
{
string Word { get; set; }
void DoIt();
}
public abstract class ExampleClass : IExample
{
public string Word { get; set; }
public abstract void DoIt();
}
I'm not sure if you've done this since you didn't post any code or errors.
Greetings everyone...
If I have the following interface:
interface IMyInterface
{
int property { get; set; }
}
And the following implementation:
class MyClass : IMyInterface
{
// anything
}
How can I hide the set method of property from the instances of MyClass... In other words, I don't want the set method of property to be public, is that possible?
It would be easy to do with abstract class:
abstract class IMyInterface
{
int property { get; protected set; }
}
Then I could only set the property within the class that implements the abstract class above...
Don't have the set in the interface to begin with. You can still implement it as private.
You can't "hide" it, it's part of the contract. If you don't want it to be part of the contract, don't define it.
If you use the following interface the set method will be unavailable when classes are manipulated via the interface:
interface IMyInterface
{
int property { get; }
}
You could then implement the class like this:
class MyClass : IMyInterface
{
int property { get; protected set; }
}
If some implementations will only implement some parts of an interface, it may be a good idea to subdivide the interface into the parts which each implementation will either implement completely or not at all, and then define interfaces which inherit all the common combinations of them. Adapting your example:
interface IMyReadableInterface
{
int property { get; }
}
interface IMyFullInterface : IMyReadableInterface
{
new int property { get; set; }
}
Classes which want to support read-write access should implement IMyFullInterface; those which want to only support read access should only implement IMyReadableInterface. This segregation will not require any extra work for implementations of either interface which are written in C# and implement property implicitly. Code which implements property in VB, or explicitly implements property in C#, will have to define two implementations of property--a read-only one and a read-write one, but such is life. Note that while one could define an IMyWritableInterface which just had a setter, and have IMyFullInterface inherit both IMyReadableInterface and IMyWritableInterface, IMyFullInterface would still have to define a read-write property of its own, and when using explicit implementation one would then have to define three properties (I really don't understand why C# can't use a read-only and write-only property together as thought they were a read-write property, but it can't).
Assuming you need the setter to be part of the interface but for some reason it does not make sense for it to be used on a particular implementer (in this case MyClass) you can always throw an exception in the setter (such as an InvalidOperationException). This will not protect you at compile time, only at run time. It is a bit strange though, as code that operates on the interface has no idea whether calling the setter is allowed.
There are certainly cases where you want the interface to have a set and then hide it in some concrete class.
I believe the code below shows what we want to accomplish. I.e. the implementation hides the setter, but any IMyInterface aware component will have access to it.
public static void Main()
{
var myClass = new MyClass();
myClass.Property = 123; // Error
((IMyInterface)myClass).Property = 123; // OK
}
It's basically the same pattern you often see for IDisposable.Dispose() where you have an Explicit Interface Implementation. Here's an example for completeness.
public interface IMyInterface
{
int Property { get; set; }
}
public class MyClass : IMyInterface, IDisposable
{
public int Property { get; private set; }
int IMyInterface.Property
{
get => Property;
set => Property = value;
}
void IDisposable.Dispose() {}
}
Too much typing :(
C# doesn't help us much here. Ideally, it would be possible to have an explicit interface implementation for the setter:
// In C# 10 maybe we can do this instead:
public class MyFutureClass : IMyInterface
{
public int Property { get; IMyInterface.set; }
}
See C# feature proposal here.
There is no protected or private in interface, everything is public. Either you don't define any set or use it as public.
I have specified a couple of interfaces, which I am implementing as entities using Entity Framework 4. The simplest demonstration code I can come up with is:
public class ConcreteContainer : IContainer
{
public EntityCollection<ConcreteChild> Children { get; set; }
}
public class ConcreteChild : IChild
{
}
public interface IContainer
{
IEnumerable<IChild> Children { get; set; }
}
public interface IChild
{
}
I receive the following compiler error from the above:
'Demo.ConcreteContainer' does
not implement interface member
'Demo.IContainer.Children'.
'Demo.ConcreteContainer.Children'
cannot implement
'Demo.IContainer.Children'
because it does not have the matching
return type of
'System.Collections.Generic.IEnumerable'
My current understanding is that this is because IEnumerable (which is implemented by EntityCollection) is covariant but presumably not contravariant:
This type parameter is covariant. That is, you can use
either the type you specified or any type that is more
derived. For more information about covariance and contravariance,
see Covariance and Contravariance in Generics.
Am I correct, & if so, is there any way I can achieve my goal of specifying the IContainer interface purely in terms of other interfaces rather than using concrete classes?
Or, am I misunderstanding something more fundamental?
The generic variance in .NET 4 is irrelevant here. The implementation of an interface has to match the interface signature exactly in terms of types.
For example, take ICloneable, which looks like this:
public interface ICloneable
{
object Clone();
}
It would be nice to be able to implement it like this:
public class Banana : ICloneable
{
public Banana Clone() // Fails: this doesn't implement the interface
{
...
}
}
... but .NET doesn't allow this. You can sometimes use explicit interface implementation work around this, like so:
public class Banana : ICloneable
{
public Banana Clone()
{
...
}
object ICloneable.Clone()
{
return Clone(); // Delegate to the more strongly-typed method
}
}
However, in your case you can't ever do that. Consider the following code, which would be valid if ConcreteContainer was considered to implement IContainer:
IContainer foo = new ConcreteContainer();
foo.Children = new List<IChild>();
Now your property setter is actually only declared to work with EntityCollection<ConcreteChild>, so it clearly can't work with any IEnumerable<IChild> - in violation of the interface.
As far as I understand, you must implement an interface - you can't assume that a covariant/contra-variant member would be picked up as a substitute.
Even if it was permissible, note that setter for children is an issue. Because it will allow to set property of type EntityCollection<ConcreteChild> with value of any other type such as List<ConcreteChild> or EntityCollection<ConcreteChild2> because both are implementing IEnumerable<IChild>.
In current design, I will implement IContainer privately in ConcreteContainer and check the input value in IEnumerable.Children setter for a compatible type. Another way to approach this design is to have generic interfaces such as:
public interface IContainer<T> where T:IChild
{
IEnumerable<T> Children { get; set; }
}
So you need to implement this interface, right?
public interface IContainer
{
IEnumerable<IChild> Children { get; set; }
}
But in the real class, you want the property to be of type EntityCollection<ConcreteChild>. Here’s how you can do this:
public class ConcreteContainer : IContainer
{
// This is the property that will be seen by code that accesses
// this instance through a variable of this type (ConcreteContainer)
public EntityCollection<ConcreteChild> Children { get; set; }
// This is the property that will be used by code that accesses
// this instance through a variable of the type IContainer
IEnumerable<ConcreteChild> IContainer.Children {
get { return Children; }
set {
var newCollection = new EntityCollection<ConcreteChild>();
foreach (var item in value)
newCollection.Add(item);
Children = newCollection;
}
}
}