I just found out after trying to implement an immutable interface property, that C# apparently does not support the behavior that I am looking for. What I planned was quite simple, objects would subscribe to an interface that held an immutable generic property, that means a property without a setter. However, to my surprise, a subscribing class can still add the setter without any sort of error. In fact, it appears as though C# interfaces entirely ignore the signature of a property altogether. All it cares about is the property type and name.
For example:
namespace Some.Arbitrary.Framework
{
public interface IIdentifiable<T>
{
/// <summary>
/// Classes that subscribe to this interface
/// may still add a 'set;' signature.
/// </summary>
T Identifier { get; } // the lack of 'set;' is ignored
}
}
I've seen many posts on StackOverflow saying that C# does not support this behavior, but my question is: why does C# not support immutable properties in interfaces like I have described? Is there a fundamental design flaw behind what I am intending to do?
Because interfaces define a contract. When you say that a contract requires a get, you say that the implementer must provide a get, you cannot say "you can only do this", so, given:
public interface IData
{
string Data { get; }
}
You are actually saying
public interface IData
{
string GetData();
}
The client can always do this:
public class Data : IData
{
public string Data { get; set; }
}
Because it's actually doing this:
public class Data : IData
{
public string GetData() {}
public void SetData(string data) {}
}
So, Data implements the IData contract, as far as IData cares.
The most similar structure you could use is this:
public interface IIdentifiable<T>
{
T Identifier { get; }
}
public abstract class IdentifiableBase<T> : IIdentifiable<T>
{
T Identifier { get; protected set; }
}
And make your classes implement IdentifiableBase<T>, or you could just keep the IdentifiableBase class and leave the interface behind altogether.
Related
I have a little problem when it comes to making Generics work in C#:
I have the following class:
public abstract class OphControl<TDataModel> : Control
where TDataModel : OphDataModel, new()
{
/// <summary>
/// The data model for this control.
/// </summary>
public TDataModel DataModel { get; private set; }
public IEnumerable<OphControl<OphDataModel>> OphControls {
get {
return Controls.Cast<Control>().Where(control => control is OphControl<OphDataModel>).Cast<OphControl<OphDataModel>>();
}
}
protected OphControl() {
DataModel = new TDataModel();
}
}
My problem is, that the OphControls property won't compile, because OphDataModel is abstract, and therefore I suppose it cannot be sure at compile-time that OphDataModel has a zero-args public constructor (notice the new() constraint on TDataModel).
I'm not even sure if this is the right way to go about it. What I really want is something like OphControl<? extends OphDataModel> from Java. I tried adding out to my TDataModel in the class declaration, but it tells me only delegates and interfaces can have covariant type parameters.
So how do I get around this pickle?
Here are a few things of confusion. First, OphControl<TDataModel> does not derive from OphControl<OphDataModel> and you are correct about the out keyword (covariance/contravariance), they can only be used with delegates and interfaces. Since it is not a base class, you may not be able to cast it.
Unlike java, c# does not support generic wildcards (...<? extends ...>). The work around is to create a non-generic version and make it the base class.
public abstract class OphControl : Control { ... }
public abstract class OphControl<TDataModel> : OphControl
where TDataModel : OphDataModel, new() { ... }
Unfortunately, the base class cannot have the DataModel property because c# does not support covariant return types either.
At least you can have
public IEnumerable<OphControl> OphControls {
get {
return Controls.OfType<OphControl>();
}
}
I thing you just need to change the generic parameter on OphControls:
public IEnumerable<OphControl<TDataModel>> OphControls {
get {
return Controls.Cast<Control>().Where(control => control is OphControl<TDataModel>).Cast<OphControl<TDataModel>>();
}
}
This should compile, but without knowing more about your classes it's hard to say if this is right
In certain scenario like a MVVM view-model, I sometimes needs to have private setter as the view-model exposes a state that can only be modified internally.
So is this wrong to need a private setter on an interface? (and I mean not particularly in the described scenario) If not, why does the C# compiler does not allow it?
Thanks.
By definition, an interface is a contract for other code to use, not for private members. However, you can specify read-only properties in interfaces and implement a private setter in the concrete class:
public interface IFoo
{
string MyReadonlyString { get; }
}
public class FooImplementation : IFoo
{
public string MyReadonlyString { get; private set; }
}
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.
Why doesnt this work?
public class ClassOptions {}
public interface Inode {
ClassOptions Options {get;}
}
public class MyClass : Inode {
public ClassOptions Options { get; set; }
}
public class ClassDerivedOptions : ClassOptions {
}
public class MyDerivedClass : Inode {
public ClassDerivedOptions Options { get; set; } << does not implement INode...
}
[ the compiler message tells me why it breaks but i'd like to know the reasoning behind why the compiler doesnt let this through - also if there are any work arounds? - thanks]
It doesn't work because the INode interface explicitly calls for an Options property of type ClassOptions. C# doesn't support return type covariance (which is what you're asking for in this case).
For what it's worth, there's also a language feature request on Microsoft Connect specifically for return type covariance:
Need covariant return types in C# / all .NET languages
If you look at the page, they also mention that the common work-around is to use Explicit Interface Implementation:
public class MyDerivedClass : INode
{
public ClassDerivedOptions Options { get; set; }
public ClassOptions INode.Options { get { return Options; } }
}
As Justin notes, the feature you want is called "return type covariance" and it is not supported in C#, or, for that matter, in the CLR type system.
Though it is frequently requested, it is extremely unlikely (*) that this feature will be implemented any time soon. Since it is not supported in the CLR, in order to implement it we would simply have to generate all the helper methods that do the call forwarding for you. Since you can already do that "manually" with a small amount of code, there is little value added by the compiler doing it for you. (And as another question today notes, people sometimes get confused or irritated when the compiler generates a method to do interface forwarding on your behalf.)
Don't get me wrong; I can see how it comes in handy, and I've used this feature in C++. But every time it has come up in a C# program, I've found I can work around its absence pretty easily.
(*) Of course five years ago I would have said exactly the same thing about named and optional parameters, and now they're in C# 4. It is possible for an unlikely feature to be implemented, but the demand has to be pretty darn high.
It doesn't work because an interface defines a contract and when you implement this contract method signatures must match exactly. A possible workaround is to use a generic interface:
public class ClassOptions
{ }
public class ClassDerivedOptions : ClassOptions
{ }
public interface INode<T> where T : ClassOptions
{
T Options { get; }
}
public class MyClass : INode<ClassOptions>
{
public ClassOptions Options { get; set; }
}
public class MyDerivedClass : INode<ClassDerivedOptions>
{
public ClassDerivedOptions Options { get; set; }
}
The standard way to deal with this situation is to implement the interface explicitly:
public class MyDerivedClass : Inode {
// New, more specific version:
public ClassDerivedOptions Options { get; set; }
// Explicit implementation of old, less specific version:
ClassOptions Inode.Options
{
get { return Options; }
}
}
This is how most old IList implementations worked before generics, for example: specifying a more specific T this[int index] property and then explicitly implementing object IList.this[int index], throwing an exception when the set got called with an object of the wrong type.
In the example code you posted, you don't even need an explicit set, as that is not a member of your Inode interface.
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.