This question already has answers here:
Implementing multiple generic interfaces - type error
(5 answers)
Inheritance from multiple interfaces with the same method name
(9 answers)
Closed 4 years ago.
Say I have an interface WorksWithType<T> and class MyClass that implements both WorksWithType<TypeA> and WorksWithType<TypeB>.
If my interface looks like
public interface WorksWithType<T> {
void DoSomething(T foo);
void DoSomethingElse();
}
it is easy to implement two different DoSomething method overloads in MyClass.
public class MyClass : WorksWithType<TypeA>, WorksWithType<TypeB> {
{
public void DoSomething(TypeA fooA) { ... }
public void DoSomething(TypeB fooB) { ... }
...
}
However, there doesn't seem to be a way to implement overloads of DoSomethingElse. In my mind I feel as though I should be able to change the signature on the interface to be
void DoSomethingElse<T>();
and then overload the class with
public void DoSomethingElse<TypeA>() { ... }
public void DoSomethingElse<TypeB>() { ... }
What is the correct approach here, if there is one?
Assuming you want the two implementations of DoSomethingElse to be different , you need to use explicit interface implementation to distinguish between the calls:
public class TypeA {}
public class TypeB {}
public interface IWorksWithType<T>
{
void DoSomething(T foo);
void DoSomethingElse();
}
public class MyClass : IWorksWithType<TypeA>, IWorksWithType<TypeB>
{
public void DoSomething(TypeA fooA) {}
public void DoSomething(TypeB fooB) {}
// Note the syntax here - this indicates which interface
// method you're implementing
void IWorksWithType<TypeA>.DoSomethingElse() {}
void IWorksWithType<TypeB>.DoSomethingElse() {}
}
You don't have to make both of them use explicit interface implementation. For example:
public class MyClass : IWorksWithType<TypeA>, IWorksWithType<TypeB>
{
public void DoSomething(TypeA fooA) {}
public void DoSomething(TypeB fooB) {}
// Explicit interface implementation
void IWorksWithType<TypeA>.DoSomethingElse() {}
// Implicit interface implementation
public void DoSomethingElse() {}
}
If you don't need the implementations to be different, you can just have three methods of course:
public class MyClass : IWorksWithType<TypeA>, IWorksWithType<TypeB>
{
public void DoSomething(TypeA fooA) {}
public void DoSomething(TypeB fooB) {}
// Implementation of both IWorksWithType<TypeA>.DoSomethingElse()
// and IWorksWithType<TypeB>.DoSomethingElse()
public void DoSomethingElse() {}
}
That's assuming you to want the type parameter to be on the interface. You could put it on the method instead, but that really represents a very different interface - and you wouldn't be able to say that MyClass can only call DoSomethingElse for types TypeA and TypeB, for example.
By the current C# specifications (draft 6) https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/introduction
The signature of a method must be unique in the class in which the method is declared. The signature of a method consists of the name of the method, the number of type parameters and the number, modifiers, and types of its parameters. The signature of a method does not include the return type
(emphasis mine)
So, unfortunately,
void MyMethod<TypeA>()
and
void MyMethod<TypeB>()
would not have different signatures, so you cannot, per the specs, define both of them : they differ only by the type of their type parameter, but not by the number of their type parameter
Other answers have already pointed out how you could work around this (the explicit interface implementation is a good idiomatic option in my opinion)
Related
I would like this program to compile, and then print the output below:
public interface IFoo
{
void Bar();
}
public class FooBase : IFoo
{
void IFoo.Bar()
{
Console.WriteLine("Hello from base class.");
}
}
public class Foo : FooBase, IFoo
{
void IFoo.Bar()
{
(base as IFoo).Bar(); // doesn't compile
Console.WriteLine("Foo added some behavior!");
}
}
public static class Program
{
public static void Main(string[] args)
{
var foo = new Foo() as IFoo;
foo.Bar();
}
}
Desired output:
Hello from base class.
Foo added some behavior!
Obviously, the code above doesn't compile, because it's an invalid way to use the base keyword. Is there a way to accomplish this, without changing the implementation in the base class to a non-explicit one?
You can simply have the explicit interface implementation in the base class call a protected method in the class for its implementation. This allows other derived classes to still call that protected method while still explicitly implementing the interface (and also not publicly exposing the interface's method through the type itself, which presumably is the actual goal).
Here is a piece of my code:
public interface IA<in TInput>
{
void Method(IB<TInput> entities);
}
public interface IB<in T> { }
I can't figure out why I get following compile error:
"Parameter must be input-safe. Invalid variance: The type parameter |TInput| must be contravariantly valid on "IB< in T>".
Any help will be appreciated.
The designator of contravariance in C# (i.e. in) is intuitive only at the immediate level, when you make a method that "takes in" a parameter of generic type. Internally, however, contravariance means an inversion of a relation (Q&A with an explanation) so using in inside IA makes it incompatible with IB.
The problem is best illustrated with an example. Consider class Animal and its derived class Tiger. Let's also assume that IB<T> has a method void MethodB(T input), which is called from IA's Method:
class A_Impl<T> : IA<T> {
T data;
public void Method(IB<TInput> entities) {
entities.MethodB(data);
}
}
Declaring IA<in TInput> and IB<in TInput> means that you can do
IA<Animal> aForAnimals = new A_Impl<Animal>();
IA<Tiger> aForTigers = aForAnimals;
IA<in TInput> has a method that takes IB<TInput>, which we can call like this:
aForTigers.Method(new B_Impl<Tiger>());
This is a problem, because now A_Impl<Animal> passes an Animal to MethodB of an interface that expects a Tiger.
You would have no problem with IB<out T>, though - both with covariance and contravariance:
public interface IB<out T> {
// ^^^
}
// This works
public interface IA<in TInput> {
void Method(IB<TInput> x);
}
// This works too
public interface IC<out TInput> {
void Method(IB<TInput> x);
}
I'm realizing now that covariance is not available in abstract classes but is there anyway that I can utilize it here so that I can continue with this pattern.
Basically want the ability to create an instance of the first generic argument and pass the object which creates this object itself.
The below will fail at runtime because SpecialProcessor cannot be assigned to ProcessorBase with respect to generic types.
Appreciate any suggestions.
public class ProcessorUser<T> where T : ProcessorBase
{
public void ReceiveCommand()
{
Activator.CreateInstance(typeof (T), this);
}
}
public abstract class ProcessorBase
{
protected ProcessorBase(ProcessorUser<ProcessorBase> param)
{
}
}
public class SpecialProcessor : ProcessorBase
{
public SpecialProcessor(ProcessorUser<ProcessorBase> param)
: base(param)
{
}
}
Actually, from your less-than-complete code example, it's not clear at all a) what you are trying to do, and b) what "fails at runtime". You didn't show any code that calls the ReceiveCommand() method, so it's impossible to see in what way that code might fail.
That said, the usual way to gain access to variance in C# is through delegate or interface types. So you can declare a covariant interface to be implemented by ProcessorUser<T>, and then use that interface in the constructor declarations instead of the actual type. For example:
interface IProcessorUser<out T> where T : ProcessorBase
{
void ReceiveCommand();
}
class ProcessorUser<T> : IProcessorUser<T> where T : ProcessorBase
{
public void ReceiveCommand()
{
Activator.CreateInstance(typeof(T), this);
}
}
abstract class ProcessorBase
{
protected ProcessorBase(IProcessorUser<ProcessorBase> param)
{
}
}
class SpecialProcessor : ProcessorBase
{
private IProcessorUser<SpecialProcessor> _param;
public SpecialProcessor(IProcessorUser<SpecialProcessor> param)
: base(param)
{
_param = param;
}
public void ReceiveCommand() { _param.ReceiveCommand(); }
}
Note that I added the ReceiveCommand() method to the SpecialProcessor class just so I could see something execute at run-time. And that something does in fact work. But there's no way for me to know whether in your scenario, this is what you wanted to happen. You'd have to provide a good, minimal, complete code example that clearly shows what you are trying to do and what difficulty you are having doing it, if you want a clear, precise answer to that aspect of it.
(By the way, this really doesn't have anything to do with abstract classes. There's not even anything in your code example that is actually abstract, other than the class declaration itself, and the general principle applies to any class, not just abstract ones).
This question gives the answer that Java's #Override has the C# equivalent of the override keyword on methods. However, since Java 1.6 the #Override annotation can be applied to interfaces also.
The practical use for this is that in Java you get compile errors when a class claims it implements an interface method when it no longer does (e.g. if the interface method is removed). Is there equivalent functionality in C#?
Some code examples:
Java:
public interface A {
public void foo();
// public void bar(); // Removed method.
}
public class B implements A {
#Override public void foo();
#Override public void bar(); // Compile error
}
C#:
public interface IA {
void Foo();
// void Bar(); // Removed method.
}
public class B : A {
public override void Foo(); // Doesn't compile as not 'overriding' method
public void Bar(); // Compiles, but no longer implements interface method
}
There is similar functionality: explicit interface implementation.
public interface IA {
void foo();
// void bar(); // Removed method.
}
public class B : IA {
void IA.foo() {}
void IA.bar() {} // does not compile
}
The problem is that if you do this you cannot call the methods through the this pointer (from inside the class) or through an expression that evaluates to a B -- it is now necessary to cast to IA.
You can work around that by making a public method with the same signature and forwarding the call to the explicit implementation like so:
public class B : IA {
void IA.foo() { this.foo(); }
public void foo() {}
}
However this isn't quite ideal, and I 've never seen it done in practice.
Not really, although VB.Net does.
You could implement the method explicitly and have that call the normal public version:
public void bar() { ... }
void IA.bar() { bar(); }
As stated, you cannot get that kind of control from an interface alone in C#. You could get it from an abstract class however. For the purpose of completeness, here is what you could do:
public interface IA
{
void Foo();
//void Bar(); - removed
}
public abstract class A : IA
{
virtual void Foo()
{ }
// Removed method
//virtual void Bar()
//{ }
}
public class B : A
{
public override void Foo()
{ }
//throws an error like the one you were receiving regarding no method to override.
public override void Bar()
{ }
}
The #Override for interface in Java means 'implements'. When in Java a class implements an interface method and that method's signature is changed or the method is removed from the interface later the java compiler starts complaining about it.
This way it prevents the method to become 'dead code', you either have to remove the #Override annotation (so the method becomes a normal method) or remove or change the method to match the interface again. This is a very nice feature to keep your code clean. I would like C# to have this feature too.
I use explicit implementing as much as I can.
By the way: Resharper shows it when a method implements an interface method.
Could somebody kindly explain this to me, in simple words:
there is no way to constrain a type to
have a static method. You cannot, for
example, specify static methods on an
interface.
many thanks in advance to you lovely people :)
With generics, you can add a constraint that means the generic-type supplied must meet a few conditions, for example:
where T : new() - T must have a public parameterless constructor (or be a struct)
where T : class - T must be a reference-type (class / interface / delegate)
where T : struct - T must be a value-type (other than Nullable<TSomethingElse>)
where T : SomeBaseType - T must be inherited from SomeBaseType (or SomeBaseType itself)
where T : ISomeInterface - T must implement ISomeInterface
for example:
public void SomeGenericMethod<T>(T data) where T : IEnumerable {
foreach(var obj in data) {
....
}
}
it is SomeBaseType and ISomeInterface that are interesting here, as they allow you to demand (and use) functions defined on those types - for example, where T : IList gives you access to Add(...) etc. HOWEVER! simply: there is no such mechanism for things like:
constructors with parameters
static methods
operators / conversions
arbitrary methods not defined via a base-type or interface
so: you can't demand those, and you can't use them (except via reflection). For some of those dynamic can be used, though.
so, basically:
public class A{}
public class B{
public static void Foo() {}
}
You can't write a generic constraint for T in:
public class C<T> {}
Such that you restrict to accept only A or B based on the presence or non-presence of the static method Foo().
Imagine the following not working code:
interface IWithStatic
{
void DoIt(); // non-static
static void DoItStatic(); // static
}
class C1 : IWithStatic
{
void DoIt() {} // non-static
static void DoItStatic(){} // static
}
class C2 : IWithStatic
{
void DoIt() {} // non-static
static void DoItStatic(){} // static
}
And, in a program :
IWithStatic myObj = GetWithAnyMethod(); // Return a C1 or C2 instance
myObj.DoIt(); // working, as the runtime type is used (either C1 or C2);
but with the static... how can the compiler interpret this :
IWithStatic.DoItStatic(); // Not knowing which type to use
Do you see what's the problem now ?
It is not possible to have:
public interface IInterface {
static void Method();
}
This is because you are not allowed/able to constrain implementing classes to methods being static.