Is there a way to define the type in a generic class like List to have contain objects which only implent multiple interfaces? Possibly class type and interfaces.
For example:
List<myObjectBase, IDisposable, IClonable> myList;
Not sure if I understood correctly, but how about this:
class MyList<T> : List<T>
where T : myObjectBase, IDisposable, IClonable
{
}
This way you can only add objects to the list which derive from the base and implement those interfaces.
No. In that case you would have to express that in the following way:
public class CommonStuff : MyObjectBase, IDisposable, IClonable {}
Then you can write:
List<CommonStuff> myList;
No, multiple generic parameters are not supported.
It wouldn't make much sense either. There would be no benefit of using the generic List<T> class over something like an ArrayList. You would lose all of the type safety benefits and you'd wind up still having to cast things all over the place.
The better option would be to create a composite class that handles all of the things you want to do...and then use that:
public class CommonBase : MyBaseClass, ICloneable, IDisposable
{
}
And then use that as your generic parameter:
var newList = new List<CommonBase>();
One approach that may be helpful is to define an interface ISelf<out T> whose one member, Self, simply returns "this" as a T; then for any interface IWhatever that might be combined, define a generic version IWhatever<out T> which inherits both the IWhatever and ISelf<T>. In that case, a class Whizbang which implements IFoo<Whizbang> and IBar<Whizbang> will implicitly implement ISelf<Whizbang>, IFoo<IBar<Whizbang>>, IBar<IFoo<Whizbang>>, etc. A routine which needs something that implements both IFoo and IBar can accept a parameter of type IFoo<IBar>; that parameter will implement IFoo; its Self property will implement IBar. Any object which implements multiple interfaces using this pattern may be cast to a nested interface type of the given form using some or all of the interfaces, listed in any order.
Below is the simplest solution for adding multiple interfaces that worked for me.
List<ICommonInterface> myList = new List<ICommonInterface>()
myFirstClass m1C = new myFirstClass();
mySecondClass m2C = new mySecondClass();
myList.Add(m1C);
myList.Add(m2C);
foreach (var item in myList)
{
item.Clone();
item.Dispose();
}
class myFirstClass : ICommonInterface
{
// implement interface methods
}
class mySecondClass : ICommonInterface
{
// implement interface methods
}
interface ICommonInterface : IDisposable, IClonable
{
}
interface IDisposable
{
void Dispose(){}
}
interface IClonable
{
void Clone(){}
}
You can use an ArrayList and can check the Type of an object in this list - maybe it is handier.
if(list[i] is Type)
Related
In .NET Core C#
I' trying something like this:
(IInterface<IParameter>)instance
Where instance is new Implementation<Parameter>()
And Implementation : IInterface & Parameter : IParameter
The issue is with the casting of the generic parameter. When I provide Parameter instead of IParameter it works but at compile time there is no way to know which type that implements IParameter will be used. All of these objects will be created via reflection.
So is there any way this cast works? Or some other way to implement this like providing no generic type parameter like you can with typeof.
EDIT Thanks to Ziriax
A Fully Working Example:
interface IInterface
{
void Run(TInput input);
}
abstract class AbstractClass<TInput> : IInterface
where TInput : IParameter
{
public abstract void Execute(TInput input);
public void Run(IParameter input)
{
Execute((TInput)input);
}
}
interface IParameter {}
class Implementation : AbstractClass<Parameter>
{
public void Run(Parameter input)
{
}
}
class Parameter : IParameter {}
class Program
{
static void Main()
{
object instance = new Implementation();
var castInstance = (IInterface) instance;
castInstance.Run(new Parameter());
}
}
Why don't you add a non-generic interface too:
interface IInterface
{
void Run(IParameter input);
}
And then let your generic interface extend this non-generic one.
Obviously your implementations should cast the IParameter, someone needs to cast it... You could make an abstract base class that does this for you, so not every implementation has to do this.
You might also be interested in the double dispatch pattern, although I'm not sure this will work in your case.
As you have it now, this cannot work. Your Implementation class implements IInterface<Parameter>, so its Run method only accepts a parameter of the concrete Parameter type, whereas the IInterface<IParameter> requires that its Run method accepts an instance of any type that implements IParameter.
If the type of cast you're trying to do were allowed, I could define a different class that implements IParameter, e.g.:
public class DifferentParameter : IParameter { ... }
And then do:
castInstance.Run(new DifferentParameter());
But your Implementation's Run method can't take DifferentParameter!
.NET therefore prevents you from performing the cast itself.
There are situations in which this kind of cast is allowed - if your interface were instead to be defined as:
interface IInterface<out TOutput>
where TOutput : IResult
{
TOutput Run();
}
By making the generic parameter out, it makes the interface covariant. This restricts the use of the type parameter as the result of method calls, but for covariant interfaces, casts like yours are allowed.
You can find plenty of documentation on both covariance and contravariance in the .NET documentation.
I have often wanted to create a list of objects where each object must implement a number of interfaces. For example, I'd like to do something similar to the following:
List<T> where T : IConvertible, IComparable _myList;
Another option I considered was to create a third interface that implements these two so that any object that implements these two interfaces inherently implements mine.
public interface IConvertibleAndComparable
: IConvertible, IComparable { }
List<IConvertibleAndComparable> _myList;
With this I would be able to add any object that implements both IConvertible and IComparable, including double and int, as well as my own objects. Explicitly implementing IConvertibleAndComparable is not required since it does not add any new functionality beyond the interfaces in inherits.
I understand that the first snippet is illegal and the second, while legal, does not do what I want. Is it possible to achieve what I am trying to do? If not, would either of these be a candidate for a future C# feature?
(Note: This would be legitimate application for empty interfaces.)
Edit
In a more general sense, I'd like to perform one of the following:
private MyGenericClass<T> where T : IA, IB, ... _myClass;
where I can declare all of the restrictions on T that I need, or
public interface IMyCombinedInterface : IA, IB, ... {}
private MyGenericClass<IMyCombinedInterface> _myClass;
where any type that implements IA, IB, and ... inherently (or implicitly) implements IMyCombinedInterface (only when IMyCombinedInterface doesn't explicitly declare new functionality).
You can, as a workaround, do a kind of superposition wrapper, and store it in the list. Look here for the idea.
To your example you could do:
public class Junction
{
public IConvertible Convertible { get; private set; }
public IComparable Comparable { get; private set; }
private Junction() { }
public static Junction Create<T>(T value) where T : IConvertible, IComparable
{
return new Junction
{
Convertible = value,
Comparable = value
};
}
}
And then add Junctions to your list:
var l = new List<Junction>();
l.Add(Junction.Create(1));
While working on another project, I had an idea for another workaround. While it may not quite work for the example I posted, it could work for the original intent of the question, which was to create a combination interface for IEnumerable<T> and INotifyCollectionChanged (as #DanielHilgarth pointed out). The workaround is as follows:
Create an empty interface which implements the combination of interfaces you need.
public interface INotifyingEnumerable<T> :
IEnumerable<T>,
INotifyCollectionChanged
{
}
Create a new class which inherits from a class implementing both of these and implements the new interface.
public class MyObservableCollection<T> :
ObservableCollection<T>,
INotifyingEnumerable<T>
{
}
Now you can use the interface INotifyingEnumerable<T> as the property type, which using your implementation.
The reason this doesn't quite cover the original requirements, however, is that it doesn't allow ObservableCollection<T> to be directly assigned. The feature requested in the original post would allow this.
On MSDN - C# Programming guide Constraints on Type Parameters, it says:
where T : interface_name
The type
argument must be or implement the
specified interface. Multiple
interface constraints can be
specified. The constraining interface
can also be generic.
Could somebody kindly explain, what it means to have a generic interface? And explain how that can be a constraint and what it provides?
A simple example and a simple explanation is highly appreciated.
Many thanks in advance : )
You can use a generic interface as a constraint. For example:
class MyClass<T> where T:IEnumerable<string>
you can even substitute the generic parameter of the type you define into your constraint:
class MyClass<T> where T:IEnumerable<T>
An example of a generic interface is IEnumerable<T>. It represents some collection you can enumerate. The type of the items in the collection is not relevant to the interface, so it allows you to specify that with a generic parameter.
You can for example create a class like this:
class Foo<T, E> where T : IEnumerable<E>
{ }
This way, the generic parameter T can only be a collection of type E. The constraining interface is generic as well. You can also do this:
class Foo<T> where T : IEnumerable<string>
{ }
In which case you're not allowing any type of collection, only collections of strings. You can go pretty crazy with this, like this:
class Foo<T> where T : IEnumerable<T>
{ }
Where T has to be some collection that contains collections of T.
One use is when you want a function or class to work generically but want to constrain which types can be used with it (so you don't have to make multiple overloads for instance).
Completely arbitrary code example:
interface IAnimal { }
interface IShape { }
class Tiger : IAnimal { }
class Wolf : IAnimal { }
class Circle : IShape { }
class Rectangle : IShape { }
public void MakeSound<T>(T animal) where T : IAnimal
{
}
public void Draw<T>(T shape) where T : IShape
{
}
This isn't actually how you would structure this type of functionality mind you :) You can use other constraints too:
public void someFunction<T>(T input) where T : IShape, new() // constraint to IShape, allow construction of new T with parameterless constructor
All in all they are useful for generic functions constrained to certain types.
The sentence only states that the constraining interface can not only be a class or interface, but a generic one too.
For instance, such a constraint is a valid one:
public class Controller<TModel,TRepository> where TRepository: Repository<TModel>{...}
Is there anyway to force a constraints for a generic definition to implement a "generic interface" ... that is, I want the class to support passing an interface and a generic class constraining it so that the class implements the interface. For example if I say:
MyGenericClass<IMyInterface, MyImplementation>.DoSomething();
That should be constrained so that MyImplementation implements IMyInterface
As far as I know that can be achieved by
public class Dynamic_Loader<T, S> where S: T
Now, is there anyway to also force T to be an interface?
Edit: The purpose of this was to have something like:
private static List<T> interfaceList = new List<T>();
public static List<T> InterfaceList {get { return interfaceList;}}
public static void Add(S input) { interfaceList.Add(input);}
and have the list restricted to only interfaces (since it is supposed to return implementations of certain interfaces)
Do you mean, can a constraint also be put on T like where T : interface?
If so, then no: this list pretty much covers your options.
What you have is as close as it gets, I believe.
Out of curiosity, what would be your reason for wanting to constrain T to be an interface?
Or do you mean can a constraint also be put on T for T to implement some specific interface?
If so, then yes: simply have two where clauses (e.g., where S : T where T : U).
where T: IMyOtherInterfaceForT
Example:
public class Test<T, V>
where T : V
where V : IEnumerable<int>
{
}
You could do something like this to enforce it at runtime instead of compile time.
public class Test<T> where T : class
{
public Test()
{
Type t = typeof( T );
if( !t.IsInterface )
throw new ArgumentException( "T must be an interface type" );
}
}
I have an interface with a single generic type parameter:
public interface IDriveable<T> where T : ITransmission { ... }
I also have a class that has a type parameter that needs to be of that interface type:
public class VehicleFactory<T> where T : /* ??? */
There is a problem here with this declaration. I can't put "IDriveable", because that has no type parameters and doesn't match the type signature of IDriveable. But I also don't want to put IDriveable<U> either, because then VehicleFactory has to know what kind of IDriveable it's getting. I want VehicleFactory to accept any kind of IDriveable.
The proposed solution a coworker had was to use:
public class VehicleFactory<T, U> where T : IDriveable<U>
But I don't like this, since it's redundant. I have to say the "U" type twice:
var factory = new VehicleFactory<IDriveable<AllWheelDrive>, AllWheelDrive>();
What should go in the question marks?
What is VehicleFactory going to do with T? Does it actually need the constraint in order to work, or is it just for the sake of developer sanity checking?
One common way round this is to declare a non-generic interface (IDriveable) and then make your generic one extend that:
public interface IDriveable {}
public interface IDriveable<T> : IDriveable {}
public class VehicleFactory<T> where T : IDriveable
If you do want the factory to be able to do things with T, you could put any interface members from IDriveable<T> which don't care about T into the nongeneric IDriveable.
Does this work for you?
public class VehicleFactory<T, U> where T : IDriveable<U>
This will let the factory know what types the driveables are.
You can define the VehicleFactory with 2 types generics and anign one of them to the interface. something like:
public class VehicleFactory<T1,T2> where T1 : IDriveabel<T2>
I hope that ist not what you ment with the Idriveable. I guess the U is a specific type. Like String, etc.
You can shorten often used cases like this:
interface IDriveableAllWheel : IDriveable<AllWheelDrive>
{}
var factory = new VehicleFactory<IDriveableAllWheel, AllWheelDrive>();
or even
class AllWheelFactory : VehicleFactory<IDriveableAllWheel, AllWheelDrive>
{}
also see a kvb's answer in Calling Generic Property In Generic Class From Interface Implemented By Generic Class for possible workaround.