Using generics in interfaces in C# - c#

I have an app written in C#. My app has a class that looks like the following:
public class FinderA
{
public IEnumerable<FinderA> GetItems()
{
return FinderA.FindAll();
}
}
I want to require other classes to do something similar. I cannot use a base class because my actual implementation is already using a base class. For that reason, I want to create an interface. Currently, I'm trying the following:
public interface IFinder
{
IEnumerable<T> GetItems();
}
When I use this approach, I get a compile-time error that says: "The type or namespace name 'T' could not be found (are you missing...". To overcome this, I add <T> to the end of the interface name so it looks like this:
public interface IFinder<T>
{
IEnumerable<T> GetItems();
}
This is turn generates another compile-time error that says: "Using the generic type 'IFinder' requires 1 type arguments.". My challenge is, I want the interface to be generic. I do not want to pass in a type. Is there a way around this? If so, what/how?

There is no way around this; you'll need to actually supply the generic type argument when declaring that a class implements the interface.

You can do this at the method level instead of as a generic type on the interface itself.
public interface IFinder
{
IEnumerable<T> GetItems<T>();
}
Your code can then call it like such:
IFinder finder = // Create finder instance
IEnumerable<MyClass> discoveredClasses = finder.GetItems<MyClass>();
If you want to ensure that MyClass is a class that implements IFinder, you can constrain the method.
public interface IFinder
{
IEnumerable<T> GetItems<T>() where T : IFinder;
}
That will cause the following to generate a compiler error:
public class Foo
{
}
public class Bar
{
Bar()
{
IFinder finder = // Create finder.
// This fails because <T> (Foo) does not implement IFinder.
IEnumerable<Foo> fooItems = finder.GetItems<Foo>();
}
}
but it will allow the following to compile
public class MyFinderA : IFinder
{
IEnumerable<T> GetItems<T>() where T : IFinder
{
return // Create collection of T
}
public class Bar
{
Bar()
{
IFinder finder = // Create finder.
// This works as <T> (MyFinderA) is an IFinder implementation
IEnumerable<MyFinderA> finderItems = finder.GetItems<MyFinderA>();
}
}

If you want your interface to be generic but you are not willing to supply a type argument, you can return an IEnumerable<object>.
public interface IFinder {
IEnumerable<object> GetItems();
}
All class types extend from System.Object so that should suffice for any type in your applicacion (enums and structs would get boxed)
Please note that this approach requires the interface's consumer to cast the generic object instances to the appropiate types.
The alternate (and recommended) approach is to make the interface use type parameters, but for that you must supply the appropiate type arguments

Related

Cannot create a list of generic interfaces

i have this code:
public interface IDataScopeType { }
public interface IDataProcessor<in T> where T : IDataScopeType { }
public class DataScopeType : IDataScopeType { }
public class DataProcessor : IDataProcessor<DataScopeType> { }
public class Executor
{
List<IDataProcessor<IDataScopeType>> processors;
public Executor()
{
processors = new List<IDataProcessor<IDataScopeType>>
{
new DataProcessor(),
};
}
}
Please explain why the compiler shows an error "Cannot convert from DataProcessor to IDataProcessor" when I try to add a new item to the list? Or am I doing something wrong?
The List<> item type is IDataProcessor<IDataScopeType>, the class DataProcessor inherits from IDataProcessor<DataScopeType> which is a different type despite that DataScopeType inherits from IDataScopeType, because, for example, when you have implemented DataScopeType you could add a data members and/or methods of which IDataProcessor<DataScopeType> will be aware of, so it has access to DataScopeType, and not to the less concrete interface IDataScopeType. To fix the issue there is a couple of ways you can do that:
Change the DataProcessor generic type parameter to be the interface type, like that: public class DataProcessor : IDataProcessor<IDataScopeType> { }
OR, Declare the list with the type that match the base class type of DataProcessor, like that: List<IDataProcessor<DataScopeType>>.

How To Define A Generic Type Parameter That Is Self Referencing

Consider the following class/interface definitions:
public abstract class Foo<TFoo> : IFoo
where TFoo: Foo<TFoo>
{
public TFoo This => (TFoo)this;
}
public interface IFoo { }
Suppose I also had a method defined where I wanted to pass in a TFoo<> into it:
public static void Test<T>() where T : IFoo { }
Is this possible without defining a class that removes the self-referencing type constraint?
public abstract class Foo2 : Foo<Foo2> { }
Is there a way to call the Test method without the non-self-referencing class?
Test<Foo<?>>();// What goes here <?>
Test<Foo2>(); // This works but I don't want to create a Foo2 class..
If you are asking whether or not you can skip providing the Generic type, i.e:
Test<Foo<>>
then no you can't, because it is inferred at compile time.
Generally, everything concerning generics and type parameters is specified at compile-time and you can't omit to provide the Generic type parameter or provide a default value.

.NET Core Cast to interface with generic type parameter of type which implements interface

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.

Cast Generic instance to Base class

I am trying to attempt something such as
void Main()
{
Temp<Bar> Test = new Foo<InheritedBar>();
}
abstract class Temp<T> where T : Bar
{
}
class Foo<T> : Temp<T> where T : Bar
{
}
abstract class Bar
{
}
class InheritedBar : Bar
{
}
The cast does not work, with the error Cannot implicity convert type Foo<InheritedBar> to Temp<Bar>.
However,
Temp<InheritedBar> Test = new Foo<InheritedBar>();
and
Temp<Bar> Test = new Foo<Bar>();
Both work. Why even though InheritedBar inherits from Bar, it can't be cast to it through generics?
I am using the generic type in a wpf Page, which can not be created as a generic so I can't pass T as its type. I only want the functionality at the time of this of Temp, not any of the derived versions functionality. Is there a better way to do this?
The concept you are attempting to utilize is covariance (full explanation on MSDN)
The short answer is that you need to use mark your generic parameter with out; but you can only do that on an interface (changing from an abstract class in your case). Also, depending on your method parameters and return values, you may not be able to mark your interface as covariant.
interface Temp<out T> where T : Bar
{
}

C# interface access implementers type for generic

Is there a way for an interface to be automatically generic for the implementer class?
For example, I want to create an interface that will return a delegate off of the implementing class. (Actually I want it to return an array of multiple functions, but I'm simplifying here.)
The following should work:
public interface IParentFinder<ChildClass>
{
Func<ChildClass, int> GetParentIdFunction();
}
And then to implement it, I have to write this:
public class AClass : IParentFinder<AClass>
{
int id;
int parentId;
static public Func<AClass, int> GetParentIdFunction()
{
return c => c.parentId;
}
}
However I only ever want this interface to be used where the generic template matches the implementing class. Is there some convenience that I can use so that instead I can just write this:
public class AClass : IParentFinder
{
...
}
I was hoping there would be some baseType parameter I could use to specify the base type for generics. For example:
public interface IParentFinder
{
Func <baseType, int> GetParentIdFunction();
}
edited for clarity
As any other parameter, either method or generic parameters, a generic interface type parameter requires an argument. A parameter without an argument isn't a parameter at all.
Maybe C# vNext could implement something like default generic parameter value:
public class A<T> where T : default(string) { ... }
...or who knows what. But, for now, a generic parameter requires an argument. And, after all, this wouldn't fit what you're looking for in your case - auto-magically-infered generic parameters... -.
No, there is no feature in C# that does this.

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