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>>.
Related
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
I have an interface
public interface IStrategy<T> where T : BaseModel
{
T GetModel(Guid userId);
}
and a concrete class inheriting the interface specifying that it should be a ConcreteModel
public class ConcreteStrategy: IStrategy<ConcreteModel>
{
ConcreteModel GetModel(Guid userId) { ... }
}
Now in the following method I can pass a new instance of ConcreteStrategy and everything works
public class Manager
{
public TModel GetContentModel<TModel>(IStrategy<TModel> strategy, Guid userId)
where TModel : ModelBase
{
return strategy.GetContentModel(userId);
}
}
But if I try to assign it to a property like this I get an error
public class Strategies
{
public static IStrategy<ModelBase> MyStrategy { get; set; }
}
Strategies.MyStrategy = new ConcreteStrategy();
Is there a way I can achieve this in C# ?
I want to be able to make a factory method that encapsulates the logic for which strategy to use and just return an instance of some type of strategy class (like ConcreteStrategy).
The error I am getting is:
Cannot implicitly convert type IStrategy<ModelBase> to IStrategy<ConcreteModel>
You need to make your interface covariant:
public interface IStrategy<out T> where T : BaseModel
Note that it will work only if T only appears in an output position in the interface (which is the case in the code you have shown, but I don't know if it's your real code).
I'm approaching a problem while still having some ignorance regarding Generics and their proper declarations / uses. I get the premiss, but some of the ins-n-outs of generics still elude me.
Given the following code (does not compile and contains code-smell):
public interface IUIConcern<out T> where T : IUIConcernExtension
{
string Name { get; }
Func<T> Extend();
}
public class UIConcern
{
public static void Register<T>(string concernName, IUIConcern<T> uiConcern) where T : IUIConcernExtension
{
Concerns.Add(uiConcern);
}
public static List<IUIConcern<T>> Concerns{
get {
// Logic...
}
set {
// Logic...
}
}
}
... I have a few questions:
Why do I have to specify this part public static void Register<T>(string concernName, IUIConcern<T> uiConcern) where T : IUIConcernExtension
with a constraint when I have already constrained the T in the declaration public interface IUIConcern<out T> where T : IUIConcernExtension
How can I have a property that holds a List<> of my IUIConcern<T> without knowing T other than knowing it will be derived from IUIConcernExtension?
Again, I realize this doesn't compile and is not correct, just looking to see how I can hold a list of generic items that may have many different type of IUIConcern<> elements.
Thank you!
You need to have a base interface, for instance:
public interface IUIConcern
{
string Name { get; }
}
public interface IUIConcern<out T> : IUIConcern where T : IUIConcernExtension
{
Func<T> Extern();
}
How you would define Concerns and Register would depend on how you treat T. Alternatively if you only deal with instances where you know T, you could use a Dictionary<Type, List<IUIConcern>> to hold anything, or potentially drop the base interface and just store using object depending on what you need in your controller code.
The problem is not located at the interface, but the problem is because of your generic implementation using static methods and properties.
The answer from Guvante was correct when saying that you need to define the IUIConcernExtension, but that is of course very logical, so im assuming you have just omitted that part since it does not matter for the issue you are facing.
The problem in the code is that you have created a class that has static methods and procedures, with the generic definition not laying at class level, but at methods level, because of this, the property that has and the Method cannot assume you are always with the same type!!
Lets say you call call :
Register<string>("something", UIConcern<string>)
but before that you have already called:
Register<Type>("something", UIConcern<Type>)
How could the compiler allows you to that?! So the answer is to define the generic type at class level, with this all properties and methods will be of same .
Also you need to use a private member for your list, since you doing everything static, the correct code should be:
interface IUIConcernExtension
{
string Name { get; }
}
public interface IUIConcern<out T> where T : IUIConcernExtension
{
Func<T> Extend();
}
public class UIConcern<T> where T : IUIConcernExtension
{
private static List<IUIConcern<T>> _Concerns = new List<IUIConcern<T>>();
public static void Register(string concernName, IUIConcern<T> uiConcern)
{
Concerns.Add(uiConcern);
}
public static List<IUIConcern<T>> Concerns
{
get { return _Concerns; }
set { _Concerns = value; }
}
}
using System.Collections.Generic;
public sealed class LoLQueue<T> where T: class
{
private SingleLinkNode<T> mHe;
private SingleLinkNode<T> mTa;
public LoLQueue()
{
this.mHe = new SingleLinkNode<T>();
this.mTa = this.mHe;
}
}
Error:
The non-generic type 'LoLQueue<T>.SingleLinkNode' cannot be used with type arguments
Why do i get this?
If you want to use IEnumerable<T>, as your post's title suggests, you need to include using System.Collections.Generic;.
As for the SingleLinkNode class, I don't know where you got it, it's not part of the .NET framework that I can see. I'd guess that it isn't implemented using generics, and you'll need to add a bunch of casts from object to T everywhere.
I'm pretty sure you haven't defined your SingleLinkNode class as having a generic type parameter. As such, an attempt to declare it with one is failing.
The error message suggests that SingleLinkNode is a nested class, so I suspect what may be happening is that you are declaring members of SingleLinkNode of type T, without actually declaring T as a generic parameter for SingleLinkNode. You still need to do this if you want SingleLinkNode to be generic, but if not, then you can simply use the class as SingleLinkNode rather than SingleLinkNode<T>.
Example of what I mean:
public class Generic<T> where T : class
{
private class Node
{
public T data; // T will be of the type use to construct Generic<T>
}
private Node myNode; // No need for Node<T>
}
If you do want your nested class to be generic, then this will work:
public class Generic<T> where T : class
{
private class Node<U>
{
public U data; // U can be anything
}
private Node<T> myNode; // U will be of type T
}
This compiles for me:
public sealed class SingleLinkNode<T>
{
}
public sealed class LoLQueue<T> where T : class
{
private SingleLinkNode<T> mHe;
private SingleLinkNode<T> mTa;
public LoLQueue()
{
this.mHe = new SingleLinkNode<T>();
this.mTa = this.mHe;
}
}
You'll need to post your SingleLinkNode class for further answers...
John
I am trying to do the following in C# 4.0:
I have a Base Class and 2 derived classes
public class Base {}
public class DerivedClass1 : Base {}
public class DerivedClass2 : Base {}
I want to do something like this, but it doesn't work.
How to I tell a Generic List to accept a Base Class and the derived classes of the base class.
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2(list);
}
}
public class Class_2
{
public Class_2(List<Base> list)
{
}
}
In Java I can do something like this
public class Class_2
{
public Class_2(List<? extends Base> list)
{
}
}
Does something like that exists in C#
I hope my question is clear, its just about the generics of the List.
Thanks a lot in Advance :)
General case:
function Foo<T>(List<T> list) where T : Base {
...
}
plus for interfaces and delegates, C# allows co/contravariance.
For e.g. IEnumerable<T> and IList<T>, your code will therefore work! Note that if this was allowed for List<T>s, you could insert a Derived1 into a list of Derived2s by using the common base class, which would break type safety. So simply stick to the above, readonly interfaces.
I think you mean either:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2<DerivedClass2>(list);
}
}
public class Class_2<T> where T : Base
{
public Class_2(List<T> list)
{
}
}
Or, if you want the constructor to be generic, and not the class:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
Class_2.Create(list);
}
}
public class Class_2
{
public static Class_2 Create<T>(List<T> list) where T : Base
{
// Stuff
return new Class_2(/*Stuff*/);
}
public Class_2()
{
}
}