I'm trying to access a generic typed property in a child class. In the below example I recreated my problem. Is there a workaround for this problem, or is it simply not possible? Thanks in advance!
EDIT: It's not possible to declare the collection as A<Model> or A<T>.
public abstract class Model {
public int Id { get; }
}
public interface I<T> where T: Model {
ICollection<T> Results { get; }
}
public abstract class A { }
public class A<T> : A, I<T> where T : Model {
public ICollection<T> Results { get; }
}
public class Example {
A[] col;
void AddSomeModels() {
col = new A[] {
new A<SomeModel>(),
new A<SomeOtherModel>()
}
}
void DoSomethingWithCollection() {
foreach (var a in col) {
// a.Results is not known at this point
// is it possible to achieve this functionality?
}
}
}
You can't do what you intend without some compromises.
First of all, you need to make your interface I<T> covariant in T:
public interface I<out T> where T : Model
{
IEnumerable<T> Results { get; }
}
The first compromise is therefore that T can only be an output. ICollection<T> isn't covariant in T so you'd need to change the type of Results to IEnumerable<T>.
Once you do this, the following is type safe and therefore allowed:
public void DoSomethingWithCollecion()
{
var genericCol = col.OfType<I<Model>>();
foreach (var a in genericCol )
{
//a.Results is now accessible.
}
}
Related
Is it possible to pass a generic object as an argument with a class type that is derived from the expected class type?
The code below results in a compiler error, cannot convert from ItemObject<Equipment> to ItemObject<Item>, which I find odd because Equipment inherits from Item.
using System.Collections.Generic;
public abstract class Item { }
public abstract class Equipment : Item { }
public class ItemObject<T> where T : Item { }
public class EquipmentManager
{
ItemObject<Equipment> equipment = new ItemObject<Equipment>();
public void Unequip()
{
Inventory.instance.AddItem(equipment);
}
}
public class Inventory
{
public static Inventory instance;
List<ItemObject<Item>> items = new List<ItemObject<Item>>();
public void AddItem(ItemObject<Item> _item)
{
items.Add(_item);
}
}
You could achieve that using covariance, which would allow you to use a more derived type. However that can only be applied to interfaces, so you would need to do something like this:
public abstract class Item { }
public abstract class Equipment : Item { }
public interface IItemObject<out T> where T : Item { }
public class ItemObject<T> : IItemObject<T> where T : Item { }
public class EquipmentManager {
IItemObject<Equipment> equipment = new ItemObject<Equipment>();
public void Unequip() {
Inventory.instance.AddItem(equipment);
}
}
public class Inventory {
public static Inventory instance;
List<IItemObject<Item>> items = new List<IItemObject<Item>>();
public void AddItem(IItemObject<Item> _item) {
items.Add(_item);
}
}
More reading on covariance and contravariance.
The problem here isn't that Equipment is inherited from Item. The issue is you are creating a specific type from a generic class. ItemObject<T>-> ItemObject<Equipment>. There is no explicit or implicit method to convert an ItemObject<Equipment> to an ItemObject<Item>.
Where the implicit conversion would work is if you had an ItemObject<Item> and tried to add an Equipment in place of an Item. In this case, the Equipment would be implicitly cast as an Item before being passed to ItemObject<Item>'s method.
I am dealing with generics here but I have this strange situation where I trying to assign an instance of a class to a generic property.
class Context<A,T> where A: Answer<T>
{
void SomeMethod()
{
A answer; // suppose it have a value;
answer.context=this; // produce CS00029 error
}
}
class Answer<T>
{
Context<Answer<T>,T> context {get;set;}
}
To whom interested, I found a solution from someone outside this site, and that is his suggestion:
public class Context<A, T>: IContext<A, T> where A : Answer<T>
{
void SomeMethod()
{
A answer = Activator.CreateInstance<A>();
answer.context = this;
}
}
public class Answer<T>
{
public IContext<Answer<T>, T> context { get; set; }
}
public interface IContext<out A, T> {}
The Solution Link
I have problem.
For example, considering these 4 classes.
public abstract ParentContainer<T> where T: Parentfoo
{
public List<T> fooList;
}
//Let there be many different ChildContainers with different types.
public class ChildContainer : ParentContainer<ChildFoo>
{
}
public abstract class ParentFoo
{
public string name;
}
public class ChildFoo : ParentFoo
{
}
How can I write a method which accepts a List of any arbitrary ChildContainers as a parameter,
which do operation on their fooLists?
Is it even possible?
Additional Explanation, there are many different childs of ParentContainer,
each with a List of a different child of foo.
public class ChildContainerB : ParentContainer<ChildFooB>{}
public class ChildContainerC : ParentCOntainer<ChildFooC>{}
...
public class ChildFooB : ParentFoo;
public class ChildFooC : ParentFoo;
Then I need a method something like this
//X means it can be any arbitrary ChildContainer
public void method(List<ChilContainerX> list)
{
foreach(ChildContainerX x in list)
{
print(x.fooList.Last().name);
}
}
So what you are asking isn't possible because you need a concrete type for the List<>. There are a couple of workarounds though.
Use List<object>. This is obviously not great because as it means you lose type checking completely and could end up with anything being added to the list. So for that reason, I wouldn't recommend this approach.
Make ParentContainer<> implement a marker interface. For example:
public interface IParentContainer
{
}
public abstract class ParentContainer<T> : IParentContainer
where T: ParentFoo
{
//snip
}
And now you can have your list like this:
var containers = new List<IParentContainer>
{
new ChildContainer(),
new ChildContainer2()
};
Unless I misunderstand the question, you want something like this ...
public void DoStuffWith<T>(List<ParentContainer<T>> containers) where T : Parentfoo
{
//TODO: implement
}
This would work on objects of type ...
List<ParentContainer<ParentFoo>>
List<ParentContainer<ChildFoo>>
where ChildFoo : ParentFoo
and solves the issue of "List<ParentContainer<ParentFoo>> does not implement IEnumerable<ParentContainer<ChuldFoo>>" which I suspect is the compiler you are seeing.
Taking this a step further something like ....
public void DoStuffWith<ContainerT,ElementT>(List<ContainerT<ElementT>> containers)
where ContainerT : ParentContainer
where ElementT : Parentfoo
{
//TODO: implement
}
... likely over complicates the issue but i suspect is what you are trying to achieve.
At this point I would question the data structures you have and give them some common parent for example ...
public class ParentContainer<T> : IEnumerable<T> { ... }
public class ChildContainer<T> : ParentContainer<T>, IEnumerable<T> { ... }
Since both implement IEnumerable you can now do ...
public void DoStuffWith<T>(IEnumerable<T> containers) where T : ParentFoo
{
//TODO: implement
}
This avoids the need to be concerned with the collection type at all.
class Program
{
static void Main(string[] args)
{
List<ParentContainer<ChildFoo>> ca = new List<ParentContainer<ChildFoo>>();
ProcessContainers processor = new ProcessContainers();
ca.Add(new ChildContainerA { fooList = new List<ChildFoo>() });
ca.Add(new ChildContainerA { fooList = new List<ChildFoo>() });
ca.Add(new ChildContainerA { fooList = new List<ChildFoo>() });
ca.Add(new ChildContainerB { fooList = new List<ChildFoo>() });
processor.Process(ca);
}
}
public abstract class ParentContainer<T> where T: ParentFoo
{
public List<T> fooList;
}
//Let there be many different ChildContainers with different types.
public class ChildContainerA : ParentContainer<ChildFoo>
{
}
public class ChildContainerB : ParentContainer<ChildFoo>
{
}
public class ProcessContainers
{
public void Process<T>(List<ParentContainer<T>> childContainers) where T : ParentFoo
{
foreach(var item in childContainers)
{
foreach(var foo in item.fooList)
{
foo.Porcess();
}
}
}
}
public abstract class ParentFoo
{
public string name;
public abstract void Porcess();
}
public class ChildFoo : ParentFoo
{
public override void Porcess()
{
//Do some processing
}
}
i have a litte problem and i need some help :)
For example i have a simle abstract class
public abstract class BaseDefinition
{
public int Id { get;set; }
public string Name { get;set; }
}
and other base class
public abstract class BaseParentClass
{
public string Name { get;set; }
public string Schema { get;set; }
}
and first generic abstract class
public abstrac class BaseParentClass<T> :
BaseParentClass where T : BaseDefinition
{
public IList<T> Objects {get;set;}
}
and first implementations
public class ClassADefintion : BaseDefinition
{
public bool IsChanged {get;set;}
}
public class ClassAObject : BaseParentClass<ClassADefinition>
{
public bool OtherField {get;set;}
}
public class ClassBDefintion : BaseDefinition
{
public bool IsBBBChanged {get;set;}
}
public class ClassBObject : BaseParentClass<ClassBDefinition>
{
public bool OtherBBBBField {get;set;}
}
Sorry for class name, but I can't create anything better (it's only example)
As We see, now is all OK :).
I have some methods who returns a IEnumerable of generic implementation
IEnumerable<ClassBObject> ClassBObjectCollection;
IEnumerable<ClassAObject> ClassAObjectCollection;
Now i must create a method, who can take a generic objects in IEnumerable
public void DoWork(IEnumerable<BaseParentClass<BaseDefinition>> objects)
{
foreach(var baseObj in objects)
{
foreach(var baseDef in baseObj.Objects)
{
// do some work
}
}
}
How i remember BaseObject<BaseDefinition> != ClassAObject, but compiler doesn't put on screen any errors. I remember in .NET in generic interface We can use IN and OUT T, so i try make this
public interface IBaseParentClass<out T> where T : BaseDefinition
{
IList<T> Objects {get;set;}
}
Yup, You can't make a List of <out T>. Somebody have any idea for this problem ?
I can get this fields values by reflection, but i have abstract class and interface so i think is a better way.
I don't have a compiler at hand, but I think it should be possible to rewrite DoWork as such:
public void DoWork<T>(IEnumerable<BaseObject<T>> objects)
where T : BaseDefinition
{
foreach(var baseObj in objects)
{
foreach(var baseDef in baseObj.Objects)
{
// do some work
}
}
}
I am not sure whether the compiler will be able to infer T for you, try it out.
Another possibility may be that if you enumerate those objects anyway, to make Objects of Type IEnumerable(Of T).
I have a generic class that has one type parameter (T). I needed to store a collection of these generic objects that are of different types, so I created an interface that the generic class implements as suggested here. There is a property in the generic class of type T that I need to access when iterating through the generic list that contains the collection of Interface objects. So far the only way I have been able to get the value is to call a method using reflection.
interface ISomeClass {
//?
}
class SomeClass<T> : ISomeClass {
T ValueINeed { get; set;}
}
class ClassThatHasListOfGenericObjects{
List<ISomeClass> _l = new List<ISomeClass>();
public AddToList<T>(T someClass) : where T : ISomeClass {
_l.Add(someClass);
}
public SomeMethod(){
foreach(ISomeClass i in _l){
i.ValueINeed; //I don't know how to access the property in the generic class
}
}
}
As I see it you have two options. The easy option is to expose the value (as an object) on the interface (and possibly its type as well). Here's how that would look:
interface ISomeClass
{
object ValueINeed { get; set; }
// Only needed if you care about static type rather than using ValueINeed.GetType()
Type TypeOfValue { get; }
}
class SomeClass<T> : ISomeClass
{
public T ValueINeed { get; set; }
public Type TypeOfValue { get { return typeof(T); } }
object ISomeClass.ValueINeed { get { return ValueINeed; } set { ValueINeed = (T)value; } }
}
This has the disadvantage that there's a bit of casting going on and you might need to invoke reflection to do certain things with the value. It has the advantage that it's easy to understand and implement.
The other alternative would be to encode an "existential type" which truly represents a SomeClass<T> for some unknown T (like a SomeClass<?> in Java). This is much more complicated and hard to follow, but avoids any casts:
interface ISomeClassUser<X>
{
X Use<T>(SomeClass<T> s);
}
interface ISomeClassUser
{
void Use<T>(SomeClass<T> s);
}
interface ISomeClass
{
X Apply<X>(ISomeClassUser<X> user);
void Apply(ISomeClassUser user);
}
class SomeClass<T> : ISomeClass
{
public T ValueINeed { get; set; }
public X Apply<X>(ISomeClassUser<X> user) { return user.Use(this); }
public void Apply(ISomeClassUser user) { user.Use(this); }
}
// Assumes you want to get a string out, use a different generic type as needed
class XmlUser : ISomeClassUser<string>
{
public string Use<T>(SomeClass<T> s)
{
string str = "";
// do your conditional formatting here, branching on T as needed
// ...
return str;
}
}
class ClassThatHasListOfGenericObjects
{
List<ISomeClass> _l = new List<ISomeClass>();
XmlUser user = new XmlUser();
public string SomeMethod()
{
string s = "";
foreach (ISomeClass i in _l)
{
s += i.Apply(user);
}
return s;
}
}
Add ValueINeed to the interface and you'll be able to call it in SomeMethod().
I think you might just need a little refactoring. Looks like you're almost there
interface ISomeClass<T> {
T ValueINeed { get; set; }
}
class SomeClass<T> : ISomeClass {
T ValueINeed { get; set;}
}
class ClassThatHasListOfGenericObjects{
List<ISomeClass> _l = new List<ISomeClass>();
public AddToList<T>(T someClass) : where T : ISomeClass {
_l.Add(someClass);
}
public SomeMethod(){
foreach(ISomeClass i in _l){
i.ValueINeed; //this will work now, since it's in the interface
}
}
}
The elements' types you are using is of ISomeClass, so if want to access a member property you need to either cast i to SomeClass or add the property deceleration to the interface
interface ISomeClass {
T ValueNeeded
{
get;
set;
}
}
Note that you still need to implement the property in SomeClass.