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
Related
Currently I am in the phase of refactoring my code after it has been unit tested, and I have some concerns about the refactoring from a design point of view with regards to type safety. My original code looked a bit like this:
Interfaces
public interface IBase
{
int ID { get; set; }
}
public interface IFirstSub : IBase
{
string Description { get; set; }
}
public interface ISecondSub : IBase
{
decimal Total { get; set; }
}
public interface IThirdSub : IBase
{
int Count { get; set; }
}
public interface IBaseContainer
{
void Add(IBase baseParam);
}
Implementations
public class FirstContainer : IBaseContainer
{
public void Add(IBase baseParam)
{
if (!(baseParam is IFirstSub || baseParam is ISecondSub))
{
throw new ArgumentException(nameof(baseParam));
}
// Do Something
}
}
public class SecondContainer : IBaseContainer
{
public void Add(IBase baseParam)
{
if (!(baseParam is IThirdSub))
{
throw new ArgumentException(nameof(baseParam));
}
// Do Something
}
}
With my original implementation of FirstContainer and SecondContainer, it was repeating the same logic at the start of the Add method, so I thought I would refactor the code to look something like this:
public abstract class BaseContainer : IBaseContainer
{
private readonly List<Type> _types = new List<Type>();
protected BaseContainer(params Type[] baseTypes)
{
_types.AddRange(baseTypes);
}
public void Add(IBase baseParam)
{
if (_types.All(type => !type.IsInstanceOfType(baseParam)))
{
throw new ArgumentException(nameof(baseParam));
}
DoSomething(baseParam);
}
protected abstract void DoSomething(IBase baseParam);
}
public class ThirdContainer : BaseContainer
{
public ThirdContainer() : base(typeof(IFirstSub)) { }
protected override void DoSomething(IBase baseParam)
{
// Do Something
}
}
With this refactoring done, it successfully removes the duplication of the code from the start of the Add method, but my main concern with the refactoring is the fact that the call to the base constructor base(typeof(IFirstSub)) is not really type safe. By that, I mean I can call the base constructor like base(typeof(object)) for example, and it will compile. For the purposes of my project, I'd like to constrain the types to ones that inherit IBase, and enforce at compile time.
Is there anyway to overcome this limitation, or would a new design be needed in order to achieve this?
No it's not type safe
Passing and validating types at run-time is not type-safe, as type-safety is a compile-time concept. In my opinion your refactoring effort does not improve the code, and in fact does something quite weird.
Function overloading
If you need a method that accepts either of two types, you can use function overloading:
public class FirstContainer : IBaseContainer
{
public void Add(IFirstSub param)
{
// Do Something
}
public void Add(ISecondSub param)
{
// Do Something
}
}
The compiler will automatically choose the right prototype for you, and will not allow anything other than an IFirstSub or ISecondSub.
Create another interface
Another approach requires you to add an interface for the types that have something in common, like this:
interface ICanBeHeldInFirstContainer
{ }
public interface IFirstSub : IBase, ICanBeHeldInFirstContainer
{
string Description { get; set; }
}
public interface ISecondSub : IBase, ICanBeHeldInFirstContainer
{
decimal Total { get; set; }
}
Then you do this:
public class FirstContainer : IBaseContainer
{
public void Add(ICanBeHeldInFirstContainer param)
{
// Do Something
}
}
or this:
public class FirstContainer : IBaseContainer
{
public void Add<T>(T param) where T : ICanBeHeldInFirstContainer
{
// Do Something
}
}
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.
}
}
I have these classes:
public class BaseGame<T> where T : BaseQuestion
{
//some fileds and methods
}
public class BaseQuestion
{
//some fileds and methods
}
public class Question : BaseQuestion
{
//some fileds and methods
}
public class SampleGame : BaseGame<Question>
{
//some fileds and methods
}
Whenever i want to cast BaseGame to SampleGame i get Error.
void SetValue(BaseGame<BaseQuestion> _game)
{
SampleGame = (SampleGame) _game;
}
Cannot implicitly convert type BaseGame to SampleGame.
How To Fix this?
Thanks.
Change your code to this:
public interface IBaseGame<out T> where T : BaseQuestion
{
}
public class BaseGame<T> : IBaseGame<T>
where T : BaseQuestion
{
}
public class BaseQuestion
{
}
public class Question : BaseQuestion
{
}
public class SampleGame : BaseGame<Question>
{
}
And then your method:
void SetValue(IBaseGame<BaseQuestion> _game)
{
var SampleGame = (SampleGame) _game;
}
Why was your code broken?
You cannot do the cast because BaseGame was not covariant. This means that the compile cannot gaurantee that the cast is safe. Take for example, the following example:
public class BaseGame<T> where T : BaseQuestion
{
private List<T> Questions { get; set; }
public void AddQuestion(T question) { Questions.Add(question); }
}
var game = new BaseGame<Question> { Questions = new List<Question> };
game.AddQuestion(new Question());
Okay, so now your game has a list of Question, with one question in it. Now let's cast it:
public class BrokenQuestion : BaseQuestion
{
}
public class BrokenGame : BaseGame<BrokenQuestion>
{
}
var brokenGame = (BrokenGame)game;
brokenGame.Add(new BrokenQuestion());`
Oops! We just added a BrokenQuestion to a List<Question>. No good.
Defining the template argument as out T instead of T means we're making a contract that we will not accept sub classes of T, but we can return them. This means that it's now illegal to expose AddQuestion(T question). Now, the compiler can be sure that there will never be a BrokenQuestion added to List<Question>.
For more information, see here
Can someone explain to me why this is incorrect in C#:
namespace NamespaceA
{
public class ClassA<T1>
{
T1 mT1;
public T1 Type1
{
get { return mT1; }
}
}
public class IOForClassA
{
public interface ICanOutput
{
void OutputFunction();
}
public static void Output(ClassA<ICanOutput> aClassA_WithOutputCapabilities)
{
aClassA_WithOutputCapabilities.Type1.OutputFunction();
}
}
}
namespace NamespaceB
{
public class ClassB
{
public class OutputableClassA : NamespaceA.IOForClassA.ICanOutput
{
public void OutputFunction()
{
}
}
public ClassB()
{
NamespaceA.ClassA<OutputableClassA> aOutputableA = new NamespaceA.ClassA<OutputableClassA>();
NamespaceA.IOForClassA.Output(aOutputableA);
}
}
}
This will result in a compile error of:
Argument 1: cannot convert from 'NamespaceA.ClassA"<"NamespaceB.ClassB.OutputableClassA">" to NamespaceA.ClassA"<"NamespaceA.IOForClassA.ICanOutput">"
... but NamespaceB.ClassB.OutputableClassA implements NameSpaceA.IoForClassA.ICanOutput, so I don't see why this is a problem...
I am trying to allow a user to create a ClassA of any type he/she wishes. However, if they wish that ClassA to be "outputable" its templated type must implement a specific interface.
You are hitting up on a covariance/contravariance problem. Basically, only interfaces can allow generic types to be of a more derived type, and only if they are explicitly specified using the in/out keywords (outlined in the linked article).
Reading your code, it seems as if you want to call a known named member of a class without knowing its type, which is something that type constraints might be a little better suited for. You might have to change your design a bit though.
namespace NamespaceA
{
public class ClassA<T1> where T1 : IOForClassA.ICanOutput
{
T1 mT1;
public T1 Type1
{
get { return mT1; }
}
}
public class IOForClassA
{
public interface ICanOutput
{
void OutputFunction();
}
public static void Output<T>(ClassA<T> aClassA_WithOutputCapabilities) where T : IOForClassA.ICanOutput
{
aClassA_WithOutputCapabilities.Type1.OutputFunction();
}
}
}
namespace NamespaceB
{
public class ClassB
{
public class OutputableClassA : NamespaceA.IOForClassA.ICanOutput
{
public void OutputFunction()
{
}
}
public ClassB()
{
NamespaceA.ClassA<OutputableClassA> aOutputableA = new NamespaceA.ClassA<OutputableClassA>();
NamespaceA.IOForClassA.Output(aOutputableA);
}
}
}
#syazdani is correct about it being a covariance problem (and he links to a useful article), but his example doesn't then use covariance, which would achieve what you are looking for. Although his answer is correct (and likely preferrable as it has the added benefit of bringing along the actual type in case you need it into the Output method, I figured I would show you how you could also do this with a covariant interface in case it helps you in any other case where you may use them:
namespace NamespaceA
{
public interface IClassA<out T1>
{
T1 Type1 { get; }
}
public class ClassA<T1> : IClassA<T1>
{
T1 mT1;
public T1 Type1
{
get { return mT1; }
}
}
public class IOForClassA
{
public interface ICanOutput
{
void OutputFunction();
}
public static void Output(IClassA<ICanOutput> aClassA_WithOutputCapabilities)
{
aClassA_WithOutputCapabilities.Type1.OutputFunction();
}
}
}
namespace NamespaceB
{
public class ClassB
{
public class OutputableClassA : NamespaceA.IOForClassA.ICanOutput
{
public void OutputFunction()
{
}
}
public ClassB()
{
NamespaceA.ClassA<OutputableClassA> aOutputableA = new NamespaceA.ClassA<OutputableClassA>();
NamespaceA.IOForClassA.Output(aOutputableA);
}
}
}
OutputableClassA is derived from ICanOutput. But it does not mean that ClassA<OutputableClassA> is also derived from ClassA<ICanOutput>. Thats why your code is not working.
Link Covariance and Contravariance in Generics might be helpful.
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.