This question already has answers here:
How to determine if a type implements a specific generic interface type
(14 answers)
Check if a type implements a generic interface without considering the generic type arguments
(2 answers)
Closed 1 year ago.
Background
My code base currently has the following interfaces and classes:
// Both DisplayValue _classes_
public class DisplayValue<T>
{
public T Value { get; set; }
}
public class DisplayValue : DisplayValue<object> {}
// Both IValueWidget _interfaces_
public interface IValueWidget<T>
{
DisplayValue<T> Value { get; set; }
}
public interface IValueWidget: IValueWidget<object>
{
new DisplayValue Value { get; set; }
}
// the BaseWidget all other widgets implement
public interface IBaseWidget
{
public string Name { get; set; }
}
The code base also has a bunch of Widgets, all of which implement IBaseWidget. Some also implement IValueWidget or IValueWidget<T>, which leads me to my problem.
Problem
Say I have the following method:
public T2 DoStuff<T2>(T2 widget) where T2 : IBaseWidget
{
// this should check if the widget is the correct type
if (widget is IValueWidget)
{
}
}
In that method, I would like to have the condition work with either IValueWidget or any generic version of IValueWidget. I don't care whatsoever what the type of T is, only that the given T2 can be treated as such.
Also worth mentioning, literally no types other than the ones provided above are known at compile time. Everything must work at runtime.
In Java, I could do something like this but obviously C# does not have wildcards.
public T2 DoStuff<T2 extends IBaseWidget>(T2 widget) {
if (widget instanceof IValueWidget<?>) {
}
}
Notes
As far as I can tell, this is not actually a duplicate. Please don't flag it as such if you're referring to any of these, as they are similar but definitely not the same:
Check if object implements specific generic interface
Wildcard equivalent in C# generics
Get interfaces implemented by class
Related
This question already has answers here:
Cannot convert type via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion
(3 answers)
Closed 2 years ago.
I am new to using interfaces, but after reading up on it, I thought the idea of interfaces was that a class derived from an interface would be accepted anywhere the interface was accepted. Here's my code:
public interface IPersonOfInterest
{
//code requiring certain info to exist
}
public abstract class PersonOfInterest
{
public string[] GetBigPersonsInfo(List<IPersonOfInterest> FromList)
{
//code to gather a list of info that is guaranteed to be in any IPersonOfInterest
return new string[] { };
}
}
public class BigDonors : PersonOfInterest, IPersonOfInterest
{
public List<BigDonors> SuchDonors = new List<BigDonors>();
public void GimmeDemInfos()
{
string[] GetInfo = GetBigPersonsInfo(SuchDonors); //<-- compiler error here
}
}
As you can see, BigDonors is derived from the IPersonOfInterest interface. So why does it give a compiler error, saying a list of BigDonor cannot be converted to list of IPersonOfInterest? I understand that they aren't the same thing. I think I know what I'm trying to do here, but it isn't letting me do it.
EDIT: My question was quickly marked as already answered, however, the answer provided only explains the problem but doesn't really give a solution. So I'm editing this question with my solution:
For my particular case, I don't need to be able to add donors to the list, at least not in the abstract method. So Andrew Shepherd's link revealed that the problem was that, although my class could convert to the interface, a list cannot. So now I'm passing a read only list, which the compiler accepts:
public interface IPersonOfInterest
{
//code requiring certain info to exist
}
public virtual class PersonOfInterest : IPersonOfInterest
{
//Changed to IReadOnlyList<IPersonOfInterest>, instead of List<IPersonOfInterest>:
public string[] GetBigPersonsInfo(IReadOnlyList<IPersonOfInterest> FromList)
{
return new string[] { };
}
}
public class BigDonors : PersonOfInterest
{
public List<BigDonor> SuchDonors = new List<BigDonor>();
public void GimmeDemInfos()
{
//Added .AsReadOnly(), and it now compiles:
string[] GetInfo = GetBigPersonsInfo(SuchDonors.AsReadOnly());
}
}
You have identified the purpose of interfaces correctly. You need to use List<IPersonOfInterest>, because that complies with the description.
In short
BigDonor is inherited IPersonOfInterest, but List<BigDonor> was NOT inherited from List<IPersonOfInterest>. This means that you will need List<IPersonOfInterest> to be passed, but you will have the opportunity to add BigDonor elements to that List.
I'm trying to make an mock data generator which will generate mocks based on a set of rules. I've been playing around with Bogus. I don't want to have to setup my rules for my entity classes each time, I would like to generically be able to apply rules to classes if they derrive from some interface.
Imagine I have entities which reuse a generic interface:
public interface IHasGeneric<T>
where T : IHasGeneric<T>
{
string Marker { get; set; }
}
public class Foo : IHasGeneric<Foo>
{
public string Marker { get; set; }
}
public class Bar : IHasGeneric<Bar>
{
public string Marker { get; set; }
}
Note: I'm aware this doesn't depict why I have a generic which takes in itself as a parameter. However, it takes too much to explain and cannot be changed from my architecture. So please work with it as a requirement.
Now I want to create a centralized Factory For Fakers, However I'm struggling to figure out how I can apply the rules generically to any type that is going to be generated.
public class MockDataGenerator
{
public T Generate<T>()
where T : class
{
var faker = new StatefulFaker<T>();
this.ApplyDefaultRules<T>(faker);
}
public void ApplyDefaultRules<T>(StatefulFaker<T> faker)
where T : class
{
//T Cannot be used as a type parameter 'T' ... No Implicit Conversion to IHasGeneric<T>
if (typeof(T) is IHasGeneric<T>>)
{
}
}
}
When trying to cast T to see if rules can be applied I get an error
T Cannot be used as a type parameter 'T' ... No Implicit Conversion to IHasGeneric. How can I generically apply rules to types which implement an interface?
Just another small C# training app, and just another Compilation Error, but it cannot just go away from me... I am just wondering, what I am doing wrong here:
public abstract class Material
{
}
public abstract class Cloth<T> where T:Material
{
public T Prop { get; set; }
}
public class Cotton : Material
{
}
public class Dress<T> : Cloth<T> where T : Material
{
}
public class Test
{
private Cloth<Material> cloth;
public Test()
{
/* below won't compile */
cloth = new Dress<Cotton>();
}
}
I want to get the base class object from a closed constructed class. Anyone ?
When trying to compile I get the error:
Cannot implicitly convert type Dress<Cotton> to Cloth<Material>
What you want to achieve is called covariance (see the following article for samples).
Unfortunately, there's no variance support for classes: it's restricted to interfaces and delegates.
Thus and alternatively, you might design an interface called ICloth<T> with T covariant:
public interface ICloth<out T>
{
T Prop { get; set; }
}
And implement it in any of your possible cloths, including Cloth<T>.
Now type cloth as ICloth<T> and your assignment should work (i.e. cloth = new Dress<Cotton>();), because Dress<Cotton> is ICloth<out T>, which is an interface with a T covariant generic parameter.
Learn more about generic interface with variance in the following article on MSDN.
This question already has answers here:
The return type of the members on an Interface Implementation must match exactly the interface definition?
(6 answers)
Does C# support return type covariance?
(9 answers)
Closed 8 years ago.
Why can I not do the following?
public class TestClass : TestInterface
{
public ClassX Property { get; private set; }
}
public interface TestInterface
{
InterfaceX Property { get; }
}
public interface InterfaceX
{
}
public class ClassX : InterfaceX
{
}
The TestInterface Property is readonly, thus can only return InterfaceX as per the contract.
However, I get this compiler error:
'TestClass' does not implement interface member
'TestInterface.InterfaceX'. 'TestClass.InterfaceX' cannot implement
'TestInterface.InterfaceX' because it does not have the matching
return type of 'InterfaceX'.
It does not have the matching type but it has a subclass of that type.
I don't know the spec offhand, but I'm sure there's one that explicitly states that return types must match exactly for interface implementations. The closest I can find is 13.4.4:
For purposes of interface mapping, a class member A matches an interface member B when:
A and B are methods, and the name, type, and formal parameter lists of A and B are identical.
A and B are properties, the name and type of A and B are identical, and A has the same accessors as B (A is permitted to have additional accessors if it is not an explicit interface member implementation).
If "type" above means "return type" that would indicate that the return type cannot change.
You could, however, change the return type and explicitly implement the interface with the right return type:
public class TestClass : TestInterface
{
public ClassX InterfaceX { get; private set; }
InterfaceX TestInterface.InterfaceX { get { return InterfaceX; } }
}
UPDATE
According to Eric Lippert it seems to be a CLR limitation, not just a C# one.
You mentioned that you want to expose a reduced set but you want all the functionality internal to the class -- that's not what you want to use an interface for. An interface should only be about your reduced set contract, not also magically function as a full set internally, not without another helper property.
But, there is a way around this limitation while still communicating the contract a little.
interface IExpose<IToolType> where IToolType : ITool
{
IToolType Handler { get; set; }
}
class Expose : IExpose<Tool>
{
public Tool Handler { get; set; }
}
interface ITool
{
}
class Tool : ITool
{
}
Why doesnt this work?
public class ClassOptions {}
public interface Inode {
ClassOptions Options {get;}
}
public class MyClass : Inode {
public ClassOptions Options { get; set; }
}
public class ClassDerivedOptions : ClassOptions {
}
public class MyDerivedClass : Inode {
public ClassDerivedOptions Options { get; set; } << does not implement INode...
}
[ the compiler message tells me why it breaks but i'd like to know the reasoning behind why the compiler doesnt let this through - also if there are any work arounds? - thanks]
It doesn't work because the INode interface explicitly calls for an Options property of type ClassOptions. C# doesn't support return type covariance (which is what you're asking for in this case).
For what it's worth, there's also a language feature request on Microsoft Connect specifically for return type covariance:
Need covariant return types in C# / all .NET languages
If you look at the page, they also mention that the common work-around is to use Explicit Interface Implementation:
public class MyDerivedClass : INode
{
public ClassDerivedOptions Options { get; set; }
public ClassOptions INode.Options { get { return Options; } }
}
As Justin notes, the feature you want is called "return type covariance" and it is not supported in C#, or, for that matter, in the CLR type system.
Though it is frequently requested, it is extremely unlikely (*) that this feature will be implemented any time soon. Since it is not supported in the CLR, in order to implement it we would simply have to generate all the helper methods that do the call forwarding for you. Since you can already do that "manually" with a small amount of code, there is little value added by the compiler doing it for you. (And as another question today notes, people sometimes get confused or irritated when the compiler generates a method to do interface forwarding on your behalf.)
Don't get me wrong; I can see how it comes in handy, and I've used this feature in C++. But every time it has come up in a C# program, I've found I can work around its absence pretty easily.
(*) Of course five years ago I would have said exactly the same thing about named and optional parameters, and now they're in C# 4. It is possible for an unlikely feature to be implemented, but the demand has to be pretty darn high.
It doesn't work because an interface defines a contract and when you implement this contract method signatures must match exactly. A possible workaround is to use a generic interface:
public class ClassOptions
{ }
public class ClassDerivedOptions : ClassOptions
{ }
public interface INode<T> where T : ClassOptions
{
T Options { get; }
}
public class MyClass : INode<ClassOptions>
{
public ClassOptions Options { get; set; }
}
public class MyDerivedClass : INode<ClassDerivedOptions>
{
public ClassDerivedOptions Options { get; set; }
}
The standard way to deal with this situation is to implement the interface explicitly:
public class MyDerivedClass : Inode {
// New, more specific version:
public ClassDerivedOptions Options { get; set; }
// Explicit implementation of old, less specific version:
ClassOptions Inode.Options
{
get { return Options; }
}
}
This is how most old IList implementations worked before generics, for example: specifying a more specific T this[int index] property and then explicitly implementing object IList.this[int index], throwing an exception when the set got called with an object of the wrong type.
In the example code you posted, you don't even need an explicit set, as that is not a member of your Inode interface.