I apologize for a weak title, but I have no idea on how exactly to phrase my problem in 1 sentence.
In order to explain my issue, I'm first going to paste my code for better understanding.
I have a class Generic:
public class Generic<TObject, ID>
where TObject : class, Entity<ID>
where ID : IComparable
Now I want to use objects of this class, and its children, as parameters for another class. Something like this:
public class GenericService<TGeneric>
where TGeneric: Generic<TObject, ID>
where TObject: class, Entity<ID>
where ID : IComparable
But the above code gives me an error in the last 2 where clauses. Error message says something along the lines of Cannot resolve symbol TObject/ID, because I have not put them in the initial generic parameter brackets.
I did a refactor, where TObject and ID were passed as generic parameters, like this:
public class GenericService<TGeneric, TObject, ID>
where TGeneric: Generic<TObject, ID>
where TObject: class, Entity<ID>
where ID : IComparable
and it works. But I want to avoid writing 3 generic parameters if possible, since those parameters are drawn from the TGeneric parameter.
I am attempting this approach because of the following scenario. If I had a class ChildGeneric:
public class ChildGeneric : Generic<SomeEntity, SomeID>
and I want to pass it to GenericService, I want to be able to write only
GenericService<ChildGeneric> service = new GenericService<ChildGeneric>
instead of
GenericService<ChildGeneric, SomeObject, SomeID> = new GenericService<ChildGeneric, SomeObject, SomeID>
Is there a way to refactor the 2nd code snippet in order to make it work with the TGeneric parameter only?
If your GenericService class does not care about the generic arguments of the nested class you can use a non-generic interface:
public interface IGeneric
{
//API for everything you do care for in other classes
//must not use the TObject, ID types
}
just like IEnumerable<T> implements IEnumerable. Although that may just be for historical reasons.
The downside is having to keep the interface in sync with the class.
If your GenericService does care for the TObject, ID types, then you need to define them on the class like you did, or specify them in an implementing class as Camilo wrote in the comments.
Related
While I understand generic types <T> and where clause for constraint, I was confused about the following code from Unity Tower Defense Template:
public abstract class Singleton<T> : MonoBehaviour where T : Singleton<T>
What's the purpose of restricting the type to be itself?
It prevents you from doing this:
public class A
{
}
public class B : Singleton<A>
{
}
You will get a compile error. Due to the type constraint, you must write:
public class A : Singleton<A>
{
}
Off the top of my head, one possible benefit of this is allowing methods to return the specific type. This is done by fluent style methods. Another example, obviously not applicable to the example you posted, is a Copy() method that returns a copy of the object as the derived type rather than as its base type. The method signatures of the Singleton< T> class you posted likely show where it uses the T parameter and likely hint at the reason why it uses this pattern.
I have defined a generic class where T can be a specific interface or a collection of the interface.
public class BaseResponse<T> where T : IBaseResource, ICollection<T>, new()
However, when I try to create BaseResponse using IBaseResource I get the following error.
'System.Collections.Generic.List' cannot be
used as type parameter 'T' in the generic type or method
'BaseResponse'. There is no implicit reference conversion from
'System.Collections.Generic.List<.Resources.IBaseResource >' to
'Vehicle.Api.Resources.IBaseResource'.
I even tried with following as well.
public class BaseResponse<T> where T : IBaseResource, ICollection<IBaseResource>, new()
Is the way I am defining multiple constraints is wrong or can't I use the ICollection of the same interface when defining multiple constraints? If it is achievable how can I achieve this?
edit -
To further clarify what I am expecting to achieve,
I am implementing a rest API where the response will be given by BaseResponse. For example, GET with single method will include BaseResponse<Entity> and GET will include BaseResponse<List<Entity>>
As mentioned in the comments, constraints are ANDed not ORed.
Without knowing what the purpose of your implementation is or what it looks like, it's difficult to address this question.
Perhaps you can parameterize your generic on two types:
public class BaseResponse<T, U>
where T : IBaseResource, new()
where U : ICollection<T>
It is a bit unclear what you want to do, but in your first attempt you specify that T must implement IBaseResource and that the collection should implement IBaseResource as well. I assume that is not what you want. That is also what the error message shows. It shows that List<T> does not implement IBaseResource
Does this solve your problem?
public class BaseResponse<ICollection<T>> where T : IBaseResource, new()
I am building a entity Repository and I have an interface, IIdentifiable<T>. So entities which are identified by Guids, for example, implement IIdentifiable<Guid> with public Guid Id { get; }. So far, so good.
I have an interface of IRepository<T, TIdentifier> where T : IIdentifiable<TIdentifier>.
It seems to me that the TIdentifier generic argument is redundant, because that already information is held in the generic argument of T. Is there any way I can have a cleaner implementation of IRepository, where I only have to specify T in my business code?
I'm after something like 'IRepository<T> where T : IIdentifiable<T.GenericArgs[0]>.
I doubt this is possible, but just thought I'd ask. Maybe the new C# 4 stuff has something for this this?
There's no constraint on T which requires it to be a generic. And there's nothing which prevents a class from implementing IIdentifiable<T1>, IIdentifiable<T2>, ... IIdentifiable<Tn>, so even some syntax saying "the first argument of the IIdentifiable'1 interface implemented by T" wouldn't be sufficient.
You could create an IIdentifiable that has object Id { get; } then have IIdentifiable<T> : IIDentifiable with a generic Id property.
Then you can reduce the number of generic arguments to 1 like so:
IRepository<T> where T : IIdentifiable
In practice you would then use IRepository<X> with X : IIdentifiable<Guid>
This assumes that your IRepository is mainly interested in the ability of T to have a key and isn't focused on the specific type of key.
You are asking to be able to specify a contraint, based on the assumption that type has already satisfied that contraint, putting the compiler into a logical paradox.
Also, your plan makes TIdentifier unknown within IRepository which, I assume it's going to need to know.
I am working on a class library and am having some trouble with generics. I have a ITransaction interface which has a collection of ITransactionItem. Each ITranscation can be either a CapitalCall or Distribution. A CapitalCall is a ITransaction but has a few additional properties. A CapitalCallItem is a ITransactionItem with a few additional properties. A CapitalCall has a collection of CapitalCallItems. Likewise, there exists a Distribution class with a collection of DistributionItem.
I have tried making the Transaction interface generic:
interface ITransactionBase<TItem>
where TItem: ITransactionItem
{
List<TItem> ITransactionItems
{
get;
set;
}
}
This works perfectly when I implement it:
class CapitalCall : ITransactionBase<CapitalCallItem>
Now all of the items in the collection are of type CapitalCallItem.
I run into the following problem. I would like to know the associate ITransaction on a ITranscationItem. I created a property on the ITransactionItem table of type ITranscation. When I use this property, it is no longer typed to the correct class:
var capitalCall = new CapitalCall();
var trans = capitalCall.TransactionItems[0].Transaction;
// trans is now of the base type ITransaction, instead of typed to CapitalCall.
I have tried making the ITransactionLineItem interface use generics as well, but I get into a recursive generic nightmare when I try to declare it. What is the correct way to model this?
Would this work:
interface ITransaction<TAction, TItems>
where TItems : ITransactionItem<TAction, TItems>
where TAction : ITransaction<TAction, TItems>
interface ITransactionItem<TAction, TItems>
where TItems : ITransactionItem<TAction, TItems>
where TAction : ITransaction<TAction, TItems>
I am confused as to how I could then use the interface by itself- what if I want a collection of mixed ITransactionItem, without specifying a type? Also I should add that I have base Transaction / Transaction item classes that implement the interface, and CapitalCall / Dist inherit from.
Yes, this sort of mutually recursive generic declaration will work, but it will make things very complicated - I know from experience. If you want an example of something similar, look at this declaration from my protocol buffers port:
public interface IMessage<TMessage, TBuilder> : IMessage<TMessage>
where TMessage : IMessage<TMessage, TBuilder>
where TBuilder : IBuilder<TMessage, TBuilder>
IBuilder<,> has the equivalent.
This declaration also demonstrates the answer to your last question: if some parts of your interface don't need to know the exact type of transaction, you can declare them in a "less generic" base interface. So you could have:
interface ITransaction<TAction, TItems> : ITransaction
where TItems : ITransactionItem<TAction, TItems>
where TAction : ITransaction<TAction, TItems>
for example, where ITransaction is a non-generic interface.
Again though, this is not for the faint of heart. In my case I can get away with it because almost no-one uses the raw interfaces - all the implementations are autogenerated, and client code uses those non-generic implementations. I would think long and hard before inflicting this on a developer to actually use day to day...
Yes the interfaces you wrote down should work as far as I can tell. Such "recursive" declarations work well with generics, but the question is whether you really need to make those generic in the first place? Recursive declarations are something which is not often used and may therefore be hard to grasp for other people using your classes.
As for using the interface for itself, you can still make a less generic interface and also implement it.
I'm trying to add another restriction on a method within a generic class. Is this possible?
Pseudocode:
public class MyBaseClass<T> where T: class
{
public IQueryable<T> ShowThisMethod where T: class, IMyInterface
{
// stuff.
}
}
ShowThisMethod should only be available when T is IMyInterface. Also IMyInterface should then give information back (about T) so that I can access properties defined in IMyInterface inside of the method.
Help :)
By the way, this compiles (and seems "almost right"):
public class MyBaseClass<T> where T: class
{
public IQueryable<T> ShowThisMethod<T>() where T: class, IMyInterface
{
String X = T.MyInterfaceStringProperty;
}
}
More Information about my goal:
I'm using a generic base class to access a common property (DateTime "Time" property on LINQ object Dinner which is also on Lunch).
Both objects are implementing ITimeable which exposes the DateTime property.
In my base class I'd like to have a method Select() which works on IQueryable<T> and can automatically filter based on the Time property. Because I'm working off the generic T, the time property is not visible to the base class, unless I tell it that T is implementing ITimeable.
I do want the same base class to work for other non-ITimeable objects too, that's why I need the interface restriction on the Select method, and I also need it in order to access the Time property using generics.
Hope that clears the goal :)
P.S. My main concern is not visibility of the method in IntelliSense etc.. I'd just like to keep my base class working, while being able to access an interface-specified property through generics in it.
It depends on what you want.
Since the class is compiled once, and the magic with generics also relies on the runtime, there's no way to make a class that has some methods in some cases, and other methods in other cases. Either the methods are there, or they aren't.
So basically, there's no way to declare MyBaseClass so that the following happens:
MyBaseClass<Int32> bc;
bc. <-- intellisense does not show ShowThisMethod here
MyBaseClass<SomeTypeImplementingIMyInterface> bc2;
bc2. <-- intellisense DOES show ShowThisMethod here
... that is... by itself.
You can "trick" the compiler and intellisense into giving you what you're asking for, but know that this gives you other limitations and challenges that might need to be solved.
Basically, by adding an extension method to a static class declared alongside MyBaseClass, you can make intellisense, and the compiler, behave as if the method is only present for MyBaseClass when T has some specific rules, as you're asking for.
However, since the method in question will be a static method, defined outside of MyBaseClass, there's limits to how much of the internals of MyBaseClass you can access, and you can't access the method inside MyBaseClass, so it depends on what you want to accomplish and whether you can live with the limitations or not.
Anyway, here's the extension method. Note that you remove it completely from MyBaseClass at the same time:
public static class MyBaseClassExtensions
{
public static IQueryable<T> ShowThisMethod<T>(this MyBaseClass<T> mbc)
where T: class, IMyInterface
{
...
}
}
Also note that ShowThisMethod is redefining T in the context of ShowThisMethod. It is not the same T as defined by the class.
Specifying a different Type parameter where the new one inherits from the one defined by the class would be the best approach, though that ends up requring the caller to have to specify the generic Type twice.
No, it's not possible. Constraints are defined when they are declared. In this case, the method is not generic, the class is (it's a non-generic method of a generic class). So the constraints can be only declared on the class itself.
Will this do what you want? The method will be visible to anyone, but not necessarily useable...
public class MyBaseClass<T> where T: class
{
public IQueryable<R> ShowThisMethod() where R: T, IMyInterface
{
Debug.Assert(typeof(R) == typeof(T));
// stuff.
}
}
You could define another class that inherits MyBaseClass and redefine the constraint :
public MyOtherClass<T> : MyBaseClass<T> where T : class, IMyInterface
{
public IQueryable<T> ShowThisMethod()
{
// stuff.
}
}