I have two sequences of objects 'A' and 'B'. Comparing the sequences should produce a third sequence 'C' of elements that indicate whether:
the objects were "deleted" from 'A' or
"inserted" from 'B'.
All remaining elements are considered as "matched".
What I would like to do:
Declare Inserted<T>, Deleted<T>, and Matched<T> generic classes that inherit all their properties from the T base class. The generic class must be able to instantiate itself from the object it inherits.
The code:
public interface IInstantiable<T>
{
void CopyFrom(T o);
}
[Serializable]
public class Inserted<T> : T
where T : IInstantiable<T>
{
public Inserted() { }
public Inserted(T t)
{
this.CopyFrom(t);
}
}
The error:
'MyNamespace.Inserted<T>' does not contain a definition for 'CopyFrom' and no
extension method 'CopyFrom' accepting a first argument of type 'MyNamespace.Inserted<T>'
could be found (are you missing a using directive or an assembly reference?)
Further discussion:
I define my own IInstantiable interface to enforce the existence of a CopyFrom method. I cannot use the standard ICloneable interface, because it only defines a method that copies the object to a new instance, whereas I need the object to copy its members in the constructor.
The error goes away if the generic defines its own implementation of the CopyFrom method; however, this does not achieve the desired goal of specializing the CopyFrom method to handle the specific needs of the base class. Only the base class could know what properties should be copied. (Or am I missing something?)
Note: The final object should have the same public members as its base class, as the object should be capable of serialization.
Is this possible in .NET?
The answer:
What I am attempting to do is impossible, simply because the generic class cannot be an extension of the template base class. Visual Studio complains "Cannot derive from 'T' because it is a type parameter." (I hadn't noticed this error yet because I had not implemented the CopyFrom method in the generic class yet.)
If I were to change the interface into a class and supply a stub implementation in that class, I could inherit from it as suggested below; however, this introduces a new base class into my inheritance hierarchy.
public class IInstantiable<T>
{
public virtual void CopyFrom(T o) { }
}
[Serializable]
public class Inserted<T> : IInstantiable<T>
where T : IInstantiable<T>
{
public Inserted() { }
public Inserted(T t)
{
base.CopyFrom(t);
}
}
Unfortunately, I cannot use this new base class in its templatized form because I must introduce it at the root of my inheritance hierarchy. It works only if I remove the template and make it as generic as possible.
public class IInstantiable
{
public virtual void CopyFrom(Object o) { }
}
However, this still does not make my Inserted<T> generic look like the object it is initialized from, and since I cannot inherit from the same type as the type parameter, it does not suit my initial purpose.
Moving away from "fancy generics" based on the type system to more (ahem) generic annotated structures might prove to be the best solution; however, the default behavior of my selected serialization approach (XmlSerialization) does not have the automatic support that would make this configuration a viable solution. Generics will not work; use hard-coded class definitions instead.
This is indirectly what you're trying to declare in your code above.
[Serializable]
public class Inserted<T> : IInstantiable<T>
where T : IInstantiable<T>
{
public Inserted() { }
public Inserted(T t)
{
this.CopyFrom(t);
}
}
Does this make sense?
.NET doesn't allow you to inherit from a generic parameter. How could it? Generics are evaluated at runtime but it needs to know what type your class is at compile time.
If I understand correctly, you want to annotate a sequence of objects with the notion of what their state is (inserted, deleted, or matched).
You don't really need fancy generics for this; what's wrong with:
enum ChangeState { Inserted, Deleted, Matched }
struct<T> Annotated {
public T Obj;
public ChangeState;
}
You can mark this for serialization however you want (the Annotated object can serialize just fine without the same properties/fields).
Though you can encode more information in the type system, it's unclear to me what the benefit would be here. Are you sure you want to do that?
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'm trying to call a method with a definition similar to the following (simplified to avoid confusion):
public static void Register<T>(T value) where T : BaseClass, IInterface
This works fine so long as I have a class instance that defines both of those values. The problem occurs when I pass a `BaseClass' into a method and then try to use that instance in the above declaration. For example:
public class MyClass
{
public MyClass(BaseClass value)
{
Register(value);
}
}
I can pass and instance of a class that implements both BaseClass and IInterface into the constructor, but when I try to use that value in the Register method I get a compilation error stating:
The type 'BaseClass' cannot be used as type parameter 'T' in the generic type or method 'Register(T)'. There is no implicit reference conversion from 'BaseClass' to 'IInterface'.
If I change the type in the constructor like so:
public class MyClass
{
public MyClass(IInterface value)
{
Register(value);
}
}
I get an error stating:
The type 'IInterface' cannot be used as type parameter 'T' in the generic type or method 'Register(T)'. There is no implicit reference conversion from 'IInterface' to 'BaseClass'.
This seems like a bit of a catch-22. Is there a way that I can define the parameter to indicate that it must implement both BaseClass and IInterface?
As I was writing the question I came up with the answer and thought I would post it instead of deleting the question.
I just need to redefine the class:
public class MyClass<T> where T : BaseClass, IInterface
{
public MyClass(T value)
{
Register(value);
}
}
The solution given by Matt is an easy answer for situations where it is not necessary to store the passed-in object in a field or collection. If you need to persist the passed-in object, things get much harder. It's easy for a SomeClass<T> where T meets multiple constraints, to store items of class T and pass them as generics to routines with such constraints, but if a class has a method SomeMethod<TParam>(TParam thing) where TParam:IFoo,BaseBar, it won't have any field of type TParam. It could store thing into a field of type IFoo or BaseBar, but unless BaseBar implements IFoo, or all passed-in instances are going to derive from one particular BaseBar derivative which implements IFoo, there's no way to specify that a field's type will meet both constraints (if every instance does derive from one particular BaseBar derivative which implements IFoo, one could simply use that type as a single constraint, or for that matter not bother with generics at all—just use that as the parameter type).
There are ways of getting around these issues, either using Reflection, an interface pattern I call ISelf<T>, or some tricky nested callback interfaces. In some cases, though, it may be better to provide alternatives to methods that take double-constrained parameters (have the methods take a parameter of one constraint type, cast it to the other type, and accept the lack of compile-time safety).
I am using xml serialization but now came across a runtime error I haven't seen before.
"To be XML serializable, types which
inherit from IEnumerable must have an
implementation of Add(System.Object)
at all levels of their inheritance
hierarchy. ImageEditor.EffectOptions
does not implement Add(System.Object)"
It seems a little weird to be forced to implement a method via runtime exception, rather than compile time error such as missing methods from implemented interfaces.
Is this by design? Should this not be enforced via some sort of interface like XmlSerializable, etc?
Other than this I am wondering if the serializer guarantees passing a value of the right type where I can just cast it to the type, which in my case is EffectOption.
Or should I implement this Add (object) method to see if the object is of type EffectOption and if not throw an exception?
I haven't implemented this Add (object) method before but my guess is it's safer to just cast it to EffectOption and add it to EffectOptions collection.
EDIT: Here's the type itself:
public class EffectOptions : IEnumerable<EffectOption>
{
public List<EffectOption> Options { get; private set; }
//IEnumerable methods
}
Because sub classes implicitly implement interface methods because of the base class but xmlserializer is using reflection and that is why you get the error at runtime and not compile time.
Try explicitly implementing and see what happens. I have not had this issue before so I'm not sure why you are unless you're doing something custom.
If you have your sub classes explicitly implementing the interface but not doing any implementation code (letting the implicit implementation of methods happen) then remove the interface from your sub type declaration as it should still be valid due to your base type. (someone tell me if i'm off here)
I've just run into this issue and solved it by adding an add method:
public class EffectOptions : IEnumerable<EffectOption>
{
public List<EffectOption> Options { get; private set; }
public void Add(object o){
this.Options.Add(o as EffectOption); //you may want to extend the code to check that this cast can be made,
//and throw an appropriate error (otherwise it'll add null to your list)
}
//IEnumerable methods
}
I hope this helps.
I have developed some extension methods for objects, which I don't want to be used/shown in intellisense for objects which implements IEnumerable. Conceptually I want something like as follows
public static T SomeMethod<T>(this object value) where T != IEnumerable
{
}
Is it possible to impose this kind of constraint anyway in C#?
Edit
Sorry, I put the question in a wrong way. I know the allowable constraints in C#, what I want to know is that if there is any other way to accomplish this?
Just to confirm Øyvind's comment: there's no such constraint in C#. The only types of constraint are:
where T : struct (non-nullable value type constraint)
where T : class (reference type constraint)
where T : SomeClassName (conversion to a particular class constraint)
where T : ISomeInterfaceName (conversion to a particular interface constraint)
where T : U (conversion to another type parameter constraint)
where T : new() (parameterless constructor constraint)
Note that I've only separated out the specific class and interface constraints as the former is a primary constraint and the latter is a secondary constraint.
This is not supported. Legal constraints are listed here.
This is not possible.
As others mentioned what you want to do isn't going to work, but, you can look at what types of objects you want to support and limit it to some class/interface that they have in common, or you may just need to ditch the generic part and write several extension methods, so you can disallow IEnumerable.
You can't, and I agree it's a nuisance, though what I've found myself wanting is not so much what you are looking for, as overriding on the basis of the constraint (so that I could e.g. have a class and a struct version of the same method or class, and have the appropriate one used as applicable).
There are two cases where we can get by well.
One is where our reason for not wanting an extension method to be used is that it's already supplied as an instance method. In fact we get this for free; instance methods are always used instead of extension methods (though a derivedClass.method() wont' be used when you call baseClass.method() if it only exists in derivedClass).
The other case is runtime selection:
public static T SomeMethod<T>(this object value) where T != IEnumerable
{
if(typeof(T).GetInterface("IEnumerable") != null)
{
//behaviour appropriate for IEnumerable
}
else
{
//other behaviour.
}
}
It's not ideal, especially if the only "behaviour appropriate for IEnumerable" is to throw an exception, but it can be enough sometimes.
You can do something like this, but only for types that you control.
Say you want to have a method like this:
public static class XmlSerializableExtension {
public static string ToXml(this object self) {
// ...
}
}
But you don't want to pollute every object with it, only a subset of your classes.
You can achieve it like this:
public interface MXmlSerializable { }
public static class XmlSerializable {
public static string ToXml(this MXmlSerializable self) {
// ...
}
}
Now, you mark the classes that you want this method to apply to with the "mixin" interface:
public class MyClass : MXmlSerializable {
// ...
}
And it will only appear in intellisense for these classes.
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.
}
}