I have an interface, IFoo, that implements one method, Bar. IFoo accepts a generic argument T. This generic argument defines the type of the parameter accepted by Bar.
public interface IFoo<T>
{
public void Bar(T value);
}
Pretty basic. But, sometimes, it is not necessary for Bar to accept any arguments. To implement this, I have created a second interface, also called IFoo, that does not accept any generic arguments.
public interface IFoo
{
public void Bar();
}
This works, but it adds overhead anytime I might want to update IFoo, given that there are technically two different IFoos. If I want to add a second method to the interface(s) I now have to do it twice. This also impacts any classes that implement IFoo if I have a class Baz that implements IFoo and I want it to implement both versions of IFoo, I have to define one version of Baz that passes a generic to IFoo and one that does not. For instance:
public class Baz<T> : IFoo<T>
{
private IFoo<T> _command;
public void Bar(T value) {
_command.Bar(value);
}
}
public class Baz : IFoo
{
private IFoo _command;
public void Bar() {
_command.Bar();
}
}
Both classes, Baz are fairly similar. They both pass a function call to another IFoo object. But since one takes an argument, and the other does not, they have to be two different classes, despite the fact that their functionality is nearly identical. Whatever changes I make to one I am guaranteed to make to the other. It would be ideal to have one class Baz that can be instantiated with or without a generic depending on what type of argument it is meant to take and whether or not it should even take an argument at all.
// This code works with the two versions of Baz defined above.
// The goal would be to get this code to work with only one Baz class.
var intBaz = new Baz<int>();
var stringBaz = new Baz<string>();
var emptyBaz = new Baz();
intBaz.Bar(100);
stringBaz.Bar("It would be really nice if this is possible, thank you");
emptyBaz.Bar();
Is there anyway, within C#, to define an interface IFoo that determines not only what type of argument its method Bar accepts, but also whether or not Bar accepts any argument at all? Thanks!
So far I have been able to work around this problem by implementing two, nearly identical, interfaces IFoo and IFoo<T>. This "works" to an extent but makes classes that implements IFoo and IFoo<T> difficult to maintain, as any class that implements one requires a second class that implements the other. Changes to one class must be reflected in changes to the other. At best, this adds time to development. At worst, this introduces vulnerabilities for the code to become out of sync.
I would define a Unit type - a type that only has one valid value.
struct Unit {}
And use that on the generic interface whenever you want to say "I don't want arguments". This way, you can get rid of the non-generic interface.
Of course, you still have to pass new(), but since that is the only value for this type, you are not actually giving any information, or doing any thinking.
var emptyBaz = new Baz<Unit>();
emptyBaz.Bar(new());
An alternative formulation is a class that cannot be instantiated:
var emptyBaz = new Baz<Unit>();
emptyBaz.Bar(null);
sealed class Unit {
private Unit() {}
}
The only value for this type is null, so this might require you to do more null checking in your implementations.
Similar problems exist in the standard library, like (Value)Task<T> and (Value)Task etc. I don't think an "elegant" solution exists yet. If it did, the standard library would have used it.
You can use default implementation to provide the second method, which will give the implementing class an option to have a different implementation for both if needed:
public interface IFoo<T>
{
public void Bar(T value);
public void Bar() {Bar(default(T)!);}
}
Or actually provide default parameter to the interface:
public interface IFoo<T>
{
public void Bar(T value = default(T)!);
}
There is really no wayto implement a single class that is and isn't generic at the same time - you have to have two classes for that and two interfaces. You still can use default implementation in the generic interface to avoid code duplication.
Related
I am studing interfaces, and at a point I came to explicit interfaces implementation. In a tutorial video at about 2:55 it says that when a class inherits 2 different interfaces, and if those two interfaces has a method with the same name, there is an ambiguity on which interface's method will be called.
using System;
interface I1
{
void InterfaceMethod();
}
interface I2
{
void InterfaceMethod();
}
public class Program : I1, I2
{
public void InterfaceMethod()
{
Console.WriteLine("I1 Interface Method");
}
public static void Main()
{
Program p = new Program();
p.InterfaceMethod();
}
}
I am confused because, interfaces doesn't have method definitions, so why does it matter which interface's method is called? Both methods are identical with the same name.
Why does it matter which interface's method is called? Both methods are identical with the same name.
And maybe that's okay. But maybe it's not.
In this case the result of the method is to output "I1 Interface Method", indicating that in real code the equivalent does care that it's I1.
When we create methods we give them names that try to be short and clear in meaning based on the meaning a word or few words have in a natural language like English. This can result in there in fact being different (whether very different or subtly so) purposes to two methods with the same name. We would then want to have separate implementations.
It's nice when things line up so that we can indeed use the same method for both, but it's also great that we're not trapped by that when inappropriate.
We'd also have to have separate implementations for interfaces with methods with the same name and parameter signature but different return types, since C# can't distinguish between these. A common example is IEnumerable<T> since it has a GetEnumerator() method that returns IEnumerator<T> but inherits from IEnumerable which has a GetEnumerator() method that returns IEnumerator. (IEnumerator<T> and IEnumerator are also examples of this same principle).
Another case where we might want to do explicit interface implementation is when a member isn't very useful in the context of the concrete type. For example List<T> implements ICollection<T>. That interface has an IsReadOnly property that is pointless in the context of working with List<T> directly as we know it's false because List<T>s are inherently not read-only. It is important when we're working on an ICollection<T> reference though (and of course, the rules require it be implemented one way or another), so it's done as an explicit implementation. ReadOnlyCollection<T> not only does the same thing (but of course, returning true instead of false) but also makes methods like Add() which are clearly pointless given that they will always throw an exception no matter what, explicit.
It is important which is called because, although the interface methods might have the same name (and parameters and even return type), they might have completely different meanings in the context of those different interfaces. Therefore you have to be able to specify which implementation method to use for each interface.
For example:
interface IVehicle
{
int GetNumberOfWheels();
int GetNumberOfDoors();
}
interface ICheeseContainer
{
int GetNumberOfWheels();
int GetNumberOfWedges();
}
class CheeseDeliveryTruck : IVehicle, ICheeseContainer
{
// Object has to be able to return the number of wheels on the truck
// when used as an IVehicle.
// Object has to be able to return the number of cheeses in the back of
// the truck which are packaged as wheels when used as an ICheeseContainer.
}
It looks like you're using C#. The language involved is very important for this question. I would recommending adding a tag for that when asking a question like this.
I believe the reason C# treats this as an ambiguous reference, rather than just using the same implementation for both, is that
If you must declare both methods in your implementing class, then the two methods with the same name can have different return types, so the same system can be used in a wider range of problems.
interface I1 {
int Foo();
}
interface I2 {
string Foo();
}
class C : I1, I2 {
int I1.Foo() { ... }
string I2.Foo() { ... }
}
If one interface is updated to change a parameter for example, code using that interface will be easier to update, without breaking other interface implementations.
I have two legacy C# user controls I need to work together.
I have an existing dialog onto which I need to add an existing generic usercontrol.
I have tried to sample the hierarchy below
interface Foo<T> {}
interface Bar<T>
{
T DataObject { get; set; }
}
public class ClassA<T> where T : Foo<T>
{
public ClassA(T dataObject)
{
//Do stuff if T implements Bar<T> - Pseudocode ahead
if(var T is Bar<T>)
{
var x = new ClassB<T>();
//x is typesafe, and I can set DataObject
x.DataObject = dataObject;
}
}
}
public class ClassB<T> where T : Bar<T>
{
T DataObject { get; set; }
}
The existing dialog, ClassA currently don't have any generic contraints, but could easily be changed to require T implementing Foo<T>.
The userControl, ClassB is based on another interface, Bar<T>. In practice, Bar<T> objects always implements Foo<T> - in theory of course not.
Are there any construction I can use to declare an object of type ClassB, and get compile time validation?
The construction above will give me a compile error stating:
The type 'T' cannot be used as type parameter 'T' in the generic type
of method ClassB<T>. There is no implicit reference conversion from
'T' for Bar<T>
I can make the ClassB object with Reflection, setting the properties using Reflection as well - but I would prefer a compile time solution.
But in my current situation with two existing dialogs - i'm not sure I am able to.
Any help is appreciated - also just if it is stating what I expect, that it can't be done.
--EDIT
Trying to elaborate a bit.
The problem rises when I have a ClassC that implements both Foo<T> and Bar<T>
public class ClassC<T> : Foo<T>, Bar<T>
{
T DataProperty
}
If I make an instance of ClassA<ClassC>, that is T in the specific instance is ClassC - then is there a way in code I can use T in creating an instance of ClassB- in this case T in ClassA does live up to the ClassB constraints, since T is ClassC.
I havent figured out how or if possible - tend to believe I can't.
As I wrote above, I have a solution based on reflection, i'm just not fan of using reflection and getting run-time validation only. But in this case with two legacy objects that need to work together I might be running out of options.
First of all, your types are a bit weird. They are kind-of recursive, with ClassB<T> requiring a T that implements Bar<T> which has the same structure as ClassB<T>. Maybe you meant to ClassB<T> to implement Bar<T> instead of requiring it as a type parameter?
Anyway you cannot do this. In order to be able to write ClassB<T>, the compiler needs to ensure that T is a valid type parameter for ClassB<> at runtime. This can only be the case when the type parameters on ClassA<T> for T are at least as restrictive as the ones from ClassB<T>.
Unfortunately, even a hard type check which would ensure that this is the case will not allow you to write ClassB<T>.
So without being able to write ClassB<T>, you will not be able to get static type safety at compile-time. So even when you create an instance of ClassB<T> (which you can), you won’t be able to access DataProperty on it since you will not be able to cast it as a ClassB<T>.
So in order to solve this, you would either have to access DataProperty using reflection only, or call a method inside ClassA<T> that does have the type constraint. I’ll show you both solutions:
public class ClassA<T>
where T : Foo<T>
{
public ClassA(T dataObject)
{
if (typeof(Bar<T>).IsAssignableFrom(typeof(T)))
{
// method 1, calling a generic function
MethodInfo mi = typeof(ClassA<T>).GetMethod("SetBDataObject").MakeGenericMethod(typeof(Bar<T>));
mi.Invoke(this, new object[] { dataObject });
// method 2, doing it all with reflection
Type type = typeof(ClassB<>).MakeGenericType(typeof(T));
object x = Activator.CreateInstance(type);
type.GetProperty("DataObject").SetValue(x, dataObject);
}
}
public object SetBDataObject<TB> (TB obj)
where TB : Bar<TB>
{
var x = new ClassB<TB>();
x.DataObject = obj;
return x;
}
}
The first thing that probably is confusing in your code, is that you have used the same letter T as the Type parameter in both classes ClassA<T> and ClassB<T>.
I'll start by stating the obvious:
when you call var x = new ClassB<T>(); the constraint for T here is in the context of ClassA<T> (i.e. T : Foo<T>), while new ClassB<T>() expects T to match the constraint of T : Bar<T>.
It seems to me that underlying reason for the issue you are having is a design problem. It looks like you a little mix up between types and classes.
Lets walk it through:
from the Gang of Four Design Patterns book:
An objects's class defines how the object is implemented .The class
defines object's internal state and the implementation of its
operations.
In contrast, an objects's type only refers to its interface -the set
of requests to which it can respond.
An object can have many type, and object of different classes can have
the same type.
The usage of interfaces in your code implies coding against types (that's good!).
Checking for if (dataObject is Bar<T>) and upon the result constructing a ClassB<U> where !typeof(U).Equals(typeof(T) implies heavily relying on implementation (e.g. class).
If you ask me, I think you should try one of the following:
Use the factory pattern for constructing ClassB. In the dedicated factory you can add some more logics and verifications in order to decide how to construct it (from your code, it is not clear since the types do not match...).
If possible, resolve the relation between Foo<T> and Foo<T> and declare the constraints in the interfaces. In that case, both interfaces should have same constraints fto T
I have an interesting problem that I keep circling around, but I never seem to quiet find a solution.
I tend to be a defensive programmer, so I try to write code that prevents problems from happening rather than reacting to problems once they've occurred. To that end, I have the following situation. Take the following code:
public class Base {}
public Interface IBase {}
public class Derived : Base, IBase {}
public class Derived2 : Base, IBase {}
...
public class DerivedN : Base, IBase {}
public class X : Base {}
public class Y : IBase {}
I need to pass a list of objects that derive from Base and implement IBase to a collection, and I need to make sure that only objects that have both are added to the list. Additionally, there can be an arbitrary number of classes that have both, so I cannot use the derived classes as constraints.
If I make the list of type Base, then I could add a Y object. If I make it of type IBase, then objects of type X can be added (neither of which are permitted).
I could, of course create my own generic collection class that has both types and has constraints for both. But, I don't want to have to do this for all possible collection types, and it's a lot of effort to duplicate all that functionality (even if you just forward the method calls to a contained class).
I could also create a BaseWithIBase class, which derives from both Base and IBase, and use that as my collection type, but I really don't want to force another abstraction if I don't have to.
I don't want this to be a runtime check, so walking the tree and throwing exceptions is not acceptable.
Can anyone suggest a better approach to this problem?
NOTE: Base and IBase are not related, just pointing out they are both base items of different types.
EDIT:
It seems that everyone wants to insist that "you don't need to do that" and that it's "not OOP". Nothing could be further from the truth. I was attempting to remove the specific from the question to prevent these kinds of questions and comments, so I will include my real situation.
The code is an implement of a Windows Service framework, based on the .NET Frameworks ServiceProcess.ServiceBase class. I am adding my own framework on top of this, that is intended to be heavily Dependency Injection based, and highly testable.
The collection must contain objects that derive from both ServiceBase and IService. IService is my framework extension that is used in my code, and for testing. It is basically just this:
public interface IService
{
void Start();
void Stop();
}
In addition, I have a number of other interfaces:
public interface IRestartableService
{
void Restart();
}
public interface IConfigurableService
{
void Configure();
}
etc.. etc.. and a service may look like this:
public class MyService : ServiceBase, IService, IConfigurableService {}
My code requires IService, Windows requires ServiceBase, thus both are needed because I work with IService, and windows works with ServiceBase. I only require IService, the other interfaces are optional.
You can create your own wrapper collection simply:
// TODO: Work out which collection interfaces you want to implement
public class BaseList
{
// Or use List<IBase>, if that's how you'll be using it more often.
private List<Base> list = new List<Base>();
public void Add<T>(T item) where T : Base, IBase
{
list.Add(item);
}
}
By using a generic method with both constraints, you can be sure that Add can only be called with an appropriate type argument.
You could have two methods to expose the data as IEnumerable<T> - one returning IEnumerable<IBase> (using Cast<T>) and one returning IEnumerable<Base>... that would let you use LINQ on either type, but not both at the same time of course.
I suspect you may find this awkward elsewhere, however - you may find yourself littering your code with generic methods which you wouldn't typically need. While there may well be a good reason for wanting both the class part and the interface part, it would be worth taking a step back and considering whether they're really both necessary. Is there something extra you could add to the interface so that you could do away with the class constraint, for example?
There is no good answer to your question because the design itself is not really fitting OOP as implemented in C#/.NET.
If you absolutely need a collection where each element statically provides two independent interfaces, either a wrapper collection or some wrapper class like Wrapper<TFirst, TSecond, T> : IBoth<TFirst, TSecond> would solve your problem.
Example:
public interface IBoth<TFirst, TSecond> {
TFirst AsFirst();
TSecond AsSecond();
}
public class Wrapper<T, TFirst, TSecond> : IBoth<TFirst, TSecond>
where T : TFirst, TSecond
{
private readonly T _value;
public Wrapper(T value) {
_value = value;
}
public TFirst AsFirst() {
return _value;
}
public TSecond AsSecond() {
return _value;
}
}
However the real question is why do you need that. Not to say that standard OOP model is perfect, but quite often a problem can be solved much easier if original design decisions are reviewed.
Another option is to completely ignore ServiceBase in most of the code and create a ServiceBaseAdapter for communication with the code that is not interface friendly. Such adapter can just call your interface methods when its method are called.
Try something like this:
List<object> collection = new List<object>();
foreach(var obj in collection.OfType<Base>().OfType<IBase>())
{
// Do what ever you want
}
Let say I have a class like this:
public sealed class Foo
{
public void Bar
{
// Do Bar Stuff
}
}
And I want to extend it to add something beyond what an extension method could do....My only option is composition:
public class SuperFoo
{
private Foo _internalFoo;
public SuperFoo()
{
_internalFoo = new Foo();
}
public void Bar()
{
_internalFoo.Bar();
}
public void Baz()
{
// Do Baz Stuff
}
}
While this works, it is a lot of work...however I still run into a problem:
public void AcceptsAFoo(Foo a)
I can pass in a Foo here, but not a super Foo, because C# has no idea that SuperFoo truly does qualify in the Liskov Substitution sense...This means that my extended class via composition is of very limited use.
So, the only way to fix it is to hope that the original API designers left an interface laying around:
public interface IFoo
{
public Bar();
}
public sealed class Foo : IFoo
{
// etc
}
Now, I can implement IFoo on SuperFoo (Which since SuperFoo already implements Foo, is just a matter of changing the signature).
public class SuperFoo : IFoo
And in the perfect world, the methods that consume Foo would consume IFoo's:
public void AcceptsAFoo(IFoo a)
Now, C# understands the relationship between SuperFoo and Foo due to the common interface and all is well.
The big problem is that .NET seals lots of classes that would occasionally be nice to extend, and they don't usually implement a common interface, so API methods that take a Foo would not accept a SuperFoo and you can't add an overload.
So, for all the composition fans out there....How do you get around this limitation?
The only thing I can think of is to expose the internal Foo publicly, so that you can pass it on occasion, but that seems messy.
I found myself asking that same question until I started working on reusable libraries of my own. Many times you wind up with certain classes that just cannot be extended without requiring obscure or arcane sequences of calls from the implementor.
When allowing your class to be extended, you have to ask: if a developer extends my class, and passes this new class to my library, can I transparently work with this new class? Can I work properly with this new class? Is this new class really going to behave the same?
I've found that most of the time the sealed classes in the .Net Framework have certain under-the-hood requirements that you aren't aware of, and that given the current implementation cannot be safely exposed to subclasses.
This doesn't exactly answer your question, but it provides insight as to why not all classes are inheritable in the .Net Framework (and why you should probably entertain sealing some of your classes too).
I'm afraid the short answer is, you can't without doing what is required, i.e. pass the composed instance variable instead.
You could allow an implicit or explicit cast to that type (whose implementation simply passed the composed instance) but this would, IMO be pretty evil.
sixlettervariable's answer is good and I won't rehash it but if you indicated which classes you wished you could extend we might be able to tell you why they prevented it.
Is there a way?
I need all types that implement a specific interface to have a parameterless constructor, can it be done?
I am developing the base code for other developers in my company to use in a specific project.
There's a proccess which will create instances of types (in different threads) that perform certain tasks, and I need those types to follow a specific contract (ergo, the interface).
The interface will be internal to the assembly
If you have a suggestion for this scenario without interfaces, I'll gladly take it into consideration...
Not to be too blunt, but you've misunderstood the purpose of interfaces.
An interface means that several people can implement it in their own classes, and then pass instances of those classes to other classes to be used. Creation creates an unnecessary strong coupling.
It sounds like you really need some kind of registration system, either to have people register instances of usable classes that implement the interface, or of factories that can create said items upon request.
You can use type parameter constraint
interface ITest<T> where T: new()
{
//...
}
class Test: ITest<Test>
{
//...
}
Juan Manuel said:
that's one of the reasons I don't understand why it cannot be a part of the contract in the interface
It's an indirect mechanism. The generic allows you to "cheat" and send type information along with the interface. The critical thing to remember here is that the constraint isn't on the interface that you are working with directly. It's not a constraint on the interface itself, but on some other type that will "ride along" on the interface. This is the best explanation I can offer, I'm afraid.
By way of illustration of this fact, I'll point out a hole that I have noticed in aku's code. It's possible to write a class that would compile fine but fail at runtime when you try to instantiate it:
public class Something : ITest<String>
{
private Something() { }
}
Something derives from ITest<T>, but implements no parameterless constructor. It will compile fine, because String does implement a parameterless constructor. Again, the constraint is on T, and therefore String, rather than ITest or Something. Since the constraint on T is satisfied, this will compile. But it will fail at runtime.
To prevent some instances of this problem, you need to add another constraint to T, as below:
public interface ITest<T>
where T : ITest<T>, new()
{
}
Note the new constraint: T : ITest<T>. This constraint specifies that what you pass into the argument parameter of ITest<T> must also derive from ITest<T>.
Even so this will not prevent all cases of the hole. The code below will compile fine, because A has a parameterless constructor. But since B's parameterless constructor is private, instantiating B with your process will fail at runtime.
public class A : ITest<A>
{
}
public class B : ITest<A>
{
private B() { }
}
Juan,
Unfortunately there is no way to get around this in a strongly typed language. You won't be able to ensure at compile time that the classes will be able to be instantiated by your Activator-based code.
(ed: removed an erroneous alternative solution)
The reason is that, unfortunately, it's not possible to use interfaces, abstract classes, or virtual methods in combination with either constructors or static methods. The short reason is that the former contain no explicit type information, and the latter require explicit type information.
Constructors and static methods must have explicit (right there in the code) type information available at the time of the call. This is required because there is no instance of the class involved which can be queried by the runtime to obtain the underlying type, which the runtime needs to determine which actual concrete method to call.
The entire point of an interface, abstract class, or virtual method is to be able to make a function call without explicit type information, and this is enabled by the fact that there is an instance being referenced, which has "hidden" type information not directly available to the calling code. So these two mechanisms are quite simply mutually exclusive. They can't be used together because when you mix them, you end up with no concrete type information at all anywhere, which means the runtime has no idea where to find the function you're asking it to call.
So you need a thing that can create instances of an unknown type that implements an interface. You've got basically three options: a factory object, a Type object, or a delegate. Here's the givens:
public interface IInterface
{
void DoSomething();
}
public class Foo : IInterface
{
public void DoSomething() { /* whatever */ }
}
Using Type is pretty ugly, but makes sense in some scenarios:
public IInterface CreateUsingType(Type thingThatCreates)
{
ConstructorInfo constructor = thingThatCreates.GetConstructor(Type.EmptyTypes);
return (IInterface)constructor.Invoke(new object[0]);
}
public void Test()
{
IInterface thing = CreateUsingType(typeof(Foo));
}
The biggest problem with it, is that at compile time, you have no guarantee that Foo actually has a default constructor. Also, reflection is a bit slow if this happens to be performance critical code.
The most common solution is to use a factory:
public interface IFactory
{
IInterface Create();
}
public class Factory<T> where T : IInterface, new()
{
public IInterface Create() { return new T(); }
}
public IInterface CreateUsingFactory(IFactory factory)
{
return factory.Create();
}
public void Test()
{
IInterface thing = CreateUsingFactory(new Factory<Foo>());
}
In the above, IFactory is what really matters. Factory is just a convenience class for classes that do provide a default constructor. This is the simplest and often best solution.
The third currently-uncommon-but-likely-to-become-more-common solution is using a delegate:
public IInterface CreateUsingDelegate(Func<IInterface> createCallback)
{
return createCallback();
}
public void Test()
{
IInterface thing = CreateUsingDelegate(() => new Foo());
}
The advantage here is that the code is short and simple, can work with any method of construction, and (with closures) lets you easily pass along additional data needed to construct the objects.
Call a RegisterType method with the type, and constrain it using generics. Then, instead of walking assemblies to find ITest implementors, just store them and create from there.
void RegisterType<T>() where T:ITest, new() {
}
I don't think so.
You also can't use an abstract class for this.
I would like to remind everyone that:
Writing attributes in .NET is easy
Writing static analysis tools in .NET that ensure conformance with company standards is easy
Writing a tool to grab all concrete classes that implement a certain interface/have an attribute and verifying that it has a parameterless constructor takes about 5 mins of coding effort. You add it to your post-build step and now you have a framework for whatever other static analyses you need to perform.
The language, the compiler, the IDE, your brain - they're all tools. Use them!
No you can't do that. Maybe for your situation a factory interface would be helpful? Something like:
interface FooFactory {
Foo createInstance();
}
For every implementation of Foo you create an instance of FooFactory that knows how to create it.
You do not need a parameterless constructor for the Activator to instantiate your class. You can have a parameterized constructor and pass all the parameters from the Activator. Check out MSDN on this.