I've been searching for a while on this because I'm naturally forgetful and I thought it would be nice to build something (an abstract class, interface, etc.?) that would force me to implement certain bits of code in a class I was writing.
In particular, I would like to force a new class to always have a constructor that takes a single parameter typed as itself in order to make duplication of the object easier. I've seen articles/questions elsewhere that talk about this, but I'm not sure this particular question has been asked (at least that I can find) or I'm simply not understanding enough of the other articles/questions to realize it. My apologies in advance.
I'm not interested in having a constructor in an abstract class, interface, etc. actually do anything. I'm merely interested in defining the requirement for a constructor signature in a derived class.
My ideal class would look like this:
public class GoodClass
{
public GoodClass(GoodClass goodClass)
{
// copy components of goodClass to this instance
}
}
So, I first began researching interfaces and also started reading up on abstract classes. I was thinking something like the code below would work, but alas I get errors. Is what I'm trying to do even possible? Is there any other way I could accomplish my goal without putting a sticky note on my monitor? :)
abstract class SelfConstructor
{
abstract public SelfConstructor(SelfConstructor) { }
}
class NewClass : SelfConstructor
{
//Required by SelfConstructor:
public NewClass(NewClass newClass)
{
// copy components of newClass to this instance
}
}
You could write a ReSharper plugin that recognises this case and highlights the class if it doesn't have a "copy constructor". This would be a daemon stage that would process the file as it's being edited, and add highlights. You can look through the abstract syntax tree of the file, look for all instances of IConstructorDeclaration, and then get the constructor's parameters from the ParameterDeclarations property. You can check that there is a constructor that only has one parameter, and that parameter is the same type as the class it's declared in.
You can compare the types by getting the constructor's parameter's TypeUsage and trying to downcast to IUserTypeUsage. You can then use ScalarTypeName.Reference.Resolve() to get an instance of IDeclaredElement. Compare this against the class's IClassDeclaration.DeclaredElement to see if they're the same instance.
In C++, what you are talking about is a copy constructor, you actually get one by default!
C# doesn't have that concept (though of course you can define one); however, it is easier (and preferred) to simply implement ICloneable (MSDN), which requires you to implement the Clone method, that does the same thing.
Instead of:
object myObj = new CloneableObject(otherObj);
You write:
object myObj = otherObj.Clone();
The other thing you could do is force a constructor signature by not having a default:
public class BaseClass
{
//No abstract constructors!
public BaseClass(BaseClass copy)
{
}
}
Now when you derive, you have to use that overload in the constructor. Nothing will force the derived signature, but at least you have to explicitly use it:
public class DerivedClass : BaseClass
{
public DerivedClass() : base(this)
{
}
}
The above example clearly shows that it doesn't "force" you to have a copy constructor, but like a sticky note, would serve as a good reminder.
I would definitely go the interface route, as that is what is there for (and you can use an abstract implementation!).
Note that you can take advantage of Object.MemberwiseClone if you want a shallow copy for free. All objects get this, no interface required.
Related
I'm looking for a way to get wildcards to work in .NET generics.
My code is as follows:
private class Rule<TSource, TSelected> where TSource : class where TSelected : class
{
// stuff in here
}
I want to be able to create a List<> of Rules where the TSource will be the same but the TSelected may be different.
You need to make a contravariant generic interface IRule<TSource, in TSelected> and make a list of that, where in addition TSelected is going to be constrained to some meaningful class. Constraining to any reference type as in your existing code will compile, but you won't be able to do anything meaningful with anything that has to do with TSelected.
At this time there is no other way to use variant generics (unless of course you go into reflection mode with List<dynamic> or something equivalent), so if this solution does not work for you you will need to redesign.
If I read your question right, I think you'd have to do this:
public interface ISelected
{
// ISelected interface
}
// A TSelected implementation
public class Implementation1: ISelected { }
// Another
public class Implementation2 : ISelected { }
// our Rule
private class Rule<TSource, TSelected> where TSource : class where TSelected ISelected
{
}
If the TSelected classes has the same super-class, you can just make a list of Rule<TSource, TSelectedSuperClass>. I believe you can use typeof (http://msdn.microsoft.com/en-us/library/58918ffs(v=vs.71).aspx) to get the exact subclass after reading the TSelected object again.
Alternatively you can make a container class to contain both and also store the exact types.
An interface could do it instead of a super class. If the Selected share implementation however, I prefer an abstract class.
In a namespace, is it possible to provide an alias for a class? And if not, why not?
By example, if I had several libraries of things that were derived from a contained, but named base class, but wanted to alias that as "BaseClass", while retaining its actual class name (i.e. "HtmlControl").
Then consumers could always come along and extend from HtmlControls.BaseClass, without having to figure out which class it really comes from.
using SomeClass = Large.Namespace.Other.FunkyClass;
class Foo : SomeClass
{
}
There really isn't an ideal way to do this in C#/.NET. What you can do is have a public BaseClass that inherits from an internal class. You can change this inheritance internally without breaking your consumers as long as the interface to the class remains intact.
public class PublicBaseClass : SomeInternalClass {
}
Consumers inherit from PublicBaseClass, and as long as you are careful, you can change what SomeInternalClass is as you wish.
You could create a dummy class that just inherits HtmlControl without adding any other functionality:
public class BaseClass : HtmlControl {}
The closest I know of is to customize your using statement:
using BaseClass = HtmlControls.BaseClass;
This is normally used to avoid ambiguity between classes with the same name in different used namespaces, without having to fully qualify one or the other. Your devs would have to include it in every code file, so probably not a good solution for what you're doing.
As far as deriving from BaseClass without knowing what you are actually deriving from, not possible. The compiler must, at some level, know what and where the parent class is, meaning it must be statically defined somewhere in code.
Ok so I'm currently working with a set of classes that I don't have control over in some pretty generic functions using these objects. Instead of writing literally tens of functions that essentially do the same thing for each class I decided to use a generic function instead.
Now the classes I'm dealing with are a little weird in that the derived classes share many of the same properties but the base class that they are derived from doesn't. One such property example is .Parent which exists on a huge number of derived classes but not on the base class and it is this property that I need to use.
For ease of understanding I've created a small example as follows:
class StandardBaseClass {} // These are simulating the SMO objects
class StandardDerivedClass : StandardBaseClass {
public object Parent { get; set; }
}
static class Extensions
{
public static object GetParent(this StandardDerivedClass sdc) {
return sdc.Parent;
}
public static object GetParent(this StandardBaseClass sbc)
{
throw new NotImplementedException("StandardBaseClass does not contain a property Parent");
}
// This is the Generic function I'm trying to write and need the Parent property.
public static void DoSomething<T>(T foo) where T : StandardBaseClass
{
object Parent = ((T)foo).GetParent();
}
}
In the above example calling DoSomething() will throw the NotImplemented Exception in the base class's implementation of GetParent(), even though I'm forcing the cast to T which is a StandardDerivedClass.
This is contrary to other casting behaviour where by downcasting will force the use of the base class's implementation.
I see this behaviour as a bug. Has anyone else out there encountered this?
I see this behaviour as a bug.
This behavior is correct. Since your method DoSomething is constraining T to StandardBaseClass, you only have access to the specific methods of StandardBaseClass, not any methods or properties of a derived class. Since StandardBaseClass does not have a Parent property, this is invalid, and should be invalid, by design.
There are two potential options here - You can use reflection to pull out the Parent property, or use C# 4's dynamic type, and treat this as a dynamic object. Both bypass the standard type checking in the compiler, however, so will require you to do extra type checking at runtime to verify that the Parent property exists.
Create an interface that contains the Parent property. Have each class that has a Parent property implement that interace. You will then be able to create a generic method that accepts a parameter of type IHaveParent, and it will do the right thing.
For anyone that is interested an succinct answer to this situation is answered by Stephen Cleary on msdn here:
http://social.msdn.microsoft.com/Forums/en-AU/csharpgeneral/thread/95833bb3-fbe1-4ec9-8b04-3e05165e20f8?prof=required
To me this is a divergence in the class hierarchy. By this this I mean that either the base class has parent, or the derived classes with Parent are derived from an abstract child of the base.
Lol as John says, an interface as opposed to an abstract class is sufficient too.
You idea won't work because the compiler can never guarantee that the base class actually would have such a property. And it won't just select the "right" one based on if it has it or not.
The only way you can do this is using reflection and then test at runtime if the requested property exists on the inspected class. You have to judge yourself if that is a viable way to do for your project (reflection is slow and requires maximum rights).
This is correct, as the compiler only knows that it can bind to your type as a StandardBaseClass. The binding is not done at runtime (where it could potentially decide to use the StandardDerivedClass overload.
If you know that it's a StandardDerivedClass, then why not just cast it as such?
object Parent = ((StandardDerivedClass)foo).Parent;
It's a bit ugly, but you can accomplish this using a Registration system, where you register delegates for different possible derived classes that expose the 'shared' property/method and then use something like a Dictionary<Type,Func<SomeT>> to store the delegates. If you know all of the derived types ahead of time and don't have to load plug-ins or the like, you can also use the classic ugly if/else-if structure. Either way you're basically creating your own substitute for what should have been supported by the virtual method table.
I feel like this is something I should already know, but I'm just not firing on all engines today...
I have a base class with a single ctor that takes an implementation of an interface as it's only parameter. I'm using a DI framework and have my component registrations all set up and working fine.
When I inherit from this base class, unless I pass in a value to the base constructor, I have to define a parameterless ctor, which bypasses the DI.
So right now I have:
public class MyObjectBase
{
IMyRequiredInterface _InterfaceImpl;
public MyObjectBase(IMyRequiredInterface interfaceImpl)
{
_InterfaceImpl = interfaceImpl;
}
...
}
public class AnotherObject : MyObjectBase
{
public AnotherObject()
{
}
...
}
So, out of the gate this fails. I get errors when AnotherObject is instantiated indicating that there is no base class ctor that takes 0 parameters. Ok, I get that. But now I have a choice: either modify the descendant class ctor to take a similar parameter and pass that value on to the base ctor, or wire up a ctor chain in the base class that forces me to bypass DI and create a concrete implementation of the required interface and pass it in as part of the parameterless ctor declaration.
The goal is to meet the requirement of the base class without the descendant classes knowing anything about it.
Maybe I'm going about this all wrong, but it's bugging me. Any thoughts on a better way to handle this? I feel like I've got to be missing something simple...
The correct approach is:
public class AnotherObject : MyObjectBase {
public AnotherObject(IMyRequiredInterface interfaceImpl) :
base(interfaceImpl) {
}
}
You specifically asked for an approach other than this approach. Why?
The goal is to meet the requirement of the base class without the descendant classes knowing anything about it.
That's generally the wrong thing to do. Why do you want to do it?
Update:
Based on your later comment, you should probably use (and configure your container to use) property injection instead of constructor injection. That will get you all of your requirements.
Err....the whole point of inheriting from MyObjectBase is that, as it were, you get the good and the bad, as far as the behaviour goes. If you can't create a MyObjectBase without an object implementing IMyRequiredInterface, you can't create a subclass without such an object either.
So what do you do when someone doesn't hand you that. Do you have a default?
It's quite reasonable for a subclass to instantiate something that implements IMyRequiredInterface, and pass that to the superclass constructor with a super(...) call. Can you do that? (Though, as I recall, you can get a bit hung up on this in Java, having to call super before doing anything else....)
There's a code smell here. If you inherit from a class that has a parameterless constructor it means that the author of this base class intended that it cannot function properly without supplying the necessary dependency. If you inherit from it and call a base method that required this dependency your code will probably fail if the dependency is not supplied. So if you really think that you should completely override this behavior you don't have to inherit from this base class, otherwise just copy the constructor in the inherited class.
What about providing a protected constructor in the base class that takes no paramters?
class MyBase
{
readonly int _x;
public MyBase(int x)
{
_x = x;
}
protected MyBase()
{
_x = 0;
}
}
class MyChild : MyBase
{
public MyChild()
{
}
}
Most DI frameworks have the functionality to inject services into properties (Property Setter Injection) using Attributes so you can try that.
You can subclass but the subclass will have to know how to create or get the Interface Implementation (using the ServiceLocator or something).
From the DI's point of view it doesn't have anything to fulfill because the class only has an empty constructor (most use the constructor with the most params) and no attributes telling him to do anything else.
I had the same scenario where my base (concrete) class and the child class had a dependency, I was worried if these are different instances.
But, Autofac (I believe other container tools too) has .InstancePerRequest() which will share the same instance per Http request.
builder.RegisterType().As().InstancePerRequest();
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.