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.
Related
I have this abstract class:
using TypeLib=some.type.library;
namespace someSpace
{
abstract class Creator
{
abstract public TypeLib.SomeObject createObject();
}
}
Here's a concrete class:
using TypeLib=some.type.library;
namespace someSpace
{
class SpecialCreator:Creator
{
override public TypeLib.SomeObject createObject()
{
doSomethingSpecial();
return new TypeLib.SomeObject();
}
}
}
Because I will want to implement Creator several different ways, all of which require importing some.type.library because they need to return a TypeLib.someObject, is there a way that I can just have the using statement at some high level and have it be inherited by all the implementations? When I didn't include the using statement in SpecialCreator(), it didn't have access to TypeLib.
EDIT: I think this is different than the duplicate directives question. I'm not trying to consolidate different using statements into one master using that I will then add to many classes; instead, I want to put one using statement in a parent class/header file/static class/etc so that it can be accessible to many classes without having to add the same line of code to each class. Let me know if I've misunderstood the duplicate directives question.
It depends. If TypeLib is only used in base class functionality there is no reason to include 'using TypeLib;' in the files that contain the concrete implementations.
If the desired architecture is intended to encapsulate TypeLib in the abstract base class, the need to have the using statement is an indicator that encapsulation has been broken.
Martin Fowler may say the requirement to 'use' TypeLib in all the concrete implementations is the smell of tight coupling between the class tree under the base class and TypeLib. Using an adapter class may reduce the coupling by putting all TypeLib uses into the adapter.
Not unless you define all classes within a single file.
It depends on what you are trying to do.
If you are just trying to avoid having to add this in each and every page of your derived class, your'd be better off using Visual Studio's Export Template (under the File menu). Pick the Item Template and your added template will show up in the Add > New Item List. I think this would be cleanest.
Of course, there is a sneaky way to do avoid the using itself altogether, but I don't think it's good design. Notice I changed the namespace.
namespace some.type.library;
{
class SpecialCreator:Creator
{
override public SomeObject createObject()
{
doSomethingSpecial();
return new SomeObject();
}
}
}
You could also instead, derive a class in your someSpace namespace from TypeLib.SomeObject (no body for the class) and use this new class wherever you were using TypeLib.SomeObject
using is just a syntactic sugar to save you from typing fully qualified type names each time you type them. It's a hint for compiler for places where to look for type names.
So unless you actually use types from TypeLib you don't have to add using statement in each implementation even if some base class of that concrete implementation uses it in some way (i.e. derives from it). And even if your concrete implementation use types from TypeLib you can avoid using statement by specifying fully qualified type names from that library each time.
You could "alias" the type by creating a child class of it for internal use. For example:
namespace Remote.Namespace
{
public class TypeLib
{
public class SomeObject
{
}
}
}
namespace Internal.Namespace
{
public class InternalTypeLibObject : Remote.Namespace.TypeLib.SomeObject
{
}
}
And now you can use InternalTypeLibObject without having to alias the namespace and cast it to TypeLib.SomeObject.
Now for the drawbacks:
If you want to use the constructors in the parent class, you will need to provide mirror constructors and call the base constructor with constructor chaining. Also, if the parent class is sealed, this won't work.
Honestly though you shouldn't be concerned too much about a little typing. Hiding classes like this can add to confusion and should probably be avoided unless you have a reason to extend the type.
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.
I'm working on a project with the following (very simplified) structure:
BaseClass
SubClassA : BaseClass
SubClassB : BaseClass
There is a UI (with a lot of logic) which uses SubClassA, and then saves it to another component which takes BaseClass as a parameter but immediately casts the argument to SubClassB. This fails as the UI is passing in SubClassA.
UI:
MyComponent.Save(subClassA)
Component:
Save(BaseClass baseClass)
{
SubClassB subClassB = (SubClassB)baseClass;
...
the current implementation creates an instance of SubClassB in the UI and pass that across - but it leads to lots of code such as
SubClassB.Property1 = SubClassA.Property1a
I'm contemplating creating a common interface which the 2 sub classes would implement. It would be a lot of work but slowly I think I could flatten the current very deep hierarchy. Reworking either the UI or the component to use the other sub type would be just as much work as the structures are different (though many fields map). Is the interface approach the right way to go? I feel there might be something I'm missing.
If SubclassA and SubclassB are related only by their ability to Save, then yes, BaseClass would be better as an interface that both sub-classes implement.
It won't solve your immediate problem straight away: the component casting from base class to (the wrong) derived class. It looks like there could be several levels of refactoring to do here. Patching up the code so that the component casting to a SubclassA by making one for it to use is wasteful, I think. Changing the component so it can operate on a single common type would be a big win there.
Flattening a deep hierarchy would bring lots of other benefits, too - like making it simpler. If there end up being a few interfaces that they all implement, that's not necessarily a bad thing. Beware of lots of interface types hunting in packs, however.
In short, reworking both the UI and the component - and any other code, too - to work in terms of just a small number of interfaces, with no knowledge of the implementing classes, will pay dividends.
From a consumer standpoint, interfaces can do almost everything that abstract classes can do (the main exceptions being that a class can expose a field as a byref, while interfaces have no means of doing so, and that static members associated with a class can be grouped under the class name, static members related to an interface must be grouped under a different name). Except in those rare cases where it's necessary to expose a field as a byref, the primary advantage of an abstract class comes on the side of the implementation. All of the functionality associated with an interface must be provided separately in every class which implements it, even when such functionality is common to 99% of the classes which implement it. By contrast, if 99% of the concrete classes derived from an abstract class will implement a particular method the same way, it's possible for the abstract class to define that method once and let derived classes inherit it without having to pay it any heed whatsoever. Such an advantage can be nice, but there's a major catch: a class can only inherit functionality from one other class. Since interfaces don't include any functionality, they can be inherited from any number of other classes.
When one is defining an abstract class, I would suggest that one should in many cases also define an interface which includes the same public functionality (which the abstract class should implement), and avoid using variables or parameters of the abstract class type. This will allow implementations which can inherit from the abstract class to achieve the ease-of-implementation benefits that would come from doing so, but will also make it possible to define implementations which inherit from something else. Writing an implementation which inherits from some other type would be more work, but if code never uses the abstract-class type in variable, field, or parameter declarations, code which uses derivatives of the abstract class would work just as well with interface implementations that don't.
Why not make a Save() virtual within the base class - it seems like a better option. That way, if you have any common save functionality, you can use it and also give it other forms in derived classes - known as polymorphism.
class BaseClass
{
public virtual void Save()
{
//Use this keyword
}
}
class B : BaseClass
{
public override void Save()
{
base.Save();
}
}
public interface ITest
{
int ChildCount { get; set; }
}
public class Test
{
}
public class OrderPool : ITest, Test
{
public int ChildCount
{
get;
set;
}
}
The error says Base class 'Test' must come before any interfaces.
Why is it necessary to extend the class first and then implement the inteface?
Because the specification says so in section ยง17.1.2.
C# supports only single inheritance, but allows classes to implement multiple interfaces. That being the case, it's much clearer to always have the base class specified in the same place by convention, rather than mixed in with a bunch of interfaces.
Regardless of convention, the specification mandates that this is the case anyway, which is why you're seeing that error.
Remember, there's nothing in the specification that says all of your interfaces have to be named with a capital "I". - that's just convention. So if your class implemented interfaces that didn't follow that convention, and if the specification allowed you to specify the base class and interfaces in any order, you wouldn't be able to easily tell which identifier was the base class and which were interfaces. Example:
class MyDerivedClass : A, B, C, D, E // which is the base class?
{
...
}
it's called syntax
There are conventions that you must follow in order for the compiler to compile the code.
They could have chosen to allow both forms, or just the other way around, but they didn't.
The reason is probably clarity : you can define a lot of interfaces and only inherit from one class.
Technically it would have been possible to allow them all in random order, but that would make the code less readable.
Simply because the language is designed like that. The reason is probably that a class can have only one base class, but implement any number of interfaces.
Because you can extend just one class and implements more than one interface, having the class first make easier to read. And probably the grammar itself is easyer to write that way, just a pseudo grammar could be:
class CLASSNAME:baseclass? interface*
meaning optional baseclass followed by many interface, writing one grammar that allow just one class messed somewhere would be difficult without any reason.
You can only inherit from one base class but many interfaces. So if there is more than one type listed you know that the first one is a class, the others interfaces. This works regardless of class/interface naming conventions
The order makes clear sense, the base class can implement members of the interface for the derived class, therefore the compiler must know of them beforehand
I have a class that inherits from LinkButton and I want to hide OnClinentClick from my class.
Somthing like this :
public class MyClass : LinkButton
{
// some Code
}
And somewhere in code:
MyClass myclass = new MyClass();
MyClass.OnClinentClick = "";//this line must not be accessable
Hiding something from a class definition is not directly supported as it breaks OOP principles.
You could use the new operator, however, I wouldn't advise it. Personally, I would think about my design and/or use a NotSupportedException if there is no other way around it.
You can use the EditorBrowsableAttribute to prevent it from being suggested by IntelliSense, but you can't get rid of it entirely.
[EditorBrowsable(EditorBrowsableState.Never)]
public virtual string OnClientClick { get; set; }
C# only supports public inheritance. You shouldn't be inheriting from a class whose methods don't make sense for all derived classes. Consider composition instead of inheritance to solve this problem.
You can override the function (that is, replace the base implementation with your one, as long as the former is virtual), but you cannot completely prevent the clients from calling the base class function if you hide it with new, as they may always cast to the base class.
Update:
actually, you cannot change the access from public to protected/private when overriding, this won't compile (http://ideone.com/Y65Uh). Besides that, if you use new to hide the base function and make it uncallable, the original function is still visible (http://ideone.com/xiL2F). If you declare the new function public (which is perhaps not what you want), the old function can still be called by casting to the base class (http://ideone.com/A3Bji).
How about making giving it a lower visibility. One of protected and internal might be what you want. Of course that doesn't remove such a member from the derived class, but just removes them from the public interface. It also requires control over the base-class. No idea if LinkButton is one of your classes.
You could also hide the property by reintroducing a new one with the same. But that's a bad idea, and casting back to the base-class allows outsiders to access it.
And you should consider using a has-a relationship instead of an is-a. i.e. Don't derive from a base class if you don't want all its public members. This violates OOP principles such as that it should be possible to substitute the derived class where the base class is expected.
You could also override it, and make the setter throw a NotSupportedException. But that's ugly too because it will only show an error at runtime instead of compiletime. You can generate compile-time warnings with attributes such as the ObsoleteAttribute.