I'm looking at some code right now, and I am a little confused. This class is not an interface, but why those methods are not defined? Also, bedsides method signatures, there is a regular property. Very confusing!
public class MyClass
{
public string Foo(string str1);
public string Bar(string str, int i);
public string myProperty { get; set; }
}
Thanks for helping
No, that is not valid as shown, and you could prove as much by trying to compile it. For a class to contain unimplemented methods it must be declared as abstract, and the same applies to each unimplemented method.
abstract classes are conceptually similar to an interface, but they may contain implementation as well. This makes them useful for scenarios in which some methods may share a common implementation all the way down the inheritance hierarchy, but others only have meaning when implemented by a descendant type.
Methods marked as extern or partial can also lack a body, but extern methods are typically interop (so it is implemented in native code), and partial methods will be implemented in another .cs file (and can be declared only in a class marked as partial).
As an aside, the property is fine because it is automatic (syntactical sugar which means that the backing field will be created for you and the get/set methods simply return/set that field, nothing more).
You can mark the methods as abstract and then you will have an abstract class
but why would you want to do that? if you don't have any code in a base class just use an interface
Related
Interface:
interface IMyInterface{
internal int Property {get; set;}
}
Class:
public class MyClass: IMyInterface{
internal int Property {get; set;}
}
Result:
CS8704 Error: MyClass doesnot implement interface member Property.get MyClass cannot implicitly implement a non-public member.
Why I have to implement the interface explicitly?
The simple answer to "why is a language like this" is "because that's how the language designers specified it".
Now, why did they design it that way? Some of the official notes I found were these. It seems the main question was about what kind of access the implementor must have:
Would we allow non-public interface members to be implemented implicitly? If so, what is required of the accessibility of the implementing method? Some options:
Must be public
Must be the exact same accessibility
Must be at least as accessible
They decided:
For now, let's simply not allow it. Only public interface members can be implicitly implemented (and only by public members).
The "for now" never changed, so as of C# 8 an interface can have non-public virtual members but a class may only implement them explicitly.
I can speculate on a couple of reasons they may have decided against implicit overrides like this:
Non-public virtual methods in interfaces may have been considered a "rare" feature (after all, aren't interfaces supposed to document the public behavior of a class?), not worth putting a lot of resources into in terms of the semantics of implicit overrides.
Unlike with method overridding in class-to-class inheritance, an class method implementing an interface method doesn't use the override keyword. It might have been considered confusing to see a protected and/or internal method and not realize that it's fulfilling an interface contract. (Public methods are presumably considered exempt from this concern because that's the way they've always worked, and public methods are part of the class' public contract anyway so modifying / removing them would already be cause the reader to think about other parts of code that depend on it.)
Interfaces can only override other interface methods explicitly, possibly again because allowing interface-to-interface implicit implementation would be too expensive for the compiler and tooling teams and too confusing for C# users. (Especially since interface-to-interface inheritance is multiple-inheritance.) Since both this and non-public interface methods were introduced in general in C# 8, it may have made sense to make the two features match syntactically.
See also the notes on this question in the default interface method proposal.
Interface members don't have scopes like public or internal. What you have here is a default interface implementation.
So you need to remove the scope on the interface:
interface IMyInterface{
int Property {get; set;}
}
The internal property forces the implementation to be explicit such that the internal members of the interfaces will remain internal to the assembly.
It helps you to keep implementations internal (to an assembly) so that you can update code without breaking changes e.g. renaming the property.
interface IMyInterface
{
internal int Property { get; set; }
}
public class MyClass : IMyInterface
{
int IMyInterface.Property { get; set; }
}
I have this sealed class,
public sealed class A
{
public string AName {get;set;}
}
and someone can write an extension method for it like this:
public static class Extensions
{
public static void ExtensionMethodForA (this A a)
{
Console.WriteLine("A's Extension method!");
}
}
The question is, how do you prevent that ?
You don't. You can't. And you shouldn't want to.
Instance methods are always preferred to extension methods, so it should not present a conflict. Other than that, they are mere syntax / convenience. Stop trying to make life inconvenient for callers.
You might be confused by the term "extension method". It is not a method in the class or even a derived class; It is an operation on a type. It has no access to the private, protected or internal members of the class hierarchy and therefore the class is still sealed.
So, you can't and don't need to.
There would be no point. Any user could still create a static class which implemented a method which used your class type.
They would just leave out the 'this' out of the declaration, and callers would have to explicitly pass the object, rather than using the simpler . syntax. The end result would be identical.
Extension methods are just a nicer way of expressing what I've just described, which will always be possible anyway.
I read somewhere
"Abstract and Sealed modifiers are equivalent to a class which is static"
I also found that
"When you declare a static class, internally the compiler marks the class abstract and sealed, and creates a private constructor in the IL code"
so, I decided to do this:
static class A
{
public static void test()
{
Console.WriteLine("test");
}
}
Now, the class "A" cannot be inherited nor instantiated.
So, let us write a class B using abstract to prevent instantiation and using sealed to prevent inheritance.
But, this approach fails.
which should be equivalent to
public abstract sealed class B
{
private B()
{
}
public void test()
{
Console.WriteLine("test");
}
}
But I recieve an error stating "error CS0418:B': an abstract class cannot be sealed or static"` . Any ideas why this is not possible ?
Thanks in advance for your answers.
Having checked the IL of the System.Directory class (which is static), it is declared in IL as:
.class public auto ansi abstract sealed beforefieldinit System.IO.Directory
extends System.Object
{
...
Further, this article (http://msdn.microsoft.com/en-us/library/ms229038.aspx) suggests that the CLR handles static classes as abstract sealed classes to support languages that do not support directly delcaring static classes (eg C++).
Thus in conclusion, static classes in C# are syntactic sugar for sealed abstract classes with private constructors. I for one am glad of that as "static" is a lot easier to write and a lot easier to get right.
By definition a sealed class enables you to prevent the inheritance of a class or certain class members that were previously marked virtual.
Abstract keyword enables you to create classes and class members that are incomplete and must be implemented in a derived class.
(Source: http://msdn.microsoft.com/en-us/library/ms173150.aspx)
This would imply that any class marked abstract would not be able to be sealed, since you wouldn't be able to derive it anywhere.
The code you mentioned doesn't make any sense.
All answers somehow take the technical point of view. Such as: the class can't be "must inherit" and "can't inherit" at the same time. But I think that is not the main reason, as clearly, the "static" is just that.
I think David Amo's answer has touched the real answer a bit, by stating: "it is a lot easier to get right".
I am convinced that Anders Hejlsberg's idea when designing C# was to eliminate ambiguity and thus decrease a chance for error. That's why "virtual" goes with "override" (override has to be explicit, not implicit as in Java). And in this case, "abstract"+"sealed" would be the same as "static". Two ways of defining the same principle. This is:
- more error prone (imagine you have abstract somewhere and put sealed there accidently without noticing, onw compiler prevents that)
- more difficult to work with (imagine you want to search for all static classes in your project)
So my point is, this entire design leads the developers the right way of doing things.
I want to know the reason behind the design of restricting Abstract Methods in Non Abstract Class (in C#).
I understand that the class instance won't have the definition and thus they wont be callable, but when static methods are defined,they are excluded from the instance too. Why abstract methods are not handled that way, any specific reason for the same?
They could be allowed in concrete class and the deriving class can be forced to implement methods, basically that is what, is done in case of abstract methods in an abstract class.
First, I think that what you're asking doesn't logically make sense. If you have an abstract method, it basically means that the method is unfinished (as #ChrisSinclair pointed out). But that also means the whole class is unfinished, so it also has to be abstract.
Or another way to put it: if you had an abstract method on a class that wasn't abstract, that would mean you had a method that cannot be called. But that means the method is not useful, you could remove it and it would all work the same.
Now, I'll try to be more concrete by using an example: imagine the following code:
Animal[] zoo = new Animal[] { new Monkey(), new Fish(), new Animal() };
foreach (Animal animal in zoo)
animal.MakeSound();
Here, Animal is the non-abstract base class (which is why I can put it directly into the array), Monkey and Fish are derived from Animal and MakeSound() is the abstract method. What should this code do? You didn't state that clearly, but I can imagine few options:
You can't call MakeSound() on a variable typed as Animal, you can call it only using a variable typed as one of the derived classes, so this is a compile error.
This is not a good solution, because the whole point of abstract is to be able to treat instances of derived classes as the base class, and still get behaviour that's specific to the derived class. If you want this, just put a normal (no abstract, virtual or override) method into each derived class and don't do anything with the base class.
You can't call MakeSound() on an object whose runtime type is actually Animal, so this is a runtime error (an exception).
This is also not a good solution. C# is a statically typed language and so it tries to catch errors like “you can't call this method” at compile time (with obvious exceptions like reflection and dynamic), so making this into a runtime error wouldn't fit with the rest of the language. Besides, you can do this easily by creating a virtual method in the base class that throws an exception.
To sum up, you want something that doesn't make much sense, and smells of bad design (a base class that behaves differently than its derived classes) and can be worked around quite easily. These are all signs of a feature that should not be implemented.
So, you want to allow
class C { abstract void M(); }
to compile. Suppose it did. What do you then want to happen when someone does
new C().M();
? You want an execution-time error? Well, in general C# prefers compile-time errors to execution-time errors. If you don't like that philosophy, there are other languages available...
I think you've answered your own question, an abstract method isn't defined initially. Therefore the class cannot be instanciated. You're saying it should ignore it, but by definition when adding an abstract method you're saying "every class created from this must implement this {abstract method}" hence the class where you define the abstract class must also be abstract because the abstract method is still undefined at that point.
The abstract class may contain abstract member. There is the only method declaration if any method has an abstract keyword we can't implement in the same class. So the abstract class is incompleted. That is why the object is not created for an abstract class.
Non-abstract class can't contain abstract member.
Example:
namespace InterviewPreparation
{
public abstract class baseclass
{
public abstract void method1(); //abstract method
public abstract void method2(); //abstract method
public void method3() { } //Non- abstract method----->It is necessary to implement here.
}
class childclass : baseclass
{
public override void method1() { }
public override void method2() { }
}
public class Program //Non Abstract Class
{
public static void Main()
{
baseclass b = new childclass(); //create instance
b.method1();
b.method2();
b.method3();
}
}
}
You can achieve what you want using "virtual" methods but using virtual methods can lead to more runtime business logic errors as a developer is not "forced" to implement the logic in the child class.
I think there's a valid point here. An abstract method is the perfect solution as it would "enforce" the requirement of defining the method body in children.
I have come across many many situations where the parent class had to (or it would be more efficient to) implement some logic but "Only" children could implement rest of the logic"
So if the opportunity was there I would happily mix abstract methods with complete methods.
#AakashM, I appreciate C# prefers compile time errors. So do I. And so does anybody. This is about thinking out-of-the-box.
And supporting this will not affect that.
Let's think out of the box here, rather than saying "hurrah" to big boy decisions.
C# compiler can detect and deny someone of using an abstract class directly because it uses the "abstract" keyword.
C# also knows to force any child class to implement any abstract methods. How? because of the use of the "abstract" keyword.
This is pretty simple to understand to anyone who has studied the internals of a programming language.
So, why can't C# detect an "abstract" keyword next to a method in a normal class and handle it at the COMPILE TIME.
The reason is it takes "reworking" and the effort is not worth supporting the small demand.
Specially in an industry that lacks people who think out of the boxes that big boys have given them.
It's still not clear why you would want that, but an alternative approach could be to force derived classes to provide a delegate instance. Something like this
class MyConcreteClass
{
readonly Func<int, DateTime, string> methodImpl;
// constructor requires a delegate instance
public MyConcreteClass(Func<int, DateTime, string> methodImpl)
{
if (methodImpl == null)
throw new ArgumentNullException();
this.methodImpl = methodImpl;
}
...
}
(The signature string MethodImpl(int, DateTime) is just an example, of course.)
Otherwise, I can recommend the other answers to explain why your wish probably isn't something which would make the world better.
So the answers above are correct: having abstract methods makes the class inherently abstract. If you cannot instance part of a class, then you cannot instance the class itself. However, the answers above didn't really discuss your options here.
First, this is mainly an issue for public static methods. If the methods aren't intended to be public, then you could have protected non-abstract methods, which are allowed in an abstract class declaration. So, you could just move these static methods to a separate static class without much issue.
As an alternative, you could keep those methods in the class, but then instead of having abstract methods, declare an interface. Essentially, you have a multiple-inheritance problem as you want the derived class to inherit from two conceptually different objects: a non-abstract parent with public static members, and an abstract parent with abstract methods. Unlike some other frameworks, C# does permit multiple inheritance. Instead, C# offers a formal interface declaration that is intended to fill this purpose. Moreover, the whole point of abstract methods, really, is just to impose a certain conceptual interface.
I have a scenario very similar to what the OP is trying to achieve. In my case the method that I want to make abstract would be a protected method and would only be known to the base class. So the "new C().M();" does not apply because the method in question is not public. I want to be able to instantiate and call public methods on the base class (therefore it needs to be non-abstract), but I need these public methods to call a protected implementation of the protected method in the child class and have no default implementation in the parent. In a manner of speaking, I need to force descendants to override the method. I don't know what the child class is at compile time due to dependency injection.
My solution was to follow the rules and use a concrete base class and a virtual protected method. For the default implementation, though, I throw a NotImplementedException with the error "The implementation for method name must be provided in the implementation of the child class."
protected virtual void MyProtectedMethod()
{
throw new NotImplementedException("The implementation for MyProtectedMethod must be provided in the implementation of the child class.");
}
In this way a default implementation can never be used and implementers of descendant implementations will quickly see that they missed an important step.
I was reading somewhere about how to handle the issue of wanting to extend a sealed class in the .NET Framework library.
This is often a common and useful task to do, so it got me thinking, in this case, what solutions are there? I believe there was a "method" demonstrated to extend a sealed class in the article I read, but I cannot remember now (it wasn't extension methods).
Is there any other way?
Thanks
There is 'fake' inheritance. That is, you implement the base class and any interfaces the other class implements:
// Given
sealed class SealedClass : BaseClass, IDoSomething { }
// Create
class MyNewClass : BaseClass, IDoSomething { }
You then have a private member, I usually call it _backing, thus:
class MyNewClass : BaseClass, IDoSomething
{
SealedClass _backing = new SealedClass();
}
This obviously won't work for methods with signatures such as:
void NoRefactoringPlease(SealedClass parameter) { }
If the class you want to extend inherits from ContextBoundObject at some point, take a look at this article. The first half is COM, the second .Net. It explains how you can proxy methods.
Other than that, I can't think of anything.
Extension methods is one way, the alternative being the Adapter Pattern. Whereby you write a class that delegates some calls to the sealed one you want to extend, and adds others. It also means that you can adapt the interface completely into something that your app would find more appropriate.
this method may have already been mentioned above by it's formal name, but i don't know it's formal name, so here it is. This example "extends" the TextBox class (example in VB). I believe an advantage of this method is that you do not need to explicitly code or expose built-in members. Hope this is relevant:
VB Class Module "MyTextBox":
public Base as TextBox, CustomProperty as Integer
Private Sub Init(newTextBox as TextBox)
Set Base = newTextBox
End Sub
public Property Get CustomProperty2() As String
CustomProperty2 = "Something special"
End Property
To call the code, you might say:
Dim MyBox as New MyTextBox
MyBox.Init MyForm.TextBox3
from here you have access to all built-in members, plus your custom members.
Debug.Print MyBox.Base.Text
MyBox.CustomProperty = 44
For extra polish, you can make Base the default property of the class, and then you can leave out "Base" when you call properties of the Base class. You call Base members like this:
Debug.Print MyBox().Text
MyBox().Text = "Hello World"
VBA Demo
Maybe use the Decorator pattern?
Other than extension methods, this is the only sensible technique I can think of.
No, you can't extend a sealed class in any legitimate way.
TypeMock allows you to mock sealed classes, but I doubt that they'd encourage you to use the same technique for production code.
If a type has been sealed, that means the class designer has not designed it for inheritance. Using it for inheritance at that point may well cause you lots of pain, either now or when the implementation is changed at a later date.
Prefer composition to inheritance - it's a lot more robust, in my experience. See item 16 in "Effective Java (2nd edition)" for more on this.
The only way I know to "extend" a sealed class without extension methods is by wrapping it. For example:
class SuperString
{
private String _innerString;
public SuperString(String innerString)
{
_innerString = innerString;
}
public int ToInt()
{
return int.Parse(_innerString);
}
}
You'd need to expose all of the same methods/properties as the string class.
Some frameworks allow you to extend existing objects. In WPF, see Dependency Properties. For Windows Forms, see IExtenderProvider.
How about extension methods? You can "add" additional methods that way, without having to deal with the inheritance restriction.