In WinRT (C#, XAML), ScrollViewer is a sealed class, and I can't extend it, but I need to overwrite some methods (for example: ScrollToHorizontalOffset).
Is it possible to override methods of a sealed class?
No - in order to override a method, you have to derive from it, which you can't do when the class is sealed.
Basically, you need to revisit your design to avoid this requirement...
You can't inherit from a sealed class, so no inheritance, no override.
See: override C#
The override modifier is required to extend or modify the abstract or
virtual implementation of an inherited method, property, indexer,
or event.
See: sealed C#
When applied to a class, the sealed modifier prevents other classes
from inheriting from it.
As the word "sealed" itself indicates that , it is protected from being inherited or overridden
So, No inheritance implies no Overriding.
please find this link to find more details about sealed class in detail.
Sealed itself means that class can not be inherit and those class which can not be inherit means can not be access any property of that class in derived class.
Related
I read an article from some site and in that article i read this :
Abstract classes can add more functionality without destroying the child classes that were using the old version. In an interface, creation of additional functions will have an effect on its child classes, due to the necessary implementation of interface methods to classes.
I don't understand what it means. Can anyone explain this more specifically , with a Good Example ?
this is the article which i read Link
Because interfaces only define members that types must implement, adding any new member to an interface will break any class that implements the old version because it inherently doesn't implement the new member. Any time you change the definition of an interface you must change every single class that implements that interface. Adding an abstract member to an abstract class does the same for derived classes but if you add a virtual member to the abstract class then it will have no impact on derived classes. They can be changed to override that member but they don't have to.
What it means is that if you consider a Abstract Class called Phone
and it has 3 virtual functions i.e.
AddPhonePrice , AddAccessoryPrice, AddAuxillaryPrice
and if there are two child classes
1) SamsungPhone 2) Iphone
now SamsungPhone will have implementation for all 3 functions.
while Iphone will have implementation for only AddPhonePrice, since they dont provide anything else with the phone
if we make interface called IMainPhone with
AddPhonePrice , AddAccessoryPrice, AddAuxillaryPrice functions
then both SamsungPhone and Iphone will need to implement all 3 functions
irrespective of whether they need them or not.
This means that you are able to add new members (methods, properties, fields, ...) to abstract classes that do not lead to changes in the derived classes - as long as the members are not abstract this is correct. For instance, consider this example:
internal abstract class MyBaseClass
{
public abstract void DoSomething();
// This method can also be added later without having an effect on the derived classes
public virtual void DoSomethingElse()
{
// Do something else...
}
}
internal class MyDerivedClass : MyBaseClass
{
public override void DoSomething()
{
// Do something...
}
}
In this case, the derived class has to implement the method DoSomething. But you can add non-abstract functions to the abstract base class later on. However, as soon as you add another abstract member to the base class, this also affects all non-abstract derived classes because the must implement the new members.
An interface on the other hand does not define concrete implementations at all but does only contain the abstract signature that all implementors must provide. Therefore, if you add a new member (method, property) to an interface it forces all implementors of the interface to also provide an implementation of the new members.
Interface: When you add a method to the base interface class, then you have to (manually) make sure all of the derived classes implement that method.
Abstract Base: When you add a method to an abstract base class, you aren't required by the compiler to implement that method in the derived classes.
Is there a way to make a method in a base class (Custom, so editable) public to classes that inherit it, but not beyond that?
I want the given method to be "private" within all classes that 'contain' it, but to not be accessible from other classes.
A little context:
I have three very similar classes, and am creating a base class with some properties and methods that the three classes share. Some of these properties and methods are private and should be, but I can't set them as private in the base class or they can't be accessed within the three.
You are looking for the protected access modifier.
You want the protected keyword:
http://msdn.microsoft.com/en-us/library/bcd5672a(VS.71).aspx
The protected access modifier probably is what you're looking for. A protected member is accessible within its class and by derived class instances.
The template method pattern provides that the abstract base class has a not overridable method: this method implements the common algorithm and should not overridden in the subclasses. In Java the template method is declared final within the abstract base class, in C# the sealed keyword has a similar meaning, but a not overridden method can not be declared sealed.
public abstract class Base
{
protected abstract AlgorithmStep1();
protected abstract AlgorithmStep2();
public sealed void TemplateMethod() // sealed: compile error
{
AlgorithmStep1();
AlgorithmStep2();
}
}
How can I solve this problem?
Why can not prevent a method can be overridden by subclasses (in C#)?
The sealed modifier is only valid for function members which are overriding base class members, to stop them from being virtual for derived classes. Function members are non-virtual by default in C# (unlike Java). You still need the sealed modifier for a class though - classes aren't sealed by default.
Just remove the sealed modifier from your method and it should be fine.
See section 10.6.5 of the C# 4 spec for more details about sealed methods (sealed properties and events are in section 10.7.5 and 10.8.4 respectively).
When an instance method declaration includes a sealed modifier, that method is said to be a sealed method. If an instance method declaration includes the sealed modifier, it must also include the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method.
Just remove the sealed keyword. By default, methods are not overridable; subclasses cannot override them, only hide them.
C# methods are sealed by default
How to define a must inherit class? in C#
You mark the class as abstract (this is the C# analogue to the VB.NET Must Inherit).
This will ensure it can't be instantiated directly.
From the linked MSDN article:
The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.
(emphasis mine)
Use the abstract modifier.
public abstract class MyClass()
{
...
}
You can define a class as abstract, or give it a protected-only constructor. abstract is better.
If u want to create a class, that has to be inherited, you'll need to mark it with the abstract modifier.
public abstract MyClass
{
}
It's not possible enforse needness of derivation or implementation in code, if that was a question.
But:
You can define an interface to force consumer to implement it.
Or you can define abstract class with only abstract members to force consumer to override all of them.
Hope this helps.
An interface would be best.
If you need to simulate the functionality , and its not a requirement that it fail at compile time...
define a method in the base class. Throw a an exception as the only line in the implementation. You might want to make the message very very clear about what the problem is.
override the method in the super class(es) and implement them.
If you fail to implement in a super class, you will get the exception.
Not perfect, but say you are trying to port code from vb.net... this could work.
I have some user controls which I want to specify properties and methods for.
They inherit from a base class, because they all have properties such as "Foo" and "Bar", and the reason I used a base class is so that I dont have to manually implement all of these properties in each derived class.
However, I want to have a method that is only in the derived classes, not in the base class, as the base class doesn't know how to "do" the method, so I am thinking of using an interface for this. If i put it in the base class, I have to define some body to return a value (which would be invalid), and always make sure that the overriding method is not calling the base. method
Is the right way to go about this to use both the base class and an interface to expose the method? It seems very round-about, but every way i think about doing it seems wrong...
Let me know if the question is not clear, it's probably a dumb question but I want to do this right.
EDIT : Thanks to all the people with your excellent abstract suggestions, but this breaks the designer. If abstract was not a selectable option, what would you do?
Alternatively you could define the method as 'abstract' in the base class, which will not require the class to implement it. For example:
abstract class A
{
public abstract void B();
}
Of course this will force your base class to be abstract as well, but it sounds like this would work just fine for you.
See Abstract methods on MSDN.
Update
Since abstract is not an option for you due to designer issues, you could just define the method as part of your base class, and have it throw a NotImplementedException if it is called directly from the base class:
void DerivMethod()
{
// Must be implemented by derived class
throw new NotImplementedException();
}
Otherwise, using an interface would be fine, especially if the above leaves a bad taste in your mouth...
You should make your base class an Abstract class. Then the base class can implement the Interface by marking the method abstract.
http://msdn.microsoft.com/en-us/library/aa664435(VS.71).aspx
Mark the method as abstract in your base class. You'll be forced to implement it in the derived classes, but the base class will not need to have a method definition.
I agree with with others, but making your user control abstract has some issues for the designer. The designer will often not display the abstract user control.
I would implement the interface methods in the base class. You can throw a NotImplemented exception or Assert.Fail in the methods if you want to make sure the inheritors are overriding these methods properly.
Declare the function signature in the base class and use the "abstract" modifier.