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
Related
interface A1 {
void jj();
void ff() { /* */}
}
class B1 : A1 {
public void jj() { }
void ff() { /* */}
}
I am implementing two methods in the B1 class: the jj(), which does not have a default implementation in the interface A1, and the ff(), which does have a default implementation in the interface A1.
I am wondering why do not we use the override keyword when implementing a method from an interface since those methods in the interfaces are wither abstract or virtual?:
jj : abstract
ff : virtual
10 years ago, you could say that it is because the interface does not have a default implementation and that is why we do not override, we implement. BUT starting from C# 8.0 default implementations are possible and so why is that that we are not OBLIGED to use the override keyword?
As msdn docs says about Default interface members:
These preceding member declarations typically don't contain a body.
Beginning with C# 8.0, an interface member may declare a body. Member
bodies in an interface are the default implementation. Members with
bodies permit the interface to provide a "default" implementation for
classes and structs that don't provide an overriding
implementation.
and:
Beginning with C# 11, an interface may define static abstract or
static virtual members to declare that an implementing type must
provide the declared members. Typically, static virtual methods
declare that an implementation must define a set of overloaded
operators.
However, override keyword is not applied. It can be seen at this tutorial about static abstract interface methods.
why is that that we are not OBLIGED to use the override keyword
Because we are not overriding behavior, we are replacing behavior. I mean that we cannot call base keyword on concrete implementations of interfaces. It is like using new keyword instead of override
from c# 8 docs
The final override for IA.M in class C is the concrete method M declared in IA. Note that a class does not inherit members from its interfaces; that is not changed by this feature:
as the interface implementation is not inherited by its children there is no need to override it every declaration of an interface method by its child is the only implementation the child can have.
The following interface has no errors in a .Net Core Console application with C#-8.0
interface I
{
public abstract void f();
public virtual void g() => Console.WriteLine("g");
public sealed void h() => Console.WriteLine("h");
}
abstract prevents adding a definition in interface. virtual and sealed necessitate a definition in interface. sealed prevents an implementation of h in derived classes.
Do abstract, virtual and sealed, when used in interfaces, have any other meaning or applications in current implemented version of C# - 8? How and when should they be used in interfaces?
This is from the proposal:
The syntax for an interface is relaxed to permit modifiers on its
members. The following are permitted: private, protected, internal,
public, virtual, abstract, sealed, static, extern, and partial.
An interface member whose declaration includes a body is a virtual
member unless the sealed or private modifier is used. The virtual
modifier may be used on a function member that would otherwise be
implicitly virtual. Similarly, although abstract is the default on
interface members without bodies, that modifier may be given
explicitly. A non-virtual member may be declared using the sealed
keyword.
It is an error for a private or sealed function member of an interface
to have no body. A private function member may not have the modifier
sealed.
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.
Not really dealt with abstract methods that much but am looking at an abstract method inside an abstract class.
protected abstract bool Validate()
{
}
When I create the above class I get an error that tells me I need to specify a return type as per a normal method. Is this correct or am I doing something wrong?
If you declaring the abstract method then you should not give body
protected abstract bool Validate();
If it is not abstract method declaration but you giving implementation of an abstract method then you should return bool using return statement from method method to satisfy the return type in declartion.
protected abstract bool Validate()
{
//The method code
return false;
}
An abstract method declaration introduces a new virtual method but
does not provide an implementation of that method. Instead,
non-abstract derived classes are required to provide their own
implementation by overriding that method. Because an abstract method
provides no actual implementation, the method-body of an abstract
method simply consists of a semicolon, MSDN.
Abstract method should not have body. It is yielded to the derived class to implement the method.
protected abstract bool Validate();
Take a look at the documentation:
Use the abstract modifier in a method or property declaration to indicate that the method or property does not contain implementation.
Abstract methods have the following features:
An abstract method is implicitly a virtual method.
Abstract method declarations are only permitted in abstract classes.
Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no curly braces ({ }) following the signature.
http://msdn.microsoft.com/en-us/library/sf985hc5.aspx
In C# abstract methods do not have implementation, so your code should look like:
//no { and } in there
protected abstract bool Validate();
You cannot create an instance of an abstract class, you should create another class that is derived from you abstract class, and in that new class you implement this method.
As others mentioned abstract methods dont have bodies.
The reason why they do not is that classes cant have instances. That mean youll never have object of abstract class.
You have to extend abstract class and implement body in concrete class
Would anybody please tell me as the reason the following use of sealed does not compile? Whereas, if I replace sealed with final and compile it as Java, it works.
private sealed int compInt = 100;
public bool check(int someInt)
{
if (someInt > compInt)
{
return true;
}
return false;
}
That's because final in Java means plenty of different things depending on where you use it whereas sealed in C# applies only to classes and inherited virtual members (methods, properties, events).
In Java final can be applied to:
classes, which means that the class cannot be inherited. This is the equivalent of C#'s sealed.
methods, which means that the method cannot be overridden in a derived class. This is the default in C#, unless you declare a method as virtual and in a derived class this can be prevented for further derived classes with sealed again. That's why you see sealed members in C# a lot less than final members in Java.
fields and variables, which means that they can only be initialized once. For fields the equivalent in C# is readonly.
Sealed in C# can be applied only to a reference types, and has impact on inheritance tree.
In practise the type marked as sealed guranteed to be the last "leaf" in the inheritance tree, or in short, you can not derive from the type declared like a sealed.
public sealed class Child : Base
{
}
public class AnotherAgain : Child //THIS IS NOT ALLOWED
{
}
It can not be applied to a members.
Tigran's answer is not wrong while Joey's is a little incorrect.
Firstly you can look into this page: What is the equivalent of Java's final in C#?.
the sealed key word can apply to class,instance method and property but not variables, or interface's methods. Classes with sealed cannot be inherited. When sealed put on method， it must be by override in company. Every struct is sealed, so struct cannot be inherited. Check this image: