Implementing the Template Method pattern in C#

Question

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#)?

You can only seal overridden members. The lack of the keyword `abstract` or `virtual` means that the method is already sealed. — ChaosPandion, Aug 15 '12 at 17:49

score 8 · Accepted Answer · answered Aug 15 '12 at 17:50

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.

Ok, it works correctly by removing the `sealed` keyword, but I thought that there was some mechanism (such as the `final` keyword in Java) that _prevents you from hiding the inherited method_. I wonder, out of curiosity, the reason for which this mechanism is not present. — enzom83, Aug 15 '12 at 19:11
@enzom83: No, you can always *hide* the inherited method - but you can't override it, so anyone calling `TemplateMethod` on an expression of static type `Base` will always call the right method. — Jon Skeet, Aug 15 '12 at 19:15

score 1 · Answer 2 · answered Aug 15 '12 at 17:50

1

Just remove the sealed keyword. By default, methods are not overridable; subclasses cannot override them, only hide them.

answered Aug 15 '12 at 17:50

Jordão

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score 1 · Answer 3 · answered Aug 15 '12 at 17:51

1

C# methods are sealed by default

answered Aug 15 '12 at 17:51

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Implementing the Template Method pattern in C#

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