Why seal a class?

Question

I'd like to hear what is the motivation behind the bulk of sealed classes in the .Net framework. What is the benefit of sealing a class? I cannot fathom how not allowing inheritance can be useful and most likely not the only one fighting these classes.

So, why is the framework designed this way and wouldn't it be unbreaking change to unseal everything? There must be another reason but just being evil?

Answer 1

• Sometimes classes are too precious and not designed to be inherited.
• Runtime/Reflection can make inheritance assumptions about sealed classes when looking for types. A great example of this is - Attributes are recommended to be sealed for lookup runtime speed. type.GetCustomAttributes(typeof(MyAttribute)) will perform significantly faster if MyAttribute is sealed.

The MSDN article for this topic is Limiting Extensibility by Sealing Classes.

Answer 2

I found this sentence in msdn documentation: "Sealed classes are primarily used to prevent derivation. Because they can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster."

I don't know if the performance is the only advantage of sealed classes and personally I also would like to know any other reasons ...

Answer 3

Classes should either be designed for inheritance or prohibit it. There is a cost to designing for inheritance:

• It can pin down your implementation (you have to declare which methods are going to call which other methods, in case a user overrides one but not the other)
• It reveals your implementation rather than just the effects
• It means you have to think of more possibilities when designing
• Things like Equals are hard to design in an inheritance tree
• It requires more documentation
• An immutable type which is subclassed may become mutable (ick)

Item 17 of Effective Java goes into more details on this - regardless of the fact that it's written in the context of Java, the advice applies to .NET as well.

Personally I wish classes were sealed by default in .NET.

Answer 4

Performance is an important factor for example, the string class in java is final(<- sealed) and reason for this is performance only. I think another important point is to avoid the brittle base class problem described in detail here: http://blogs.msdn.com/ericlippert/archive/2004/01/07/virtual-methods-and-brittle-base-classes.aspx

If you provide a framework it is important for maintainability legacy projects and to upgrade your framework to avoid the brittle base class problem

Answer 5

Sealing allows you to realize some minor performance gains. This is less true in the world of JITs and lazy pessimization than in the world of, say C++, but since .NET is not as good as pessimization as java compilers are mostly because of different design philosophies it is still useful. It tells the compiler that it can directly call any virtual methods rather than call them indirectly through the vtable.

It is also important when you want a 'closed world' for things like equality comparison. Normally once I define a virtual method, I'm pretty much hosed for defining a notion of equality comparison that really implements the idea. On the other hand, I might be able to define it for a particular subclass of the class with the virtual method. Sealing that class ensures that equality really does hold.

Answer 6

you will be able to fine some cool stuff about sealed class and abstract class.please refer the link http://lionalblog.blogspot.com/2010/07/class-modifiers-in-c-and-vb-sealed.html