Why seal a class?
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.
- 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.
It seems that the official Microsoft guidelines on sealing have evolved since this question was asked ~9 years ago, and they moved from an opt-in philosophy (seal by default) to opt-out (don't seal by default):
X DO NOT seal classes without having a good reason to do so.
Sealing a class because you cannot think of an extensibility scenario is not a good reason. Framework users like to inherit from classes for various nonobvious reasons, like adding convenience members. See Unsealed Classes for examples of nonobvious reasons users want to inherit from a type.
Good reasons for sealing a class include the following:
- The class is a static class. See Static Class Design.
- The class stores security-sensitive secrets in inherited protected members.
- The class inherits many virtual members and the cost of sealing them individually would outweigh the benefits of leaving the class unsealed.
- The class is an attribute that requires very fast runtime look-up. Sealed attributes have slightly higher performance levels than unsealed ones. See Attributes.
X DO NOT declare protected or virtual members on sealed types.
By definition, sealed types cannot be inherited from. This means that protected members on sealed types cannot be called, and virtual methods on sealed types cannot be overridden.
✓ CONSIDER sealing members that you override. Problems that can result from introducing virtual members (discussed in Virtual Members) apply to overrides as well, although to a slightly lesser degree. Sealing an override shields you from these problems starting from that point in the inheritance hierarchy.
Indeed, if you search the ASP.Net Core codebase, you will only find about 30 occurences of sealed class, most of which are attributes and test classes.
I do think that immutability conservation is a good argument in favor of sealing.
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 ...
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