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Let s just recap the implications of all that, as it is a bit complicated. Back in 3, when we designed our first class, we talked about its public contract, and how that encapsulated the implementation details which allowed us to use it as though it was a black box. With the addition of public and protected virtual members, we re opening that black box and creating a second contract: for people who derive their own classes, which, as we just saw, is a whole lot more complex. The designers of C# decided that should be an opt-in complexity: unless we specify that a member is virtual we don t have to worry about it. Along the same lines, they ve also provided a way to ensure that we don t have to worry about anyone deriving from us at all.

Having got through all of that, you re probably rightly concerned that, simple though it is in theory, the practical implications of inheritance are actually rather complex and require a lot of documentation, testing, and imagining how people might use and abuse your virtual methods. And we have to do that for every class down the hierarchy.

Of course, similar examples could be constructed using other scripting languages supported by the victim s browser (such as VBScript) To keep the examples short, we use the placeholder evil-script; to denote a sequence of JavaScript statements the attacker might inject (possible exploit payloads were discussed in more detail in Section 1023) For each class of XSS vulnerability (which is based on the context within an HTML document in which the injection occurs, such as simple text, within an href attribute, etc), we provide the following:.

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When we designed our FireChief, we happily provided an override for the Extinguish Fire method, without giving a thought for the fact that someone else might override that method in his own derived class. In fact, we didn t even consider the possibility that anyone might derive from FireChief at all. No documentation, nothing. Now there are several members on our own base class that could be overridden by a class that derives from FireChief. Does that have any implications for our own documentation or testing Can we even tell And how could we have guessed that was going to happen when we built our FireChief class, since there was only one virtual member on the base at that time This looks like it has the potential to become a rich future source of bugs and security holes. Fortunately, we can eliminate this problem at a stroke by saying that we didn t design our FireChief to be derived from, and stopping anyone from doing so. We do that by marking the FireChief class sealed. Let s see how that looks:

Before we go into the how things work, stop the currently running test harness by selecting Debug Stop Debugging or clicking the Stop Debugging button in the VWD window (see Figure 8-20).

sealed class FireChief : Firefighter { // ... }

We apply the sealed modifier before the class keyword and after any accessibility modifiers if they are present. So, what happens if we try to derive a new class from FireChief now

class MasterChief : FireChief { }

An example that shows how the injection can be exploited that is, how the attacker could inject strings into the HTML document such that script of his choosing would execute in the user s browser Prevention techniques for avoiding XSS in each particular context (e.g., escape a specific set of characters ) Rationale explaining why these guidelines are necessary and why they indeed prevent XSS in the given context

Compile it, and you ll see the following error:

'MasterChief': cannot derive from sealed type 'FireChief'

Not only can sealing classes be very useful (and defensive), but if you decide later that you want to unseal the class and allow people to derive their own types, it doesn t (normally) break binary compatibility for the type. Sealing a previously unsealed class, however, does break compatibility.

We now have three different types of firefighter. Let s remind ourselves how they are related (see Figure 4-1).

Why, you may ask, if we are nonvirtual by default aren t we also sealed by default, with an unseal keyword Notice, for instance, that we ve been talking about classes so far value types (struct) are sealed (with no opt-out), so you can t derive from them. There s no performance hit to marking a class sealed. There are potential security advantages to marking a class sealed (no one can sneakily exploit polymorphism to insert code where you weren t expecting it). So why not make them all sealed It is certainly much less problematic to present an unsealed class than it is to present a virtual method; if there are no virtual methods, all you can do is to bolt extra bits on, which do no harm to anyone. It also conforms to the expectations of a generation of C++ and Java developers in this regard. Plenty of people argue that we should have both unsealed-by-default and virtual-bydefault, and they certainly have a point, particularly with regard to convenience; but the designers of C# took a different view. No doubt, the debate will continue.

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