Why are private virtual methods illegal in C#?

问题:

Coming from a C++ background, this came as a surprise to me. In C++ it's good practice to make virtual functions private. From http://www.gotw.ca/publications/mill18.htm: "Guideline #2: Prefer to make virtual functions private."

I also quote Eric Lippert's blog, from Knights-knaves-protected-and-internal:

Private virtual methods are illegal in C#, which irks me to no end. I would totally use that feature if we had it.

I understand that in C#, you wouldn't be able to override a private virtual method in a derived (but not nested) class. Why is this the case? In C++ the access specifier has nothing to do with whether you can override a function or not.

回答1:

I note that there are two questions here. In the future you might consider posting two questions instead of combining two questions in one. When you combine questions like this often what happens is only the first one gets answered.

The first question is "why are private virtual methods illegal in C#?"

Here are the arguments against the "private virtual methods" feature:

1. private virtual is only useful when you have a nested derived class. This is a useful pattern, but far less common than the non-nested derived class situation.

2. If you desire to restrict the ability to override the method in non-nested derived classes then you can do so by restricting the ability of non-nested classes to derive from the base class; make all the base class constructors private. Therefore private virtual is not necessary to prevent overriding; protected virtual is sufficient, because the only derived classes will be nested.

3. If you desire to restrict the ability to call a method in a non-nested derived class then you can make the method internal virtual and then tell your coworkers to not use that method. It is irritating to have this not be enforced by the compiler, but the compiler does not enforce any other semantic constraint on how a method is supposed to be used either; getting the semantics right is your business, not the compiler's, and you have to enforce that with appropriate code reviews. Therefore private virtual is not necessary to prevent calling; internal virtual plus code reviews is sufficient.

4. It is possible to implement this pattern already with existing parts:

   abstract class C
   {
       private int CF() { whatever; }
       private Func<int> f;
       public C() { f = CF; } 
       private int F() { return f(); }
       private class D : C
       {
           private int DF() { whatever; }
           public D() { f = DF; }
       }
   

   Now I have a method F which is effectively virtual, but can only be "overridden" by derived nested classes.

Since in every case either protected, internal or protected internal does the trick, private virtual is unnecessary. It's almost never the right thing to do, since you have to be already committed to using the nested derived class pattern. So, the language makes it illegal.

The arguments for are:

There have been times in real code when I've want a virtual method to be a private implementation detail of a class that I want to be extended both by non-nested internal classes and nested internal classes. Having to enforce the invariant that the internal method not be called by my coworkers is vexing; I'd like that to be enforced by the compiler without me having to jump through crazy hoops like making a field of delegate type, etc.

Also, there's simply the matter of consistency and orthogonality. It seems weird that two things that ought to be independent -- accessibility and virtualness -- have an effect on each other unnecessarily.

The arguments against the feature are pretty strong. The arguments for are pretty weak. Therefore, no such feature. I'd personally like it very much, but I totally understand why the design team has never taken me up on it. It's not worth the cost, and I would hate to not ship a better feature because we spent budget on a feature that benefits almost no one.

The second question is "Why in C# are you not able to override a private virtual method in a derived non-nested class?"

There are several reasons.

1. Because you can only override what you can see. A private method is a private implementation detail of a base class and must not be accessible.

2. Because allowing that has serious security implications. Remember, in C++ you almost always compile code into an application all at once. You have the source code for everything; everything is essentially "internal" from the C++ perspective most of the time. In C#, that's not at all the case. Third party assemblies can easily get at public types from libraries and produce novel extensions to those classes which can then be used seamlessly in place of instances of the base class. Since virtual methods effectively change the behaviour of a class, any code which depends for security reasons on invariants of that class needs to be carefully designed so that they do not depend on invariants guaranteed by the base class. Restricting accessibility of virtual methods helps ensure that invariants of those methods are maintained.

3. Because allowing that provides another form of the brittle base class problem. C# has been carefully designed to be less susceptible to the brittle base class problem than other OO languages. If an inaccessible virtual method could be overridden in a derived class then that private implementation detail of the base class becomes a breaking change if altered. Providers of base classes should be free to change their internal details without worrying overmuch that they've broken derived classes which depend on them; ideally only the public, documented interface to a type needs to be maintained when implementation details change.

回答2:

Because private methods can ONLY be accessed from the class defining them, hence a private virtual method would be useless. What you want is a protected virtual method. A protected method can be accessed by the class defining it and any subclasses.

EDIT:

By providing private and protected keywords C# allows you more granular control over your methods. That is private means completely closed and protected means completely closed apart from subclasses. This allows you to have methods that only your superclass knows about and methods that subclasses can know about.

回答3:

I would guess the reason is that internal virtual does almost the same thing as private virtual and is somewhat less confusing for those not familiar with the private virtual idiom.

Whereas only inner classes could override private virtual methods, only classess in your assembly can override internal virtual methods.

回答4:

In C# (and in the CLI, as far as i've seen), "private" has a pretty clear and unambiguous meaning: "accessible only in this class". The concept of private virtuals screws that all up, not to mention making namespaces a bit of a minefield. Why should i have to care what you've called a method i can't even see, and get a compiler warning for happening to have chosen a name you already snagged for it?

回答5:

Because C# does not have any mechanism for providing public/private/protected inheritance, which is what you are actually after.

Even in C++, private members cannot be accessed by derived classes, but they can limit the base class visibility by specifying inheritance visibility:

class Derived : /*->>*/private/*<--*/ Base {
}

C# provides a whole bunch of other things in order for you to control the visibility of your class' members. Between protected and internal, you should be able to get the hierarchy exactly as you want.

IMHO C# enforces stronger IS-A relationship via single base class inheritance, so it makes sense that if a car has an engine, a BMW subclass shouldn't be able to hide it.

C++ supports multiple inheritance which is a less stricter IS-A relationship - it's almost like a HAS-A relationship where you can bring in multiple unrelated classes. Because of the ability to bring in multiple base classes, you want tighter control over the visibility of all of them.

回答6:

Let me make this clear: C# is not C++.

C# is designed multiple decades after C++ and is built using advancing insights over the years. In my humble opinion C# is well defined and finally handles object orientation the right way (imho). It includes the internal statement for a reason and does not allow you to "virtualize" and override private methods. For a reason.

All the issues described earlier (inner classes overriding private virtual methods, using the abstract factory pattern this way etc...) can be easily written in a different way using interfaces and the internal statement. Having said that, I must say that it is fairly a matter of taste whether you like the C++ way or the C# way.

I prefer to use descriptive code (code that speaks for itself, without using comments) and I use interfaces instead of deep inheritance. Overriding private virtual methods feels like hacking or spaghetti to me, regardless if it's common practice, an often used pattern or gets the job done.

I've been developing in C++ for almost 1.5 decades and I never came across the necessity for overriding private methods... ( I can see the comments flying in :-) )