方法可以是抽象的但不是虚拟的吗？

Question

As I understand it, a method being marked abstract is implicitly virtual. The reason: Suppose the compile-time type of a given object is abstract. If one of the object's abstract methods is being called, the actual method to be executed is the one defined in the object's runtime type. Isn't it? If I'm right then the abstract method behaves as if it is also virtual.

In spite of that, I have successfully marked a C# method both abstract and virtual simultaneously:

public abstract virtual void crazy();

I suppose it means that an abstract method is not necessarily virtual and being abstract is actually orthogonal to being virtual.

What do I get wrong? How can an abstract method not be virtual?

Answer 1

You cannot mark a method as both abstract and virtual . It will result in a compiler error:

The abstract method 'Namespace.Class.Foo()' cannot be marked virtual

The rest of your question is correct: abstract methods are implicitly virtual .

Answer 2

From MSDN

• An abstract method is implicitly a virtual method.
• Abstract method declarations are only permitted in abstract classes.
• Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) following the signature.
• The implementation is provided by an overriding method, which is a member of a non-abstract class.
• It is an error to use the static or virtual modifiers in an abstract method declaration.

Answer 3

"You cannot use the virtual modifier with the static, abstract, private, or override modifiers."

• http://msdn.microsoft.com/en-us/library/9fkccyh4(v=vs.100).aspx