What is the difference between abstract and virtual in …

In C#, both abstract and virtual are used to define members that can be overridden, but there are some key differences between them.

1. Members that are abstract.
2. Abstract members must be declared in an abstract class.
3. Abstract members do not have implementation, only contain method signatures.
4. Abstract members must be implemented in the derived class.
5. Abstract classes cannot be directly instantiated and can only be instantiated through derived classes.
6. In a derived class, the override keyword is required when implementing abstract members.
7. Virtual members:
8. Virtual members can be declared in the base class and can be overridden in the derived class.
9. Virtual members contain default implementations, but can be overridden in derived classes.
10. Virtual members can be instantiated in the base class or can be instantiated through derived classes.
11. When overriding a virtual member in a derived class, the override keyword must be used.
12. If the derived class does not override a virtual member, it will use the default implementation from the base class.

In summary:

1. Abstract members must be implemented in derived classes, while virtual members can be optionally overridden.
2. Abstract classes cannot be instantiated directly, while virtual classes can be instantiated directly.
3. Abstract members do not have a default implementation, while virtual members include a default implementation.
4. Abstract members must be declared in an abstract class, while virtual members can be declared in either a regular class or an abstract class.