C# IQ-54

19. What is the role of the DataReader class in ADO.NET connections?
It returns a read-only dataset from the data source when the command is executed.
1. To do: Improve the answer.
Class Questions

20. How do you inherit from a class in C#?
Place a colon and then the name of the base class.

21. Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.

22. Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.

23. What’s an abstract class?
A class that cannot be instantiated.
An abstract class is a class that must be inherited and have the methods overridden.
An abstract class is essentially a blueprint for a class without any implementation.

24. When do you absolutely have to declare a class as abstract?
1. When at least one of the methods in the class is abstract.
2. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.

25. What’s an interface class?
It’s an abstract class with public abstract methods all of which must be implemented in the inherited classes.

26. Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it’s public by default.

27. Can you inherit multiple interfaces?
Yes, why not.

28. And if they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.

29. What’s the difference between an interface and abstract class?
In an interface class, all methods must be abstract. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed, which is ok in an abstract class.

30. What is the difference between a Struct and a Class?
Structs are value-type variables and are thus saved on the stack -> additional overhead but faster retrieval. Another difference is that structs CANNOT inherit. (questions courtesy of Eyal)
Method and Property Questions