What is an interface class?

Interfaces, like classes, define a set of properties, methods, and
events. But unlike classes, interfaces do not provide implementation.
They are implemented by classes, and defined as separate entities from
classes.

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.

What’s the difference between an interface and abstract class?

In an interface class, all methods are abstract - there is no
implementation. In an abstract class some methods can be concrete.
In an interface class, no accessibility modifiers are allowed. An
abstract class may have accessibility modifiers.

What happens if you inherit multiple interfaces and 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.

When do you absolutely have to declare a class as abstract?

1. When the class itself is inherited from an abstract class, but not
all base abstract methods have been overridden.
2. When at least one of the methods in the class is abstract.

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.

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.

How to prevent your class from being inherited by another class?

Yes. The keyword "sealed" will prevent the class from being inherited.

Write the syntax to inherit from a class in C#?

Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass