Exploring Abstract class (OOP concepts) in .NET - My Observations

Following are my observations with Abstract classes in .NET.

Feel free to write in comments if you think of more points to be added in this list, or if you want know more details about one or more points :)

1. A class must be declared with "abstract" keyword
2. It can contain complete implantation of methods
3. Methods declared with "abstract" keyword can contain only declarations in Abstract class
4. We cannot create an instance of abstract class
5. Abstract class must be inherited
6. A class derived from an abstract class must contain complete implementation of all abstract methods
7. An abstract class can be derived from another abstract class
8. Abstract class can be used as a parameter in methods
9. If a class contains an abstract method, then that class must be marked as "abstract"
10. An abstract class can have a constructor
11. An abstract class can be inherited from a non-abstract class
12. Constructor rules are also applied in case of abstract classes inheritance

P.S. - Please note all keyword and code snippets (if demonstrated above) are in C#.NET 

How constructors are called while the classes are inherited: Exploring Object Oriented Programming and Concepts

Constructor Calls in Inheritance

Case study 1:

ClassA is a base class of ClassB.

Both classes have default constructor.

Observations:
While creating a new instance of ClassB (without parameters):
- it will first call default constructor of ClassB which will call constructor of ClassA.
- it will complete executing code of default constructor of ClassA.
- after that, it will complete executing code of default constructor of ClassB.

Case study 2:

ClassA is a base class of ClassB.

ClassA has no default constructor, but only parameterized constructor.
ClassB has a default constructor.

Observations: 
- it will not allow creating ClassA without a default constructor 

- if you want to do so, you need to call a base class constructor from ClassB's constructor: 

For example, following is the constructor of class A 

public ClassA(string someVariable) { ..... some code .... }  

To allow ClassA without a default constructor, its all child classes should call base class constructor as below: 

public ClassB(): base("some value") 

or

public ClassB(string fullName): base(fullName)