##

**Classes** can be defined as user-defined data types representing an object's state (properties) and behavior (actions).  

## 
### Types of classes

There are four types of classes in C#, which are as follows:

1. Abstract Class
2. Partial Class
3. Sealed Class
4. Static Class

In this Answer, we'll only discuss the abstract class.

##      
### Abstract class

An **abstract class** is defined as a class that is declared using the `abstract` keyword and whose object is not created. This type of class provides a standard definition for the subclasses.

To access the object of this class, it must be inherited from another class.

##
### Example

If we run the following code, it will generate an error as we cannot generate an object of the class of type `abstract`.

abstract class Car
{
    public abstract void startengine();
    public void enginesound()
    {
        System.Console.WriteLine("Broom!");
    }
}
class Program
{
  static void Main(string[] args)
  {
    Car myObj = new Car();
  }
}

##
### Explanation

In the code above:

* **Line 1:** We generate a class `car` of type `abstract`. The class type is declared using a specific keyword,  `abstract`.  
* **Lines 2–8:** These are the methods specific to the class `Car`.
* **Line 9:** This class `Program` contains the `main` program that executes and generates an error signifying that we cannot create an object of the class `car`.

We can solve the problem by making a derived class. Try executing the following code. It will not generate any error.

abstract class Car
{
    public abstract void startengine();
    public void enginesound()
    {
        System.Console.WriteLine("Broom!");
    }
}
class BMW : Car
{
    public override void startengine()
    {
        System.Console.WriteLine("Engine started.");
    }
}
class Program
{
  static void Main(string[] args)
  {
    BMW myBMW = new BMW();
    myBMW.startengine();
    myBMW.enginesound();
  }
}

> Note: The code above uses C# version 7 or later.

The code above is the same as the previous one, but **lines 9–15** contain a derived class `BMW`. In **lines 20–22**, we generate an object of class `BMW` rather than that of `car`. Therefore, there is no error in the execution of the code.

What is an abstract class in C#?

An abstract class in C# defines a template for subclasses, cannot be instantiated, and is declared with the abstract keyword.