Abstract Class:
  1. An abstract class cannot be instantiated.
  2. An abstract class may contain abstract methods and also contain own concrete method.
  3. We want use the abstract class must be inherited by the non abstract class.
  4. A non-abstract class, which derived from an abstract class must include implementations for all inherited abstract methods
  5. An abstract method is implicitly a virtual method.
  6. In abstract method declaration provides no actual implementation, there is no method body; it has only the signature
  7. The implementation is provided by an overriding method (using override keyword in c#), which is a work as a member of non-abstract class.
Code:

public abstract class ClassBase //abstract classes
 {
 //Abstract class constructor
 public ClassBase()
 {
 Console.WriteLine("Abstract Constructor");
 }

//Own completed method in Abstract Class
 public void useinsub()
{
 Console.WriteLine("Call by sub-class/non abstract");
 }

//Virtual method in abstract class
 public virtual void fill()
{
 Console.WriteLine("Abstract Virtual Function");
 }

 //Pure abstract method
 public abstract void fill2();
}

 public class ClassSub : ClassBase //Sub Class(Non Abstarct)
 {
 // Sub class constructor
 public ClassSub()
 {
 Console.WriteLine("Sub Constructor");
 }

// Over ride the virtual method in abstract class
 public override void fill()
 {
 Console.WriteLine("Override the virtual Function");
 }

// Implement the abstract method, which is in abstract class
 public override void fill2()
 {
 Console.WriteLine("Implement the abstract Function");
 //Call abstract class's own completed method
 base.useinsub();
 }
}
 class Program
 {
 static void Main(string[] args)
 {
 ClassSub cs = new ClassSub();
 cs.fill();
 cs.fill2();
 }
 }