How To Use An Abstract Method In C#

We know that when a method is declared as virtual, it can be implemented in a child class but it is optional. But what if there is requirement that child class must implement a method. To achieve this, we can declare method as an abstract method. When a method is declared as an abstract method, it is mandatory for all derived classes to implement it. Otherwise, the compiler will throw an error. Uually the parent class has a body and signature of the method without any implementation. The derived or child class has the actual implementation of the method.

 

If you're new to Abstract classes, read here: Abstract Class In C# 

 

Some important points about abstract methods: 

• An abstract method is implicitly a virtual method.
• Abstract method declarations are only permitted in abstract classes.
• Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) 

Here is an example of implementation of an abstract method.

using System;
namespace OOPSProject
{
abstract class AbsParent
{
public void Add(int x, int y)
{
Console.WriteLine(x + y);
}
public void Sub(int x, int y)
{
Console.WriteLine(x - y);
}
public abstract void Mul(int x, int y);//only method declaration
public abstract void Div(int x, int y);//only method declaration
}
}
using System;
namespace OOPSProject
{
class AbsChild : AbsParent
{
//first we have to implement abstract methods of parent class
public override void Mul(int x, int y)
{
Console.WriteLine(x * y);
}
public override void Div(int x, int y)
{
Console.WriteLine(x / y);
}
static void Main()
{
AbsChild c = new AbsChild();
AbsParent p = c;
p.Add(100, 50); p.Sub(156, 78);//then we can invoke other methods
p.Mul(50, 30); p.Div(625, 25);
Console.ReadLine();
}
}
}