What is the difference between Virtual method and Abstract method?

Both virtual methods and abstract methods in C# provide mechanisms for achieving polymorphism and enabling method overriding in derived classes. However, there are significant differences between them in terms of their purpose, implementation, and usage. Here are the main differences:

  1. Purpose:
    • Virtual Methods: A virtual method is a method in a base class that provides a default implementation but allows derived classes to override and provide their own implementation. It is used when a base class wants to provide a common implementation that can be extended or modified by derived classes.
    • Abstract Methods: An abstract method is a method declared in an abstract class or an interface that has no implementation in the declaring class/interface. It is used to define a contract or an obligation for derived classes or implementing classes to provide their own implementation of the method.
  2. Implementation:
    • Virtual Methods: A virtual method has a default implementation in the base class, which can be overridden by derived classes using the override keyword. The base class provides a default behavior, but derived classes can choose to override it or use it as-is.
    • Abstract Methods: An abstract method has no implementation in the declaring class. It is marked with the abstract keyword and ends with a semicolon instead of a method body. Derived classes or implementing classes must provide their own implementation for the abstract method.
  3. Forceful Implementation:
    • Virtual Methods: Derived classes are not required to override a virtual method. They can choose to override it or leave it as-is, depending on their specific needs.
    • Abstract Methods: Derived classes or implementing classes must provide an implementation for all the abstract methods defined in the base abstract class or interface. Failure to provide an implementation results in a compilation error.
  4. Instantiation:
    • Virtual Methods: Virtual methods are associated with instances of classes. They can be called using an instance of a class or through polymorphism.
    • Abstract Methods: Abstract methods are associated with abstract classes or interfaces. They cannot be called directly since abstract classes cannot be instantiated. Instead, they must be implemented by derived classes or implementing classes.
  5. Class Type:
    • Virtual Methods: Virtual methods are associated with classes, allowing for method overriding within the class hierarchy.
    • Abstract Methods: Abstract methods are associated with abstract classes or interfaces, providing a contract for derived classes or implementing classes to fulfill.

In summary, virtual methods provide a default implementation in the base class that can be overridden by derived classes, while abstract methods define a contract without any implementation and must be implemented by derived or implementing classes. Virtual methods offer flexibility for customization, while abstract methods enforce implementation obligations. The choice between virtual and abstract methods depends on the specific requirements of the design and the desired behavior for derived or implementing classes.

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