When a derived class can overrides the base class member?

In object-oriented programming, a derived class can override a base class member when the base class member is marked as "virtual" (or "abstract" in the case of abstract classes and interfaces) and the derived class explicitly indicates its intention to override the member using the "override" keyword. This concept applies to methods, properties, and indexers. Here are the conditions under which a derived class can override a base class member:

1. The base class method must be marked as "virtual" using the 'virtual' or 'abstract' keyword.
   • •virtual methods: These are methods in the base class that are designed to be overridden by derived classes. The base class provides a default implementation, but the derived classes can provide their own implementation by using the override keyword.
   • •abstract methods: These are methods in an abstract base class or an interface that are meant to be implemented by derived classes. Derived classes must provide their own implementation for these methods using the override keyword.
2. The derived class must use the 'override' keyword before the method declaration to indicate that it's intentionally providing a new implementation.
3. The method name, return type, and parameter types in the derived class must match those in the base class.

Example in C#:

class BaseClass {
    public virtual void SomeMethod() {
        // Base class method implementation
    }
}

class DerivedClass : BaseClass {
    public override void SomeMethod() {
        // Derived class method implementation, overrides the base class method
    }
}

1. Similar to methods, properties and indexers in the base class can be marked as "virtual".
2. The derived class uses the 'override' keyword to provide a new implementation.
3. The property or indexer name, data type, and accessors must match those in the base class.

Example in C#:

class BaseClass {
    public virtual int MyProperty { get; set; }
}

class DerivedClass : BaseClass {
    public override int MyProperty {
        get {
            // Derived class property getter implementation
        }
        set {
            // Derived class property setter implementation
        }
    }
}

In summary, a derived class can override a base class member when the member is explicitly marked as "virtual" in the base class and the derived class uses the "override" keyword to provide a new implementation. This mechanism enables the concept of polymorphism, allowing different behaviors to be executed based on the actual type of the object at runtime.