When a derived class inherits from a base class, it gains all the methods, fields, properties and events of the base class. The designer of the derived class can choose whether to

• override virtual members in the base class,

• inherit the closest base class method without overriding it

• define new non-virtual implementation of those members that hide the base class implementations

A derived class can override a base class member only if the base class member is declared as virtual orabstract. The derived member must use the override keyword to explicitly indicate that the method is intended to participate in virtual invocation. The following code provides an example:

public class BaseClass
{
    public virtual void DoWork() { }
    public virtual int WorkProperty
    {
        get { return 0; }
    }
}
public class DerivedClass : BaseClass
{
    public override void DoWork() { }
    public override int WorkProperty
    {
        get { return 0; }
    }
}

Fields cannot be virtual; only methods, properties, events and indexers can be virtual. When a derived class overrides a virtual member, that member is called even when an instance of that class is being accessed as an instance of the base class. The following code provides an example:

DerivedClass B = new DerivedClass();
B.DoWork();  // Calls the new method.

BaseClass A = (BaseClass)B;
A.DoWork();  // Also calls the new method.

Virtual methods and properties enable derived classes to extend a base class without needing to use the base class implementation of a method. For more information, see Versioning with the Override and New Keywords (C# Programming Guide). An interface provides another way to define a method or set of methods whose implementation is left to derived classes. For more information, see Interfaces (C# Programming Guide) and Choosing Between Classes and Interfaces.

If you want your derived member to have the same name as a member in a base class, but you do not want it to participate in virtual invocation, you can use the new keyword. The new keyword is put before the return type of a class member that is being replaced. The following code provides an example:

public class BaseClass
{
    public void DoWork() { WorkField++; }
    public int WorkField;
    public int WorkProperty
    {
        get { return 0; }
    }
}

public class DerivedClass : BaseClass
{
    public new void DoWork() { WorkField++; }
    public new int WorkField;
    public new int WorkProperty
    {
        get { return 0; }
    }
}

Hidden base class members can still be accessed from client code by casting the instance of the derived class to an instance of the base class. For example:

DerivedClass B = new DerivedClass();
B.DoWork();  // Calls the new method.

BaseClass A = (BaseClass)B;
A.DoWork();  // Calls the old method.

Virtual members remain virtual indefinitely, regardless of how many classes have been declared between the virtual member and the class that originally declared it. If class A declares a virtual member, and class B derives from A, and class C derives from B, class C inherits the virtual member, and has the option to override it, regardless of whether class B declared an override for that member. The following code provides an example:

public class A
{
    public virtual void DoWork() { }
}
public class B : A
{
    public override void DoWork() { }
}

A derived class can stop virtual inheritance by declaring an override as sealed. This requires putting the sealed keyword before the override keyword in the class member declaration. The following code provides an example:

public class C : B
{
    public sealed override void DoWork() { }
}

In the previous example, the method DoWork is no longer virtual to any class derived from C. It is still virtual for instances of C, even if they are cast to type B or type A. Sealed methods can be replaced by derived classes by using the new keyword, as the following example shows:

public class D : C
{
    public new void DoWork() { }
}

In this case, if DoWork is called on D using a variable of type D, the new DoWork is called. If a variable of type C, B, or A is used to access an instance of D, a call to DoWork will follow the rules of virtual inheritance, routing those calls to the implementation of DoWork on class C.

A derived class that has replaced or overridden a method or property can still access the method or property on the base class using the base keyword. The following code provides an example:

public class Base
{
    public virtual void DoWork() {/*...*/ }
}
public class Derived : Base
{
    public override void DoWork()
    {
        //Perform Derived's work here
        //...
        // Call DoWork on base class
        base.DoWork();
    }
}

For more information, see base.

Note:

It is recommended that virtual members use base to call the base class implementation of that member in their own implementation. Letting the base class behavior occur enables the derived class to concentrate on implementing behavior specific to the derived class. If the base class implementation is not called, it is up to the derived class to make their behavior compatible with the behavior of the base class.