In addition to producing a new class based on an old one by adding additional features, you can modify existing behavior of the parent class.
If a method is defined in a derived class so that the name, return type, and argument list match exactly those of a method in the parent class, then the new method is said to override the old one.
The keyword virtual
In C#, a class can declare virtual methods, properties, and indexers, and derived classes can override the implementation of these function members.
The keyword virtual allows programmers to specify methods that a derived class can override, C# methods are non-virtual by default and must be explicitly declared as virtual.
The implementation of a non-virtual method is invariant: The implementation is the same whether the method is invoked on an instance of the class in which it is declared or an instance of derived class. In contrast, the implementation of a virtual method can be changed by derived classes.
The keyword override
To override a base-class method definition, a derived class must specify that the derived-class method overrides the base-class method with keyword override in the method header.
If the override modifier is not used, the new member hides the inherited member, and a compiler warning occurs. If a derived class attempts to override a non-virtual inherited member, a compiler error will occur. The following examples illustrate the using of virtual and override keywords.
The Manager class has a GetDetails method by definition because it inherits one from the Employee class. However, the original method has been replaced, or overridden, by the derived class’s version.