Virtual Methods

By declaring a base class function as virtual, you allow the function to be overridden in any derived classes:

class MyBaseClass
{
public virtual string VirtualMethod()
{
return "This method is virtual and defined in MyBaseClass";
}
}

It is also permitted to declare a property as virtual. For a virtual or overridden property, the syntax is the same as for a non-virtual property, with the exception of the keyword virtual, which is added to the definition. The syntax looks like this:

public virtual string ForeName
{
get { return fName;}
set { fName = value;}
}
private string foreName;
For simplicity, the following discussion focuses mainly on methods, but it applies equally well to properties.

The concepts behind virtual functions in C# are identical to standard OOP concepts. You can override a virtual function in a derived class, and when the method is called, the appropriate method for the type of object is invoked. In C#, functions are not virtual by default, but (aside from constructors) can be explicitly declared as virtual. This follows the C++ methodology: for performance reasons, functions are not virtual unless indicated. In Java, by contrast, all functions are virtual. C# differs from C++ syntax, however, because it requires you to declare when a derived class's function overrides another function, using the override keyword:

class MyDerivedClass : MyBaseClass
{
public override string VirtualMethod()
{
return "This method is an override defined in MyDerivedClass";
}
}

This syntax for method overriding removes potential runtime bugs that can easily occur in C++, when a method signature in a derived class unintentionally differs slightly from the base version, resulting in the method failing to override the base version. In C# this is picked up as a compile-time error, because the compiler would see a function marked as override but no base method for it to override.

Neither member fields nor static functions can be declared as virtual. The concept simply wouldn't make sense for any class member other than an instance function member.

Regards,
Praveen KVC
1 February 2010