Nano's Blog

The C# language is designed such that versioning between base and derived classes in different libraries can evolve and maintain backwards compatibility. This means, for example, that the introduction of a new member in a base class with the same name as a member in a derived class is not an error. It also means that a class must explicitly state whether a method is intended to override an inherited method, or whether a method is a new method that simply hides a similarly named inherited method.

In C#, methods are by default, not virtual. To make a method virtual, the virtual modifier has to be used in the method declaration of the base class. The derived class can then override the base virtual method by using the override keyword or hide the virtual method in the base class by using the new keyword. If neither the override keyword nor the new keyword is specified, the compiler will issue a warning and the method in the derived class will hide the method in the base class. The following example shows these concepts in action.

// versioning.cs
// CS0114 expected
public class MyBase
{
    public virtual string Meth1()
    {
        return "MyBase-Meth1";
    }
    public virtual string Meth2()
    {
        return "MyBase-Meth2";
    }
    public virtual string Meth3()
    {
        return "MyBase-Meth3";
    }
}

class MyDerived : MyBase
{
    // Overrides the virtual method Meth1 using the override keyword:
    public override string Meth1()
    {
        return "MyDerived-Meth1";
    }
    // Explicitly hide the virtual method Meth2 using the new
    // keyword:
    public new string Meth2()
    {
        return "MyDerived-Meth2";
    }
    // Because no keyword is specified in the following declaration
    // a warning will be issued to alert the programmer that
    // the method hides the inherited member MyBase.Meth3():
    // warning CS0114: 'MyDerived.Meth3()' hides inherited member 'MyBase.Meth3()'.
    // To make the current member override that implementation, add the override keyword. Otherwise add the new keyword.
    public string Meth3()
    {
        return "MyDerived-Meth3";
    }

    public static void Main()
    {
        MyDerived mD = new MyDerived();
        MyBase mB = (MyBase)mD;

        System.Console.WriteLine(mB.Meth1());
        System.Console.WriteLine(mB.Meth2());
        System.Console.WriteLine(mB.Meth3());
    }
}

Output

MyDerived-Meth1
MyBase-Meth2
MyBase-Meth3

- from msdn.com