CODEWITHSUNDEEP
c#inheritancenew-operator
Denish
Denish

Reputation: 983

inheritance in C# with new keyword

I am trying to understand inheritance in C# and that is why I am trying out some samples, but I got to know that with new keyword or without new keyword things are same.

Then why we need new keyword?

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        try
        {
            List<int> col = new List<int>();
            Class1 c1 = new Class1();
            Class2 c11 = new Class2();
            Class1 c111 = new Class3();
            c1.setmessage();
            c11.setmessage();
            c111.setmessage();
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }
   
}
class Class1
{
    private string member;

    public  void setmessage()
    {
        MessageBox.Show("Class1");
    }
}

class Class2 : Class1
{
    private string member;

    public  void setmessage()
    {
        MessageBox.Show("Class2");
    }
}

Upvotes: 1

Views: 5365

Answers (3)

Sergioet
Sergioet

Reputation: 1288

In inheritance, you can mark a method in the base class as virtual, and this gives the opportunity to the derived class to override it, providing a different implementation for the derived class.

But if a base class doesn't mark a method as virtual, then the derived class can't override the base method. If the derived class implements a method with the same name as the base, it will be implemented as an independent method from the base class. At this point the compiler gives a warning stating that the derived method "hides" inherited member from the base class. If the new keyword is used, that compiler disappears.

An example:

class Base
{
     public void DoWork()
     {
         Console.WriteLine("Base.DoesWork");
     }
}

class Derived : Base
{
    public void DoWork()  
    // warning Derived.DoWork() hides inherited member Base.DoWork(). 
    // Use the new keyword if hiding was intended
    {
        Console.WriteLine("Derived.DoesWork");
    }
}

if you use public new void DoWork() the compiler warning disappears.

Interestingly if you execute:

Base b = new Base();
b.Execute();
b = new Derived();
b.Execute();

The ouput will be "Base.DoesWork" - twice.

Upvotes: 0

Kendall Frey
Kendall Frey

Reputation: 44376

Without the new keyword, there is a compiler warning, as the compiler isn't sure you meant to hide it. You should specify new or override, so that the compiler knows that you didn't make a mistake.

This is similar to "Why is there a private keyword for class members, as they are by default private?" and "Why is there an internal keyword for classes, as they are by default internal?". The common answer is that it is usually better to be explicit than implicit.

Upvotes: 6

Oded
Oded

Reputation: 499132

The new keyword, in this context signifies shadowing of a method.

This means that you are not overriding the method from the base class, but writing a completely separate one, as far as inheritance is concerned.

A compiler issues a warning if it is not used for an overridable method that isn't being overridden as most of the time this is an error and the method should simply be overridden. Having the new keyword there shows that shadowing was an explicit decision.

See Difference between shadowing and overriding in C#?.

Upvotes: 14

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