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Friday, February 18, 2011

What do you mean by shadowing in .NET?

What do you mean by shadowing in .NET?

Assume that you have a class that derives from the base class and redefines a method in the base class. Then what process is happening? You will generally assume it as overriding meaning that derived class method overrides base class method and redefines the method.

Even when it redefines, the derived class method should have the same signature as base class method and the derived class method should include the keyword override in its method signature.
What if you don’t explicitly specify the override keyword in your derived class method? Even then does it mean as overriding. No, if you don’t explicitly use override then the derived class method is shadowing base class method. This means that derived class is creating a new method that is no way related to the base class method. But the derived class method completely shadows/hides base class method. Unlike overriding, members sharing same name in derived class can have a totally different signature since it is a new member.

Shadowing happens by default when two members share the same name but override keyword is not mentioned. However you can explicitly perform shadowing using a keyword.

The keyword differs between languages:
• In C#, shadowing is done by using new modifier
• In VB.NET, shadowing is done using shadows keyword

Given below are examples to demonstrate shadowing in both the languages.

Shadowing in C#:
In the following example, method displayMsg of baseClass and data member called member are shadowed by the derived class members using the new modifier:
class baseClass {
public int member;
baseClass(int data) {
member = data;
public void displayMsg() {
Console.WriteLine(“In baseClass..”);
class derivedClass:baseClass {
new public string member;
public derivedClass(string data) {
member = data;
new public void displayMsg(string msg) {
Console.WriteLine(“In derivedClass with msg:”+msg);
class testClass {
baseClass bObj = new baseClass(100);
Console.WriteLine(“Member value of bObj is:” + bObj.member);
derivedClass dObj = new derivedClass(“Hello”);
Console.WriteLine(“Member value of dObj is:” + dObj.member);
dObj.displayMsg(“Good Day!”);
Output of this code will be:
Member value of bObj is: 100
In baseClass..
Member value of dObj is: Hello
In derivedClass with msg: Good Day!

In this example, derived class has completely shadowed member and displayMsg of baseClass. Note that the signature of member and displayMsg is different in derivedClass when compared to baseClass. This is acceptable in case of shadowing. But in overriding, it will throw error.

Shadowing in VB.NET:
In VB.NET, shadowing is done with the help of shadows keyword as demonstrated in the example shown below:
Public Class baseClass
Public member As Integer = 100
End Class

Public Class derivedClass
Inherits baseClass
Public Shadows member As String = “Hello”
End Class

Module testModule
Sub Main()
Dim bObj as New baseClass
Dim dObj as New derivedClass
End Sub
End Module

The MessageBox will display 100 first and then Hello.

Invoking Base Class Member Using Derived Class Object:
So far in the discussion, it was told that the baseClass member is completely hidden in derivedClass. Does that mean you cannot access the baseClass member in derivedClass? No. You still have an option using references. Given below is the modified code of testClass containing code to access baseClass member using derivedClass object:
Module testModule
Sub Main()
Dim dObj As New derivedClass
Dim refBaseClass As baseClass = dObj
End Sub
End Module

Now to your surprise, you will get the output as 100. This is because you create a reference to baseClass which is assigned with derivedClass object. Whatever is the assigned object, the reference takes priority and the baseClass member gets displayed. In this case, shadowing is suppressed.

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