\\Dev's Place

Jun 27, 2008

Polymorphism, Method Hiding and Overriding in C#

Polymorphism

One of the fundamental concepts of object oriented software development is polymorphism. The term polymorphism (from the Greek meaning "having multiple forms") in OO is the characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow a variable to refer to more than one type of object.

C# Example

Let's assume the following simple classes A and B for the discussions in this text. A is the base class, B is derived from A.

Inherited Methods

A method Foo() which is declared in the base class A and not redeclared in classes B or C is inherited in the two subclasses

using System;
namespace Polymorphism
{
class A
{
public void Foo() { Console.WriteLine("A::Foo()"); }
}

class B : A {}

class Test
{
static void Main(string[] args)
{
A a = new A();
a.Foo(); // output --> "A::Foo()"

B b = new B();
b.Foo(); // output --> "A::Foo()"
}
}
}

The method Foo() can be overridden in classes B and C:

using System;
namespace Polymorphism
{
class A
{
public void Foo() { Console.WriteLine("A::Foo()"); }
}

class B : A
{
public void Foo() { Console.WriteLine("B::Foo()"); }
}

class Test
{
static void Main(string[] args)
{
A a;
B b;

a = new A();
b = new B();
a.Foo(); // output --> "A::Foo()"
b.Foo(); // output --> "B::Foo()"

a = new B();
a.Foo(); // output --> "A::Foo()"
}
}
}

There are two problems with this code.

The output is not really what we, expected. The method Foo() is a non-virtual method. C# requires the use of the keyword virtual in order for a method to actually be virtual.

Although the code compiles and runs, the compiler produces a warning:
...\polymorphism.cs(11,15): warning CS0108: The keyword new is required on 'Polymorphism.B.Foo()' because it hides inherited member 'Polymorphism.A.Foo()'

Virtual and Overridden Methods

Only if a method is declared virtual, derived classes can override this method if they are explicitly declared to override the virtual base class method with the override keyword.

using System;
namespace Polymorphism
{
class A
{
public virtual void Foo() { Console.WriteLine("A::Foo()"); }
}

class B : A
{
public override void Foo() { Console.WriteLine("B::Foo()"); }
}

class Test
{
static void Main(string[] args)
{
A a;
B b;

a = new A();
b = new B();
a.Foo(); // output --> "A::Foo()"
b.Foo(); // output --> "B::Foo()"

a = new B();
a.Foo(); // output --> "B::Foo()"
}
}
}

Method Hiding

Why did the compiler in the second listing generate a warning? Because C# not only supports method overriding, but also method hiding. Simply put, if a method is not overriding the derived method, it is hiding it. A hiding method has to be declared using the new keyword. The correct class definition in the second listing is thus:

using System;
namespace Polymorphism
{
class A
{
public void Foo() { Console.WriteLine("A::Foo()"); }
}

class B : A
{
public new void Foo() { Console.WriteLine("B::Foo()"); }
}

class Test
{
static void Main(string[] args)
{
A a;
B b;

a = new A();
b = new B();
a.Foo(); // output --> "A::Foo()"
b.Foo(); // output --> "B::Foo()"

a = new B();
a.Foo(); // output --> "A::Foo()"
}
}
}

Combining Method Overriding and Hiding

Methods of a derived class can both be virtual and at the same time hide the derived method. In order to declare such a method, both keywords virtual and new have to be used in the method declaration:

class A
{
public void Foo() {}
}

class B : A
{
public virtual new void Foo() {}
}

A class C can now declare a method Foo() that either overrides or hides Foo() from class B:

class C : B
{
public override void Foo() {}
// or
public new void Foo() {}
}

Important things to remember -->

1. If you are using same name in derived class it hides the base class method, and generate a compiler warning. Use new keyword to supress this warning and tells the compiler that ok i am hiding it intentional.
2. Else define the base as virtual and write override in derived class method.
3. If you dont sure whether you want to hide or override use both virtual and new in base class method definition.

Jun 26, 2008

Creating a DataTable | DataTable in C#

Creating a DataTable | DataTable in C#

DataTable aTable = new DataTable();
aTable.Columns.Add("ProductID", typeof(int));
aTable.Columns.Add("ProductName", typeof(string));
DataRow dr;
dr = aTable.NewRow();
dr[0] = 12;
dr[1] = "Lap Top";
aTable.Rows.Add(dr);
dr = aTable.NewRow();
dr[0] = 13;
dr[1] = "DeskTop";
aTable.Rows.Add(dr);
dr = aTable.NewRow();
dr[0] = 14;
dr[1] = "Server";
aTable.Rows.Add(dr);
dr = aTable.NewRow();
dr[0] = 15;
dr[1] = "Mac PC";
aTable.Rows.Add(dr);

You can create these rows by getting data from database also

command=new SqlCommand("select * from Product",connection);
connection.Open();
SqlDataReader reader = command.ExecuteReader();
while(reader.Read())
{
Dr=aTable.NewRow();
Dr[0] = reader.GetValue(1);
Dr[1] = reader.GetValue(2);
aTable.Rows.Add(Dr);
}

Jun 22, 2008

Haj committee India | Haj 2008 Lucknow

All about Haj, please visit http://www.hajcommittee.com

This year UP haj pilgrims will be decided on lottery draw due to more responces of limited quota assigned to UP. The lucky draw was decided to held on 21st June 08, and the final list of pilgrims will be posted on haj committee site.

Lets wait and see Whom Allah give a chance to perform Haj this year.
Finally wait is over you can get the results here

Jun 10, 2008

AMU 2008 Test courses results

AMU 2008 Test Results

B. Arch.
B.A.L.L.B.
B.C.A./B.I.T./B.C.M.
B.Ed.
B.E. Evening
B.Lib.
B. Sc. (Hons.) Indus. Chemistry
B.Tech.
C.E.T.
D.C.P.
Dip. in Gen. Nursing
L.L.M .
M.B.A./M.B.A.(IB)
M.B.B.S. / B.D.S.
M.C.A.
MD/MS/PG Dip.
M.Ed.
M.Lib.
M.A Mass Comm.
M.S.W. (Final)
XI/Dip. Engg.

All the above courses results can be found at this link.

May 31, 2008

Learn ASP.NET

Here are the links which helps you to get started learning ASP.NET

1. Learn ASP.NET
2. Free videos to understand you better Free ASP.NET Videos
3. ASP.NET FAQs Get It
4. ASP.NET
5. For any technical queries join to this forum

Note: If you know some more resources for learning ASP.NET please add a comment.

May 30, 2008

UP Board Xth Results declared

UP Board Xth Results declared on 30th May 08.
Click to Get it

Best of Luck guys !!!