C# - Interfaces

An interface is defined as a syntactical contract that all the classes inheriting the interface should follow. The interface defines the 'what' part of the syntactical contract and the deriving classes define the 'how' part of the syntactical contract.

Interfaces define properties, methods and events, which are the members of the interface. Interfaces contain only the declaration of the members. It is the responsibility of the deriving class to define the members. It often helps in providing a standard structure that the deriving classes would follow.

Abstract classes to some extent serve the same purpose, however, they are mostly used when only few methods are to be declared by the base class and the deriving class implements the functionalities.

Declaring Interfaces

Interfaces are declared using the interface keyword. It is similar to class declaration. Interface statements are public by default. Following is an example of an interface declaration:

public interface ITransactions
{
   // interface members
   void showTransaction();
   double getAmount();
}

Example

The following example demonstrates implementation of the above interface:

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace InterfaceApplication
{

   public interface ITransactions
   {
      // interface members
      void showTransaction();
      double getAmount();
   }
   public class Transaction : ITransactions
   {
      private string tCode;
      private string date;
      private double amount;
      public Transaction()
      {
         tCode = " ";
         date = " ";
         amount = 0.0;
      }
      public Transaction(string c, string d, double a)
      {
         tCode = c;
         date = d;
         amount = a;
      }
      public double getAmount()
      {
         return amount;
      }
      public void showTransaction()
      {
         Console.WriteLine("Transaction: {0}", tCode);
         Console.WriteLine("Date: {0}", date);
         Console.WriteLine("Amount: {0}", getAmount());

      }

   }
   class Tester
   {
      static void Main(string[] args)
      {
         Transaction t1 = new Transaction("001", "8/10/2012", 78900.00);
         Transaction t2 = new Transaction("002", "9/10/2012", 451900.00);
         t1.showTransaction();
         t2.showTransaction();
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result:

Transaction: 001
Date: 8/10/2012
Amount: 78900
Transaction: 002
Date: 9/10/2012
Amount: 451900

C# - Namespaces

A namespace is designed for providing a way to keep one set of names separate from another. The class names declared in one namespace will not conflict with the same class names declared in another.

Defining a Namespace

A namespace definition begins with the keyword namespace followed by the namespace name as follows:

namespace namespace_name
{
   // code declarations
}

To call the namespace-enabled version of either function or variable, prepend the namespace name as follows:

namespace_name.item_name;

The following program demonstrates use of namespaces:

using System;
namespace first_space
{
   class namespace_cl
   {
      public void func()
      {
         Console.WriteLine("Inside first_space");
      }
   }
}
namespace second_space
{
   class namespace_cl
   {
      public void func()
      {
         Console.WriteLine("Inside second_space");
      }
   }
}   
class TestClass
{
   static void Main(string[] args)
   {
      first_space.namespace_cl fc = new first_space.namespace_cl();
      second_space.namespace_cl sc = new second_space.namespace_cl();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}

When the above code is compiled and executed, it produces the following result:

Inside first_space
Inside second_space

The using Keyword

The using keyword states that the program is using the names in the given namespace. For example, we are using the System namespace in our programs. The class Console is defined there. We just write:

Console.WriteLine ("Hello there");

We could have written the fully qualified name as:

System.Console.WriteLine("Hello there");

You can also avoid prepending of namespaces with the using namespace directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace. The namespace is thus implied for the following code:

Let us rewrite our preceding example, with using directive:

using System;
using first_space;
using second_space;

namespace first_space
{
   class abc
   {
      public void func()
      {
         Console.WriteLine("Inside first_space");
      }
   }
}
namespace second_space
{
   class efg
   {
      public void func()
      {
         Console.WriteLine("Inside second_space");
      }
   }
}   
class TestClass
{
   static void Main(string[] args)
   {
      abc fc = new abc();
      efg sc = new efg();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}

When the above code is compiled and executed, it produces the following result:

Inside first_space
Inside second_space

Nested Namespaces

Namespaces can be nested where you can define one namespace inside another namespace as follows:

namespace namespace_name1 
{
   // code declarations
   namespace namespace_name2 
   {
     // code declarations
   }
}

You can access members of nested namespace by using the dot (.) operator as follows:

using System;
using first_space;
using first_space.second_space;

namespace first_space
{
   class abc
   {
      public void func()
      {
         Console.WriteLine("Inside first_space");
      }
   }
   namespace second_space
   {
      class efg
      {
         public void func()
         {
            Console.WriteLine("Inside second_space");
         }
      }
   }   
}
 
class TestClass
{
   static void Main(string[] args)
   {
      abc fc = new abc();
      efg sc = new efg();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}

When the above code is compiled and executed, it produces the following result:

Inside first_space
Inside second_space

C# - Operator Overloading

You can redefine or overload most of the built-in operators available in C#. Thus a programmer can use operators with user-defined types as well. Overloaded operators are functions with special names the keyword operator followed by the symbol for the operator being defined. Like any other function, an overloaded operator has a return type and a parameter list.

For example, look at the following function:

public static Box operator+ (Box b, Box c)
{
   Box box = new Box();
   box.length = b.length + c.length;
   box.breadth = b.breadth + c.breadth;
   box.height = b.height + c.height;
   return box;
}

The above function implements the addition operator (+) for a user-defined class Box. It adds the attributes of two Box objects and returns the resultant Box object.

Implementation of Operator Overloading

The following program shows the complete implementation:

using System;

namespace OperatorOvlApplication
{
   class Box
   {
      private double length;      // Length of a box
      private double breadth;     // Breadth of a box
      private double height;      // Height of a box

      public double getVolume()
      {
         return length * breadth * height;
      }
      public void setLength( double len )
      {
         length = len;
      }

      public void setBreadth( double bre )
      {
         breadth = bre;
      }

      public void setHeight( double hei )
      {
         height = hei;
      }
      // Overload + operator to add two Box objects.
      public static Box operator+ (Box b, Box c)
      {
         Box box = new Box();
         box.length = b.length + c.length;
         box.breadth = b.breadth + c.breadth;
         box.height = b.height + c.height;
         return box;
      }

   }

   class Tester
   {
      static void Main(string[] args)
      {
         Box Box1 = new Box();         // Declare Box1 of type Box
         Box Box2 = new Box();         // Declare Box2 of type Box
         Box Box3 = new Box();         // Declare Box3 of type Box
         double volume = 0.0;          // Store the volume of a box here

         // box 1 specification
         Box1.setLength(6.0);
         Box1.setBreadth(7.0);
         Box1.setHeight(5.0);

         // box 2 specification
         Box2.setLength(12.0);
         Box2.setBreadth(13.0);
         Box2.setHeight(10.0);

         // volume of box 1
         volume = Box1.getVolume();
         Console.WriteLine("Volume of Box1 : {0}", volume);

         // volume of box 2
         volume = Box2.getVolume();
         Console.WriteLine("Volume of Box2 : {0}", volume);

         // Add two object as follows:
         Box3 = Box1 + Box2;

         // volume of box 3
         volume = Box3.getVolume();
         Console.WriteLine("Volume of Box3 : {0}", volume);
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result:

Volume of Box1 : 210
Volume of Box2 : 1560
Volume of Box3 : 5400

Overloadable and Non-Overloadable Operators

The following table describes the overload ability of the operators in C#:

Operators	Description
+, -, !, ~, ++, --	These unary operators take one operand and can be overloaded.
+, -, *, /, %	These binary operators take one operand and can be overloaded.
==, !=, <, >, <=, >=	The comparison operators can be overloaded
&&, ||	The conditional logical operators cannot be overloaded directly.
+=, -=, *=, /=, %=	The assignment operators cannot be overloaded.
=, ., ?:, ->, new, is, sizeof, typeof	These operators cannot be overloaded.

Example:

In the light of the above discussions, let us extend the preceding example, and overload few more operators:

using System;

namespace OperatorOvlApplication
{
    class Box
    {
       private double length;      // Length of a box
       private double breadth;     // Breadth of a box
       private double height;      // Height of a box
      
       public double getVolume()
       {
         return length * breadth * height;
       }
      public void setLength( double len )
      {
          length = len;
      }

      public void setBreadth( double bre )
      {
          breadth = bre;
      }

      public void setHeight( double hei )
      {
          height = hei;
      }
      // Overload + operator to add two Box objects.
      public static Box operator+ (Box b, Box c)
      {
          Box box = new Box();
          box.length = b.length + c.length;
          box.breadth = b.breadth + c.breadth;
          box.height = b.height + c.height;
          return box;
      }
      
      public static bool operator == (Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length == rhs.length && lhs.height == rhs.height 
             && lhs.breadth == rhs.breadth)
          {
              status = true;
          }
          return status;
      }
      public static bool operator !=(Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length != rhs.length || lhs.height != rhs.height 
              || lhs.breadth != rhs.breadth)
          {
              status = true;
          }
          return status;
      }
      public static bool operator <(Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length < rhs.length && lhs.height 
              < rhs.height && lhs.breadth < rhs.breadth)
          {
              status = true;
          }
          return status;
      }

      public static bool operator >(Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length > rhs.length && lhs.height 
              > rhs.height && lhs.breadth > rhs.breadth)
          {
              status = true;
          }
          return status;
      }

      public static bool operator <=(Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length <= rhs.length && lhs.height 
              <= rhs.height && lhs.breadth <= rhs.breadth)
          {
              status = true;
          }
          return status;
      }

      public static bool operator >=(Box lhs, Box rhs)
      {
          bool status = false;
          if (lhs.length >= rhs.length && lhs.height 
             >= rhs.height && lhs.breadth >= rhs.breadth)
          {
              status = true;
          }
          return status;
      }
      public override string ToString()
      {
          return String.Format("({0}, {1}, {2})", length, breadth, height);
      }
   
   }
    
   class Tester
   {
      static void Main(string[] args)
      {
        Box Box1 = new Box();          // Declare Box1 of type Box
        Box Box2 = new Box();          // Declare Box2 of type Box
        Box Box3 = new Box();          // Declare Box3 of type Box
        Box Box4 = new Box();
        double volume = 0.0;   // Store the volume of a box here

        // box 1 specification
        Box1.setLength(6.0);
        Box1.setBreadth(7.0);
        Box1.setHeight(5.0);

        // box 2 specification
        Box2.setLength(12.0);
        Box2.setBreadth(13.0);
        Box2.setHeight(10.0);

       //displaying the Boxes using the overloaded ToString():
        Console.WriteLine("Box 1: {0}", Box1.ToString());
        Console.WriteLine("Box 2: {0}", Box2.ToString());
        
        // volume of box 1
        volume = Box1.getVolume();
        Console.WriteLine("Volume of Box1 : {0}", volume);

        // volume of box 2
        volume = Box2.getVolume();
        Console.WriteLine("Volume of Box2 : {0}", volume);

        // Add two object as follows:
        Box3 = Box1 + Box2;
        Console.WriteLine("Box 3: {0}", Box3.ToString());
        // volume of box 3
        volume = Box3.getVolume();
        Console.WriteLine("Volume of Box3 : {0}", volume);

        //comparing the boxes
        if (Box1 > Box2)
          Console.WriteLine("Box1 is greater than Box2");
        else
          Console.WriteLine("Box1 is  greater than Box2");
        if (Box1 < Box2)
          Console.WriteLine("Box1 is less than Box2");
        else
          Console.WriteLine("Box1 is not less than Box2");
        if (Box1 >= Box2)
          Console.WriteLine("Box1 is greater or equal to Box2");
        else
          Console.WriteLine("Box1 is not greater or equal to Box2");
        if (Box1 <= Box2)
          Console.WriteLine("Box1 is less or equal to Box2");
        else
          Console.WriteLine("Box1 is not less or equal to Box2");
        if (Box1 != Box2)
          Console.WriteLine("Box1 is not equal to Box2");
        else
          Console.WriteLine("Box1 is not greater or equal to Box2");
        Box4 = Box3;
        if (Box3 == Box4)
          Console.WriteLine("Box3 is equal to Box4");
        else
          Console.WriteLine("Box3 is not equal to Box4");

        Console.ReadKey();
      }
    }
}

When the above code is compiled and executed, it produces the following result:

Box 1: (6, 7, 5)
Box 2: (12, 13, 10)
Volume of Box1 : 210
Volume of Box2 : 1560
Box 3: (18, 20, 15)
Volume of Box3 : 5400
Box1 is not greater than Box2
Box1 is less than Box2
Box1 is not greater or equal to Box2
Box1 is less or equal to Box2
Box1 is not equal to Box2
Box3 is equal to Box4