C# Generics: Collection Interfaces
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Implementing IComparable

Like all collections, the List implements the Sort( ) method, which allows you to sort any objects that implement IComparable. In the next example, you'll modify the Employee object to implement IComparable:

public class Employee : IComparable<Employee>

To implement the IComparable<Employee> interface, the Employee object must provide a CompareTo() method:

public int CompareTo(Employee rhs)
{
   return this.empID.CompareTo(r.empID);
}

The CompareTo( ) method takes an Employee as a parameter. We know this is an Employee because this is a type-safe collection. The current Employee object must compare itself to the Employee passed in as a parameter and return -1 if it is smaller than the parameter, 1 if it is greater than the parameter, and 0 if it is equal to the parameter. It is up to Employee to determine what smallerthan, greaterthan, and equalto mean. In this example, you delegate the comparison to the empId member. The empId member is an int and uses the default CompareTo( ) method for integer types, which will do an integer comparison of the two values.

TIP: The System.Int32 class implements IComparable<Int32>, so you may delegate the comparison responsibility to integers.

You are now ready to sort the array list of employees, empList. To see if the sort is working, you'll need to add integers and Employee instances to their respective arrays with random values. To create the random values, you'll instantiate an object of class Random; to generate the random values, you'll call the Next( ) method on the Random object, which returns a pseudorandom number. The Next( ) method is overloaded; one version allows you to pass in an integer that represents the largest random number you want. In this case, you'll pass in the value 10 to generate a random number between 0 and 10:

Random r = new Random();
r.Next(10);

Example 9-14 creates an integer array and an Employee array, populates them both with random numbers, and prints their values. It then sorts both arrays and prints the new values.

Example 9-14. Sorting an integer and an employee array

#region Using directives

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

#endregion

namespace IComparable
{
   // a simple class to store in the array
   public class Employee : IComparable<Employee>

   {
      private int empID;

      public Employee( int empID )
      {
         this.empID = empID;
      }

      public override string ToString( )
      {
         return empID.ToString( );
      }

      public bool Equals( Employee other )
      {
         if ( this.empID == other.empID )
         {
            return true;
         }
         else
         {
            return false;
         }
      }

      // Comparer delegates back to Employee
      // Employee uses the integer's default
      // CompareTo method

      public int CompareTo( Employee rhs )
      {
         return this.empID.CompareTo( rhs.empID );
      }
   }
   public class Tester
   {
      static void Main( )
      {
         List<Employee> empArray = new List<Employee>( );
         List<Int32> intArray = new List<Int32>( );

         // generate random numbers for 
         // both the integers and the
         // employee id's

         Random r = new Random( );

         // populate the array
         for ( int i = 0; i < 5; i++ )
         {
            // add a random employee id
            empArray.Add( new Employee( r.Next( 10 ) + 100 ) );

            // add a random integer
            intArray.Add( r.Next( 10 ) );
         }

         // display all the contents of the int array
         for ( int i = 0; i < intArray.Count; i++ )
         {
            Console.Write( "{0} ", intArray[i].ToString( ) );
         }
         Console.WriteLine( "\n" );

         // display all the contents of the Employee array
         for ( int i = 0; i < empArray.Count; i++ )
         {
            Console.Write( "{0} ", empArray[i].ToString( ) );
         }
         Console.WriteLine( "\n" );

         // sort and display the int array
         intArray.Sort( );
         for ( int i = 0; i < intArray.Count; i++ )
         {
            Console.Write( "{0} ", intArray[i].ToString( ) );
         }
         Console.WriteLine( "\n" );

         // sort and display the employee array
         Employee.EmployeeComparer c = Employee.GetComparer( );
         empArray.Sort(c);

         empArray.Sort( );

         // display all the contents of the Employee array
         for ( int i = 0; i < empArray.Count; i++ )
         {
            Console.Write( "{0} ", empArray[i].ToString( ) );
         }
         Console.WriteLine( "\n" );
      }
   }
}

Output:
4 5 6 5 7 
108 100 101 103 103 
4 5 5 6 7 
100 101 103 103 108

The output shows that the integer array and Employee array were generated with random numbers. When sorted, the display shows the values have been ordered properly.

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