C# 0 The Complete Reference

Using a Non-Generic IComparer

The non-generic 

IComparer

 defines only one method, 

Compare( )

, which is shown here:

int Compare(object 

obj1

, object 

obj2

)

Compare( )

 compares 

obj1

 to 

obj2

. To sort in ascending order, your implementation must 

return zero if the objects are equal, a positive value if 

obj1

 is greater than 

obj2,

 and a negative 

value if 

obj1

 is less than 

obj2.

 You can sort in descending order by reversing the outcome of 

the comparison. The method can throw an 

ArgumentException

 if the type of 

obj

 is not 

compatible for comparison with the invoking object.

An

IComparer

 can be specified when constructing a 

SortedList

, when calling 

ArrayList.Sort(IComparer)

, and at various other places throughout the collection classes. 

The main advantage of using 

IComparer

 is that you can sort objects of classes that do not 

implement

IComparable

.

The following program reworks the non-generic inventory program so that it uses an 

IComparer

 to sort the inventory list. It first creates a class called 

CompInv

 that implements 

IComparer

 and compares two 

Inventory

 objects. An object of this class is then used in a call 

to

Sort( )

 to sort the inventory list.

// Use IComparer.

using System;

using System.Collections;

// Create an IComparer for Inventory objects.

class CompInv : IComparer {

  // Implement the IComparer interface.

  public int Compare(object obj1, object obj2) {

    Inventory a, b;

    a = (Inventory) obj1;

    b = (Inventory) obj2;

    return a.name.CompareTo(b.name);

  }

}

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class Inventory {

  public string name;

  double cost;

  int onhand;

  public Inventory(string n, double c, int h) {

    name = n;

    cost = c;

    onhand = h;

  }

  public override string ToString() {

    return

      String.Format("{0,-10}Cost: {1,6:C}  On hand: {2}",

                    name, cost, onhand);

  }

}

class IComparerDemo {

  static void Main() {

    CompInv comp = new CompInv();

    ArrayList inv = new ArrayList();

    // Add elements to the list.

    inv.Add(new Inventory("Pliers", 5.95, 3));

    inv.Add(new Inventory("Wrenches", 8.29, 2));

    inv.Add(new Inventory("Hammers", 3.50, 4));

    inv.Add(new Inventory("Drills", 19.88, 8));

    Console.WriteLine("Inventory list before sorting:");

    foreach(Inventory i in inv) {

      Console.WriteLine("   " + i);

    }

    Console.WriteLine();

    // Sort the list using an IComparer.

    inv.Sort(comp);

    Console.WriteLine("Inventory list after sorting:");

    foreach(Inventory i in inv) {

      Console.WriteLine("   " + i);

    }

  }

}

The output is the same as the previous version of the program.

Using a Generic IComparer

The

IComparer

 interface is the generic version of 

IComparer

. It defines the generic 

version of 

Compare( )

, shown here:

int Compare(T 

obj1

, T 

obj2

)

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