Know Dot Net Article

Sort ArrayList Of Objects

Use IComparable

by Les Smith

I can sort an ArrayList of Strings.  How can I sort an ArrayList of Objects?  Can it be done?  Yes, you can do it with IComparable.  I know some are so enthused about Generics, they have made statements like, "I will never use an ArrayList again."  But ArrayLists are still very powerful and have their place.  This application will show you why.

This week I am working on an application that has to sort a relatively small number of Input Lines (300 or so) on Social Security Number because there can be multiple lines of input for the same person.  The application makes a call to a Web Service for each person but should only make one call per person.  The processing of the application requires that I build objects prior to beginning the processing.  Since the input file can have multiple lines for the same person, the objects must be sorted.

Even if the first Property of the data class is the SSN, you cannot just place the objects in an ArrayList and then sort the ArrayList.  Your class must Implement IComparable and you must creat an IComparable.CompareTo function in the class.  This article will show you the code for a Console Application in both C# and VB.NET to accomplish the sorting.

Figure 1 - C# Sorting ArrayList of Objects.

using System;
using System.Collections;
namespace SortArrayListOfObjects
{
   class Class1
   {
      static ArrayList al = new ArrayList();
      [STAThread]
      static void Main(string[] args)
      {
         BuildArrayList();
         ListSortedObjects(); //list data before sorting
         al.Sort();
         ListSortedObjects();
         Console.ReadLine();
      }

      static private void BuildArrayList()
      {
         MyObject obj1 = new MyObject();
         obj1.EmpID = 1;
         obj1.SSN = "234567890";
         obj1.Name = "Les Smith";
         al.Add(obj1);
         MyObject obj2 = new MyObject();
         obj2.EmpID = 2;
         obj2.SSN = "987654321";
         obj2.Name = "Bob Jones";
         al.Add(obj2);
         MyObject obj3 = new MyObject();
         obj3.EmpID = 3;
         obj3.SSN = "123456789";
         obj3.Name = "Jim Jones";
         al.Add(obj3);
      }

      static private void ListSortedObjects()
      {
         foreach (MyObject obj in al)
        {
            Console.WriteLine(obj.SSN);
            Console.WriteLine(obj.EmpID.ToString());
            Console.WriteLine(obj.Name);
         }
      }
   }

   public class MyObject : IComparable
   {
      public string SSN;
      public int EmpID;
      public string Name;

      public int CompareTo(object obj)
      {
         MyObject Compare = (MyObject)obj;
         int result = this.SSN.CompareTo(Compare.SSN);
         if (result == 0)
            result = this.SSN.CompareTo(Compare.SSN);
         return result;
      }
   }
}

The application is stright-forward and does not need much explanation.  Since the ArrayList does not have the capability to sort objects as it would strings, I had to Implement an IComparable.CompareTo function to facilitate the sort.  Obviously, you can expand or modify the CompareTo function to sort on multiple properties of different types.

Figure 2 - VB.NET Sorting ArrayList of Objects.

Imports System
Imports System.Collections
Module Module1

   Private al As New ArrayList
   Sub Main()
      BuildArrayList()
      ListSortedObjects()
      al.Sort()
      ListSortedObjects()
      Console.ReadLine()
   End Sub

   Private Sub BuildArrayList()
      Dim obj1 As MyObject = New MyObject
      obj1.EmpID = 1
      obj1.SSN = "234567890"
      obj1.Name = "Les Smith"
      al.Add(obj1)
      Dim obj2 As MyObject = New MyObject
      obj2.EmpID = 2
      obj2.SSN = "987654321"
      obj2.Name = "Bob Jones"
      al.Add(obj2)
      Dim obj3 As MyObject = New MyObject
      obj3.EmpID = 3
      obj3.SSN = "123456789"
      obj3.Name = "Jim Jones"
      al.Add(obj3)
   End Sub

   Private Sub ListSortedObjects()
      Dim obj As MyObject
      For Each obj In al
         Console.WriteLine(obj.SSN)
         Console.WriteLine(obj.EmpID.ToString())
         Console.WriteLine(obj.Name)
      Next
   End Sub

End Module
Public Class MyObject
   Implements IComparable
   Public SSN As String
   Public EmpID As Integer
   Public Name As String

   Public Function CompareTo(ByVal obj As Object) As Integer _
      Implements System.IComparable.CompareTo
      If Not TypeOf obj Is MyObject Then
         Throw New Exception("Object is not MyObject")
      End If
      Dim Compare As MyObject = CType(obj, MyObject)
      Dim result As Integer = Me.SSN.CompareTo(Compare.SSN)

      If result = 0 Then
         result = Me.SSN.CompareTo(Compare.SSN)
      End If
      Return result
   End Function
End Class

The code for C# and VB.NET is almost identical except for the differing of the syntax of the two languages.

The following lines show the contents of the ArrayList before it is sorted.

   234567890
   1
   Les Smith
   987654321
   2
   Bob Jones
   123456789
   3
   Jim Jones

The following output shows the contents of the ArrayList after it is sorted.

   123456789
   3
   Jim Jones
   234567890
   1
   Les Smith
   987654321
   2
   Bob Jones

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