4.4. Create a Comparable Object

Problem

You need to provide a mechanism that allows two custom objects to be compared.

Solution

Determine what data you want to use for the basis of your comparison, and implement the IComparable interface.

Discussion

The IComparable interface defines a single CompareTo method that accepts an object for comparison and returns an integer. The integer can take one of the following three values:

• Zero, which means the objects are equal.

• Less than zero (typically - 1), which means that the current object is less than the object that was supplied as a parameter.

• Greater than zero (typically 1), which means that the current object is greater than the object that was supplied as a parameter.

You can evaluate the object contents on your own and decide which value to return. However, you can often use the CompareTo method of one of the contained data types to make the comparison. For example, in the code that follows, the Person object performs an alphabetical comparison (based on the last name, then the first name) on two Person instances using the String.CompareTo implementation.

Public Class Person
    Implements IComparable

    Private _FirstName As String
    Private _LastName As String
    
    ' (Property procedure and constructor code omitted.)

    Public Function CompareTo(ByVal obj As Object) As Integer _
      Implements System.IComparable.CompareTo

        If Not TypeOf obj Is Person Then
            Throw New ArgumentException("Object is not a Person")
        End If

        Dim Compare As Person = CType(obj, Person)

        ' Compare last names
        Dim result As Integer = Me.LastName.CompareTo(Compare.LastName)

        ' If last names are equal, compare first names
        If result = 0 Then
            result = Me.FirstName.CompareTo(Compare.FirstName)
        End If

        Return result
    End Function

End Class

Here’s a simple test of a comparable object:

Dim PersonA As New Person("Andrew", "Sempf")
Dim PersonB As New Person("Andrew", "Sempf")

If PersonA Is PersonB Then
    ' This never happens.
    Console.WriteLine("These Person objects point to the same data " & _
      "in memory.")
End If

If PersonA.CompareTo(PersonB) = 0 Then
    ' This always happens.
    Console.WriteLine("These Person objects represent the same person.")
End If

Once you implement CompareTo, you can sort arrays or ArrayList objects that contain your object, as described in recipe 3.8. If you need to create an

object that can be sorted in several different ways, you will need to create separate IComparer instances, as described in recipe 3.9.