Sorting a list / Implementation of IComparable

Classes that implement IComparable have a CompareTo method.  If you take a look at an integer or string or many other objects you'll see they have this method.  All the CompareTo method does is accepts another object and returns a value indicating the current objects relative position with the object passed.  The return value would be 1, 0, -1 for "greater then", "equal to", "less then".  It's this method that allows any object to be sorted becaues all you need to know in order to sort a list of objects is how one of these objects compares to another.

In order to use the interface just type "implements IComparable(of cl_country)" just under your class declration and then hit enter.  When you hit enter the appropriate method declarations for the interface will be created for you.  In this case there is only one method 'CompareTo'.  It's your responsibilty to add the code into this method that compares the current object with the object passed and return a relative value.

An example may help.

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ' Create three people objects
        Dim obj1 As New Person("FirstPerson", 29)
        Dim obj2 As New Person("SecondPerson", 47)
        Dim obj3 As New Person("ThirdPerson", 10)

        ' Create a list of people and add the objects I've created
        Dim people As New List(Of Person)
        people.AddRange(New Person() {obj1, obj2, obj3})

        ' Sort the list
        people.Sort()

        ' Display each of the people in the list showing they've been
        ' sorted is Ascending order by age
        For Each p As Person In people
            Console.WriteLine("{0} is {1} years old", p.Name, p.Age)
        Next
    End Sub
End Class

' A person with a name and age
' This class uses IComparable so it can be sorted
Public Class Person
    Implements IComparable(Of Person)

    Public Age As Integer       ' Persons age
    Public Name As String       ' Persons name


    ' Constructor
    Public Sub New(ByVal name As String, ByVal age As Integer)
        Me.Age = age
        Me.Name = name
    End Sub

    ' CompareTo method is part of the IComparable interface
    ' This method will return the relative value of this object with the passed
    ' object.  1 = Greater then   0 = Equal To   -1 = Less then
    ' I'll be sorting on age so this method will compare the age fields
    Public Function CompareTo(ByVal other As Person) As Integer Implements System.IComparable(Of Person).CompareTo
        ' I've actually used more code here then I need to drive the
        ' example home.  You can clearly see how I've done the comparison
        ' with this if statement.
        If Me.Age > other.Age Then
            Return 1
        ElseIf Me.Age = other.Age Then
            Return 0
        Else
            Return -1
        End If

        ' In this case I really could have used the integers CompareTo method
        ' and simply used the next commented line of code instead of
        ' the if statement above.

        'Return Me.Age.CompareTo(other.Age)

        ' The if statement above will cause my peson objects to be sorted in Ascending order
        ' However this could easily be changed with the following commented code.
        ' Notice how I'm now passing a -1 when this object's age is greater then the passed object's age
        ' and a 1 when this object is less then the other object.  By reversing the return value I'll reverse
        ' the sort order

        'If Me.Age > other.Age Then
        '    Return -1
        'ElseIf Me.Age = other.Age Then
        '    Return 0
        'Else
        '    Return 1
        'End If
    End Function
End Class