Textbox and Class - vb.net

Question

Have trouble connecting exception to textbox input - Program should only accept "IL", "MO" or "WI" - But it's accepting every input. Everything I tried I'm hitting a brick wall - I know the solution is there, I just need some help. Thanks
Form Codes
Public Class Form1
Dim packages(4) As Packages
Dim pkg As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
packages(0) = New Packages("Z111", "Eau Claire", "WI")
packages(1) = New Packages("Z121", "Vicki Vale", "IL")
packages(2) = New Packages("Z131", "Sammy Davis", "MO")
packages(3) = New Packages("Z141", "Jon Smith", "IL")
packages(4) = New Packages("Z151", "Suzie Cassidy", "WI")
DisplayInfo()
End Sub
Sub DisplayInfo()
txtID.Text = packages(pkg).PackageID
txtCity.Text = packages(pkg).DestinationCity
txtState.Text = packages(pkg).DestinationState
lblPackageCount.Text = "Package # " & pkg + 1
End Sub
Private Sub btnFirst_Click(sender As Object, e As EventArgs) Handles btnFirst.Click
pkg = 0
DisplayInfo()
End Sub
Private Sub btnPrevious_Click(sender As Object, e As EventArgs) Handles btnPrevious.Click
If pkg > 0 Then
pkg -= 1
DisplayInfo()
End If
End Sub
Private Sub btnNext_Click(sender As Object, e As EventArgs) Handles btnNext.Click
If pkg < packages.Length - 1 Then
pkg += 1
DisplayInfo()
End If
End Sub
Private Sub btnLast_Click(sender As Object, e As EventArgs) Handles btnLast.Click
pkg = packages.Length - 1
DisplayInfo()
End Sub
Private Sub btnSaveChanges_Click(sender As Object, e As EventArgs) Handles btnSaveChanges.Click
Try
packages(pkg).PackageID = txtID.Text
packages(pkg).DestinationCity = txtCity.Text
packages(pkg).DestinationState = txtState.Text
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
End Class
Class Codes
Public Class Packages
Private packID As String
Private desCity As String
Private desState As String
Sub New(id As String, city As String, state As String)
packID = id
desCity = city
desState = state
End Sub
Public Property PackageID As String
Get
Return packID
End Get
Set(value As String)
packID = value
End Set
End Property
Public Property DestinationCity As String
Get
Return desCity
End Get
Set(value As String)
desCity = value
End Set
End Property
Public Property DestinationState As String
Get
Return desState
End Get
Set(value As String)
If desState = "IL" Then
desState = value
ElseIf desState = "WI" Then
desState = value
ElseIf desState = "MO" Then
desState = value
Else
Throw New Exception("Can not deliver to this state")
End If
End Set
End Property
End Class

Answer 1

Your immediate problem is you're checking the property value instead of the passed value. Something like this should work:
Set(value As String)
'This creates a temporary string array of available states and checks if _
the value passed is contained in the array
If {"IL","WI", "MO"}.Contains(value)Then
desState = value
Else
Throw New Exception("Can not deliver to this state")
End If
End Set
If this is something you may need to change you could make the array a property.