Checking if index is null and providing an error? - vb.net

Question

I have the following code:
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click ' calculate button
If String.IsNullOrEmpty(TextBox1.Text) Then
MsgBox("Please enter a value to convert!")
End If
If currentIndex = vbNull Then
MsgBox("Please select a conversion!")
End If
Select Case currentIndex
Case 1
Label2.Text = TextBox1.Text & " Celsius = " & Math.Round(((TextBox1.Text * 1.8) + 32), 2) & " Fahrenheit"
Case 2
Label2.Text = TextBox1.Text & " Fahrenheit = " & Math.Round(((TextBox1.Text - 32) / 1.8), 2) & " Celsius"
Case 3
Label2.Text = TextBox1.Text & " Kelvin = " & Math.Round((TextBox1.Text - 273.15), 2) & " Celsius"
Case 4
Label2.Text = TextBox1.Text & " Celius = " & Math.Round((TextBox1.Text + 273.15), 2) & " Kelvin"
Case 5
Label2.Text = TextBox1.Text & " Kelvin = " & Math.Round((((TextBox1.Text - 273.15) * 1.8) + 32), 2) & " Fahrenheit"
Case 6
Label2.Text = TextBox1.Text & " Fahrenheit = " & Math.Round(((((TextBox1.Text - 32) * 5) / 9) + 273.15), 2) & " Kelvin"
Case 8
Label2.Text = TextBox1.Text & " Miles P/H = " & Math.Round((TextBox1.Text * 1.609344), 2) & " Kilometers P/H"
Case 9
Label2.Text = TextBox1.Text & " Kilometers P/H = " & Math.Round((TextBox1.Text / 1.609344), 2) & " Miles P/H"
Case 11
Label2.Text = TextBox1.Text & " Kilograms = " & Math.Round((TextBox1.Text * 2.20462), 2) & " Pounds"
Case 12
Label2.Text = TextBox1.Text & " Pounds = " & Math.Round((TextBox1.Text / 2.20462), 2) & " Kilograms"
Case 14
Label2.Text = TextBox1.Text & " Meters = " & Math.Round((TextBox1.Text * 3.2808399), 2) & " Feet"
Case 15
Label2.Text = TextBox1.Text & " Feet = " & Math.Round((TextBox1.Text / 3.2808399), 2) & " Meters"
End Select
End Sub
As you can see, I have a variable (currentIndex) and have a select case statement checking them against the various conversions, however my problem lies in the piece of code above this.
If currentIndex = vbNull Then
MsgBox("Please select a conversion!")
End If
I require it to spit an error message out if the index is null, however I cannot work out a way to do this. 0 cannot be used as this is the first entry in the index, and vbNull etc do not seem to work. Can anybody point me in the right direction? Thanks.
EDIT:
This is how current index is created:
Dim currentIndex As Integer
and this is how it is assigned:
Private Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
' Get the currently selected index of the item in the list box.
currentIndex = ListBox1.FindString(currentItem)
End Sub

Answer 1

I would suggest using SelectedIndex instead.
In your code:
If ListBox1.SelectedIndex = -1 Then
MsgBox("Please select a conversion!")
End If
Here is the reference for the property: http://msdn.microsoft.com/en-us/library/system.windows.forms.listbox.selectedindex.aspx

Answer 2

If currentIndex is an index from dropdown box then value of -1 means that nothing was selected.