Try to cast the object collection items back to integers:
Dim FB As Integer = Enumerable.Range(0, Integer.MaxValue).Except(ListBox1.Items.Cast(Of Integer)()).FirstOrDefault()
It may not work, though, depending on how you populated the listbox.
Edit: after some second toughts, loading into memory a range of over 2 billions of integers may not be the best idea. You could try this instead:
Public Function GetFirstAvailable() As Integer
Dim list as New List(Of Integer)(ListBox1.Items.Cast(Of Integer)())
For i As Integer = 0 To Integer.MaxValue
If Not list.Contains(i)
Return i
End If
Next
Return -1
End Function
Dim FB As Integer = GetFirstAvailable()
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