VB Seat booking file outputting incorrectly - vb.net

Question

I am creating a seat booking program for coursework in which a user selects which row he/she wishes to book seats in, and then the number of seats they wish to book.
Public Class Form1
Private Sub ListBox2_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListBox2.SelectedIndexChanged
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim rowNumber As Integer
Dim SeatsData As String(,) = {{"booked", "A2", "A3", "A4", "A5", "A6", "A7", "A8", "A9", "A10"}, {"booked", "booked", "booked", "booked", "booked", "booked", "booked", "booked", "booked", "booked"}, {"C1", "C2", "C3", "C4", "C5", "C6", "C7", "C8", "C9", "C10"}, {"D1", "booked", "booked", "D4", "D5", "D6", "D7", "D8", "D9", "D10"}, {"E1", "E2", "booked", "booked", "booked", "E6", "E7", "E8", "E9", "E10"}}
If ListBox2.Text = "A" Then
rowNumber = 0
ElseIf ListBox2.Text = "B" Then
rowNumber = 1
ElseIf ListBox2.Text = "C" Then
rowNumber = 2
ElseIf ListBox2.Text = "D" Then
rowNumber = 3
ElseIf ListBox2.Text = "E" Then
rowNumber = 4
End If
FindSeats(rowNumber, ListBox1.Text, SeatsData)
End Sub
Function FindSeats(ByVal RowNumber As Integer, ByVal NumSeats As Integer, SeatsData As Array) As String
Dim i As Integer = 0
Dim arrayPos As Integer = 0
Dim largestStreak As Integer = 0
Dim FirstSeat As String = 0
Dim LastSeat As String = 0
Dim foundSeats As Boolean = False
Dim returnMsg As String = ""
Do While foundSeats = False
Dim seatChar As String = SeatsData(RowNumber, arrayPos)
arrayPos = arrayPos + 1
If seatChar = "booked" Then
i = 0
Else
If i = 0 Then
FirstSeat = seatChar
End If
i = i + 1
If i > largestStreak Then
largestStreak = i
End If
End If
If i = NumSeats Then
LastSeat = seatChar
If FirstSeat = LastSeat Then
returnMsg = "Found seat: " + FirstSeat
Else
returnMsg = "Found seats: " + FirstSeat + " - " + LastSeat
End If
MsgBox(returnMsg)
Label3.Text = returnMsg
foundSeats = True
Exit Do
End If
If arrayPos = 10 Then
returnMsg = "Not enough available seats, maximum available seats: " + CStr(largestStreak)
MsgBox(returnMsg)
Label3.Text = returnMsg
Exit Do
End If
Loop
End Function
End Class
The issue I am having is that when a user selects the number of seats they want but not which row they want them for, the program automatically books seats in row A. How do I fix this? Also when A user selects a row but not a number of seats (or vice versa) the form goes to code and outputs the following error message: "An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll
Additional information: Conversion from string "" to type 'Integer' is not valid."
How do I fix these errors? I am very new to VB so if any of this is obvious then I apologise!

Answer 1

You get the exception because you call FindSeats like this:
FindSeats(rowNumber, ListBox1.Text, SeatsData)
Note that the signature of FindSeats is actually
Function FindSeats(RowNumber As Integer, NumSeats As Integer, SeatsData As Array) As String
So you pass a string ListBox1.Text as NumSeats parameter, which is an Integer. Since you use Option Strict Off, VB.Net tries to implicitly cast the string in ListBox1.Text into an Integer.
This fails if ListBox1.Text is empty, and that's what the exception message it telling you:
Conversion from string "" to type 'Integer' is not valid.
First, turn Option Strict to On, then you'll see all those nifty implicit conversions that will bite you at runtime. Go on and replace them with safe explicit conversions that make sense.
Think about how you want to handle the case when the user clicks Button1 without selecting something in ListBox1 before.
Maybe you want to use Int32.TryParse to handle such cases, or maybe preselect a valid value in ListBox1