(VB.NET) Object reference not set to an instance of an object - vb.net

Question

This program works like this: User enters building name and number of floors, that gets validated and accepted. Then user enters rates for each floor until it goes through all floors, that data gets added to a listbox, then user enters a desired floor and it adds the rent and other info to another listbox lower down. As soon as I enter my number of floors and click on the button to save the info, the program runs into an error under btnEnterBuilding Click event where it says dblRates(intHowMany) = dblRent. The error is
"An unhandled exception of type 'System.NullReferenceException' occurred in WindowsApplication5.exe
Additional information: Object reference not set to an instance of an object."
Any help would be greatly appreciated, thanks!
Option Explicit On
Option Strict On
Option Infer Off
Public Class Form1
Dim dblRates() As Double
Dim intHowMany As Integer = 0 'points to the next avail entry in the array
Private Function ValidateString(ByVal strText As String, strInput As String, strValue As String) As Boolean
If strText = Nothing Then
MessageBox.Show(strText & " Must Be Supplied", "Error")
Return False
Else
Return True
End If
End Function
Private Function ValidateInteger(ByVal strText As String,
ByVal strIn As String,
ByRef intValue As Integer,
ByVal intMinValue As Integer,
ByVal intMaxValue As Integer) As Boolean
If strIn = Nothing Then
MessageBox.Show(strText & " Must Be Supplied", "Error")
Return False
Else
If Integer.TryParse(strIn, intValue) = False Then
MessageBox.Show(strText & " Must Be A Whole Number", "Error")
Return False
Else
If intValue < intMinValue Or intValue > intMaxValue Then
MessageBox.Show("Outside of Number of " & strText & " Limits", "Error")
Return False
Else
Return True
End If
End If
End If
End Function
Private Function ValidateDouble(ByVal strText As String,
ByVal strDbl As String,
ByRef dblValue As Double,
ByVal dblMinValue As Double,
ByVal dblMaxValue As Double) As Boolean
If strDbl = Nothing Then
MessageBox.Show(strText & " Must Be Supplied", "Error")
Return False
Else
If Double.TryParse(strDbl, dblValue) = False Then
MessageBox.Show(strText & " Must Be A Whole Number", "Error")
Return False
Else
If dblValue < dblMinValue Or dblValue > dblMaxValue Then
MessageBox.Show("Outside of Number of " & strText & " Limits", "Error")
Return False
Else
Return True
End If
End If
End If
End Function
Private Sub Form1_Load(sender As Object,
e As EventArgs) Handles Me.Load
Me.grpBuilding.Enabled = True
Me.grpRents.Enabled = False
Me.grpDesiredFloor.Enabled = False
End Sub
Private Sub btnRents_Click(sender As Object,
e As EventArgs) _
Handles btnRents.Click
Dim strName, strFloors As String
Dim intFloors As Integer
strName = txtName.Text
strFloors = txtFloors.Text
intFloors = CInt(strFloors)
If ValidateString("Building name", Me.txtName.Text, strName) = True Then
If ValidateInteger("Number of floors", Me.txtFloors.Text, intFloors, 3, 20) = True Then
Me.grpBuilding.Enabled = False
Me.grpRents.Enabled = True
Me.grpDesiredFloor.Enabled = False
End If
End If
End Sub
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
Close()
End Sub
Private Sub btnEnterBuilding_Click(sender As Object,
e As EventArgs) _
Handles btnEnterBuilding.Click
Dim intFloors As Integer
Dim dblRent As Double
Dim strRent As String
strRent = txtRent.Text
dblRent = CDbl(strRent)
If ValidateDouble("Rent", Me.txtRent.Text, dblRent, 1000.0, 2500.0) = True Then
dblRates(intHowMany) = dblRent
Me.txtRent.Focus()
Me.txtRent.SelectAll()
Me.ListBox1.Items.Add("Floor No. " & intHowMany.ToString("N0") &
" -- Rent Is: " & dblRent.ToString("N$"))
If intHowMany < intFloors Then
intHowMany += 1
Else
Me.grpBuilding.Enabled = False
Me.grpRents.Enabled = False
Me.grpDesiredFloor.Enabled = True
End If
Else
Me.txtRent.Focus()
Me.txtRent.SelectAll()
End If
End Sub
Private Sub btnCompute_Click(sender As Object, e As EventArgs) Handles btnCompute.Click
Dim intFloors, intFloor As Integer
Dim strName, strFloors As String
strName = txtName.Text
strFloors = txtFloors.Text
intFloors = CInt(strFloors)
If ValidateInteger("Desired Floor", Me.txtFloor.Text, intFloor, 3, 20) = False Then
MessageBox.Show("Please enter a valid floor number", "Error",
MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
Me.lstDisplay.Items.Add("Building Name: " & strName & " # of Floors: " & intFloors.ToString)
Me.lstDisplay.Items.Add("Desired Floor: " & intFloor.ToString)
Me.lstDisplay.Items.Add(" Rent: " & intFloors.ToString)
End If
End Sub
End Class

Answer 1

You can't dim dblRates() as double like this without giving it initial values. You will need to dim dblRates(<some amount>) as Double and then redim it if necessary to add more values to it. Or you could Dim dblRates() as double = {0} but if you still want to add more values to the array, you will still need to redim it as the second options would just create an array of 1 element.