Calculating Profit in Visual Basic - vb.net

Question

This program is supposed to read in values from a text file and get a sum of all these values. It is then to use information gathered from a series of check boxes and text boxes to calculate the final profit.
As the code is written now, the profit is only correct if all check boxes are selected, but I need it to be correct if one, two, or all three are checked. Here is the current code
Option Strict On
Imports System.IO
Public Class Form1
Dim sum As Double
Dim fileRead As Boolean
Dim profit As Double
Private Sub menOpen_Click(sender As Object, e As EventArgs) Handles menOpen.Click
Dim ofd As New OpenFileDialog
ofd.Filter = "text files |*.txt|All Files|*.*"
ofd.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory)
If ofd.ShowDialog() = Windows.Forms.DialogResult.OK Then
Dim selectedFileName As String = System.IO.Path.GetFileName(ofd.FileName)
If selectedFileName.ToLower = "profit.txt" Then
Dim line As String
Using reader As New StreamReader(ofd.OpenFile)
While Not reader.EndOfStream
line = reader.ReadLine
Dim value As Integer
If Integer.TryParse(line, value) Then
sum = sum + value
fileRead = True
End If
Console.WriteLine(line)
End While
End Using
Else
MessageBox.Show("You cannot use that file!")
End If
End If
End Sub
Private Sub menExit_Click(sender As Object, e As EventArgs) Handles menExit.Click
Me.Close()
End Sub
Private Sub radSales_CheckedChanged(sender As Object, e As EventArgs) Handles radSales.CheckedChanged
If radSales.Checked Then
profit = sum
End If
End Sub
Private Sub radSandO_CheckedChanged(sender As Object, e As EventArgs) Handles radSandO.CheckedChanged
If radSandO.Checked Then
If Trim(txtWages.Text) = "" Then
txtWages.Text = CStr(0)
End If
profit = (sum - CDbl(txtWages.Text) - CDbl(txtRent.Text) - CDbl(txtUtilities.Text))
End If
End Sub
Private Sub menComputeProfit_Click(sender As Object, e As EventArgs) Handles menComputeProfit.Click
If fileRead = False Then
MessageBox.Show("The file profit.txt has not been read in yet, the profit will be set to zero.")
sum = 0
End If
If chkWages.Checked Then
profit = CDbl(("$" & Val(sum) - (Val(txtWages.Text) + Val(txtRent.Text) + Val(txtUtilities.Text))))
End If
If chkRent.Checked Then
profit = CDbl(("$" & Val(sum) - (Val(txtRent.Text) + Val(txtWages.Text) + Val(txtUtilities.Text))))
End If
If chkUtilities.Checked Then
profit = CDbl(("$" & Val(sum) - (Val(txtUtilities.Text) + Val(txtWages.Text) + Val(txtRent.Text))))
End If
txtAnswer.Text = profit.ToString
End Sub
End Class
Any help would be greatly appreciated.

Answer 1

One of those text inputs is most likely empty (as the error states). Even if it wasn't empty, it could still be an invalid Double value.
In order to safely check if the string can be converted to a double, you can use Double.TryParse, like this:
If Double.TryParse(value, number) Then
Console.WriteLine("'{0}' --> {1}", value, number)
Else
Console.WriteLine("Unable to parse '{0}'.", value)
End If

Answer 2

Before you convert a string to double, check that the string is not empty. This is what error says. Afterwards you should check that the string in the box is actually numeric.

Answer 3

You are using TextChanged Event for txtAnswer
Private Sub txtAnswer_TextChanged(sender As Object, e As EventArgs) Handles txtAnswer.TextChanged
txtAnswer.Text = CStr(profit)
End Sub
So the above code is going to fire itself each time it changes! Just pop profit into txt Answer each time it is calculated
If chkWages.Checked Then
profit = CDbl(("$" & Val(sum) - Val(txtWages.Text)))
txtAnswer.text = profit
End If
If chkRent.Checked Then
profit = CDbl(("$" & Val(sum) - Val(txtRent.Text)))
txtAnswer.text = profit
End If
If chkUtilities.Checked Then
profit = CDbl(("$" & Val(sum) - Val(txtUtilities.Text)))
txtAnswer.text = profit
End If
A bit untidy but that should do it... you will need to do this whenever profit changed. And get rid of the TextChanged event.
UPDATED ANSWER FOR CLARITY
Ok I never add currency symbols so I'm not sure what CDbl(("$" & Val(Sum) - Val(txtWages.Text))) will do as it may treat it as a string character "$" rather than a currency.
Get rid of the "$" and just add the values... if you want to then stick the $ in front once calculation is complete say txtAnswer = $ & profit.toString