Multiplication application, visual basic. invalid cast exception - vb.net

Question

Public Class Form1
Dim randomNumberOne As New Random
Dim randomNumberTwo As New Random
Dim ChildGrade As Integer
Dim strChildGrade As Integer
Dim strcorrectCounter As Integer
Dim Correctcounter As Integer = 0
Dim WrongCounter As Integer = 0
Dim totalQuestions As Integer = 0
Dim percent As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
ChildGrade = InputBox("Enter your age ", "age")
grade()
FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
SecondNumberlbl.Text = randomNumberOne.Next(0, 12)
End Sub
Private Sub grade()
If ChildGrade <= 4 Then
Gradelbl.Text = "Preschool"
ElseIf ChildGrade = 5 Then
Gradelbl.Text = "Kindergarden"
ElseIf ChildGrade <= 10 Then
Gradelbl.Text = "Elementary"
ElseIf ChildGrade <= 13 Then
Gradelbl.Text = "Middle School"
ElseIf ChildGrade <= 18 Then
Gradelbl.Text = "High School"
ElseIf ChildGrade > 18 Then
Gradelbl.Text = "NA"
End If
End Sub
Private Sub RandomNumberToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles RandomNumberToolStripMenuItem.Click
NumbersGroupBox.Enabled = False
FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
End Sub
Private Sub SingleTableToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles SingleTableToolStripMenuItem.Click
NumbersGroupBox.Enabled = True
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
FirstNumberlbl.Text = "1"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
FirstNumberlbl.Text = "2"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
FirstNumberlbl.Text = "3"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
FirstNumberlbl.Text = "4"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button5_Click(sender As Object, e As EventArgs) Handles Button5.Click
FirstNumberlbl.Text = "5"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
FirstNumberlbl.Text = "6"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button7_Click(sender As Object, e As EventArgs) Handles Button7.Click
FirstNumberlbl.Text = "7"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button8_Click(sender As Object, e As EventArgs) Handles Button8.Click
FirstNumberlbl.Text = "8"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button9_Click(sender As Object, e As EventArgs) Handles Button9.Click
FirstNumberlbl.Text = "9"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button10_Click(sender As Object, e As EventArgs) Handles Button10.Click
FirstNumberlbl.Text = "10"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
FirstNumberlbl.Text = "11"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub Button12_Click(sender As Object, e As EventArgs) Handles Button12.Click
FirstNumberlbl.Text = "12"
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Private Sub CheckAnswer()
If TextBox1.Text <> (FirstNumberlbl.Text * SecondNumberlbl.Text) Then
MsgBox("Please try again", MsgBoxStyle.Information, "Answer")
WrongCounter = WrongCounter + 1
If WrongCounter = 2 Then
MsgBox("The correct answer is " & FirstNumberlbl.Text * SecondNumberlbl.Text, MsgBoxStyle.Information, "Correct answer")
WrongCounter = 0
FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
End If
TextBox1.Text = ""
End If
End Sub
Private Sub rightAnswer()
If FirstNumberlbl.Text * SecondNumberlbl.Text = TextBox1.Text Then
Correctcounter = Correctcounter + 1
NumberCorrectlbl.Text = Correctcounter
TextBox1.Text = ""
FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
End If
totalQuestions = totalQuestions + 1
End Sub
Private Sub Enterbtn_Click(sender As Object, e As EventArgs) Handles Enterbtn.Click
CheckAnswer()
rightAnswer()
TotalCountlbl.Text = totalQuestions
PercentCorrectlbl.Text = ((Correctcounter * 100) / (totalQuestions * 100)) * 100
End Sub
End Class
I'm creating a multiplication application in VB that keeps count of how many questions the user gets right and how many they get wrong. When I run the program, it works fine in counting the amount right and totaling up the percentage, but when I test for when they get it wrong I get this error. enter image description here
if the image isn't visible, the error says,
An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll
Additional information: Conversion from string "" to type 'Double' is not valid
The program points to the problem being in the if statement in the rightAnswer() sub class but i'm not exactly sure what is wrong with it.

Answer 1

The problem is you are trying to do calculations with strings. Strings are text, not numbers.
This is not a full answer but enough to get you to understand the problem and change your code to fix it.
Don't do any calculations on the contents of text boxes and/or labels. Use the correct variables for such tasks. Use the controls for simply retrieving and displaying. This will save you a lot of heart-ache in the long run.
So for example, don't do:
If TextBox1.Text <> (FirstNumberlbl.Text * SecondNumberlbl.Text) Then
You're checking a number in the TextBox to a calculation based on two labels, they are all text!
So think along the lines of doing calculations on a variable fit for purpose, an integer for example:
Example:
Dim FirstNumber As Integer
Dim SecondNumber As Integer
Use these for the calculations, then, display the result as text.
If you are expecting a floating point number, a decimal, change the variable type as needed.
In your 'CheckAnswer' subroutine you should starting thinking along the lines of:
If Not String.IsNullOrWhiteSpace(TextBox1.Text) Then
'We have something in the TextBox (you can check to allow only numbers to be entered in the TextBox.KeyDown/Press event)
If Not Integer.Parse(TextBox1.Text) = (FirstNumber * SecondNumber) Then
'Do what you need ...
End If
End If
There's a couple of other things too that could simplify your code.
You don't need 12 button events all doing the same thing.
In you Form1_Load Event put this:
Dim bTags() As Button = {Button1, Button2, Button3, Button4, Button5, Button6, Button7, Button8, Button9, Button10, Button11, Button12}
For t As Integer = 0 To bTags.Count - 1
bTags(t).Tag = t + 1
Next
Then delete all your 12 Button Click events and replace them with:
Private Sub Buttons_Click(sender As Object, e As EventArgs) Handles Button9.Click, Button8.Click, Button7.Click, Button6.Click, Button5.Click, Button4.Click, Button3.Click, Button2.Click, Button12.Click, Button11.Click, Button10.Click, Button1.Click
Dim selectedButton As Button = CType(sender, Button)
FirstNumberlbl.Text = selectedButton.Tag.ToString
SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
TextBox1.Focus()
End Sub
Job done...
Also, you grade subroutine would be better of as a Select Case function:
In your Form1_Load replace: grade() with Gradelbl.Text = Grade() and your grade() subroutine replaced with:
Private Function Grade() As String
Select Case ChildGrade
Case < 5 : Return "Preschool"
Case = 5 : Return "Kindergarden"
Case < 11 : Return "Elementary"
Case < 14 : Return "Middle School"
Case < 19 : Return "High School"
Case Else : Return "N/A"
End Select
End Function
I hope some of the advice will be of help.

Answer 2

What you need to do is cast the values of the textbox. For example:
Convert.toInt32(FirstNumberlbl.Text)
or
Dim firstNumber as Integer
Integer.TryParse(FirstNumberlbl.Text, firstNumber)
Or assign a variable to the textbox and cast the variable instead of the textbox.