why does "Conversion from string "" to type 'Decimal' is not valid." keep appearing

[silverkaito]

i'm creating gpa and cgpa calculator for my school project. and i'm a newbie to this visual basic thingy. so do help me, pls.

here's my code

Public Class Form1
    Dim CreditHour1 As Integer
    Dim CreditHour2 As Integer
    Dim CreditHour3 As Integer
    Dim CreditHour4 As Integer
    Dim CreditHour5 As Integer
    Dim CreditHour6 As Integer
    Dim CreditHour7 As Integer
    Dim TotalCreditHours As Integer
 
 
    Private Sub ButtonTotal_Click(sender As System.Object, e As System.EventArgs) Handles ButtonTotal1.Click
 
        CreditHour1 = TextBoxCreditHour1.Text
        CreditHour2 = TextBoxCreditHour2.Text
        CreditHour3 = TextBoxCreditHour3.Text
        CreditHour4 = TextBoxCreditHour4.Text
        CreditHour5 = TextBoxCreditHour5.Text
        CreditHour6 = TextBoxCreditHour6.Text
        CreditHour7 = TextBoxCreditHour7.Text
        TotalCreditHours = CreditHour1 + CreditHour2 + CreditHour3 + CreditHour4 + CreditHour5 + CreditHour6 + CreditHour7
        LabelCreditHours2.Text = TotalCreditHours
 
 
    End Sub
 
    Private Sub ButtonExit_Click(sender As System.Object, e As System.EventArgs) Handles ButtonExit.Click
        Close()
    End Sub
 
    Private Sub ButtonTotal2_Click(sender As System.Object, e As System.EventArgs) Handles ButtonTotal2.Click
 
        Dim TotalGradePoints1 As Double
        Dim TotalGradePoints2 As Double
        Dim TotalGradePoints3 As Double
        Dim TotalGradePoints4 As Double
        Dim TotalGradePoints5 As Double
        Dim TotalGradePoints6 As Double
        Dim TotalGradePoints7 As Double
        Dim TotalTotalGradePoints As Double
 
        TotalGradePoints1 = TextBoxTotalGradePoints1.Text
        TotalGradePoints2 = TextBoxTotalGradePoints2.Text
        TotalGradePoints3 = TextBoxTotalGradePoints3.Text
        TotalGradePoints4 = TextBoxTotalGradePoints4.Text
        TotalGradePoints5 = TextBoxTotalGradePoints5.Text
        TotalGradePoints6 = TextBoxTotalGradePoints6.Text
        TotalGradePoints7 = TextBoxTotalGradePoints7.Text
        LabelTotalGradePoints2.Text = TotalTotalGradePoints
 
        CreditHour1 = TextBoxCreditHour1.Text
        CreditHour2 = TextBoxCreditHour2.Text
        CreditHour3 = TextBoxCreditHour3.Text
        CreditHour4 = TextBoxCreditHour4.Text
        CreditHour5 = TextBoxCreditHour5.Text
        CreditHour6 = TextBoxCreditHour6.Text
        CreditHour7 = TextBoxCreditHour7.Text
 
        Select Case ComboBox1.Text
            Case "A"
                TotalGradePoints1 = 4 * CreditHour1
            Case "A-"
                TotalGradePoints1 = 3.75 * CreditHour1
            Case "B+"
                TotalGradePoints1 = 3.5 * CreditHour1
            Case "B"
                TotalGradePoints1 = 3.0 * CreditHour1
            Case "B-"
                TotalGradePoints1 = 2.75 * CreditHour1
            Case "C+"
                TotalGradePoints1 = 2.5 * CreditHour1
            Case "C"
                TotalGradePoints1 = 2.0 * CreditHour1
            Case "C-"
                TotalGradePoints1 = 1.75 * CreditHour1
            Case "D"
                TotalGradePoints1 = 1 * CreditHour1
            Case "F"
                TotalGradePoints1 = 0 * CreditHour1
 
        End Select
 
        Select Case ComboBox2.Text
            Case "A"
                TotalGradePoints2 = 4 * CreditHour2
            Case "A-"
                TotalGradePoints2 = 3.75 * CreditHour2
            Case "B+"
                TotalGradePoints2 = 3.5 * CreditHour2
            Case "B"
                TotalGradePoints2 = 3.0 * CreditHour2
            Case "B-"
                TotalGradePoints2 = 2.75 * CreditHour2
            Case "C+"
                TotalGradePoints2 = 2.5 * CreditHour2
            Case "C"
                TotalGradePoints2 = 2.0 * CreditHour2
            Case "C-"
                TotalGradePoints2 = 1.75 * CreditHour2
            Case "D"
                TotalGradePoints2 = 1 * CreditHour2
            Case "F"
                TotalGradePoints2 = 0 * CreditHour2
 
        End Select
 
        Select Case ComboBox3.Text
            Case "A"
                TotalGradePoints3 = 4 * CreditHour3
            Case "A-"
                TotalGradePoints3 = 3.75 * CreditHour3
            Case "B+"
                TotalGradePoints3 = 3.5 * CreditHour3
            Case "B"
                TotalGradePoints3 = 3.0 * CreditHour3
            Case "B-"
                TotalGradePoints3 = 2.75 * CreditHour3
            Case "C+"
                TotalGradePoints3 = 2.5 * CreditHour3
            Case "C"
                TotalGradePoints3 = 2.0 * CreditHour3
            Case "C-"
                TotalGradePoints3 = 1.75 * CreditHour3
            Case "D"
                TotalGradePoints3 = 1 * CreditHour3
            Case "F"
                TotalGradePoints3 = 0 * CreditHour3
 
        End Select
 
        Select Case ComboBox4.Text
            Case "A"
                TotalGradePoints4 = 4 * CreditHour4
            Case "A-"
                TotalGradePoints4 = 3.75 * CreditHour4
            Case "B+"
                TotalGradePoints4 = 3.5 * CreditHour4
            Case "B"
                TotalGradePoints4 = 3.0 * CreditHour4
            Case "B-"
                TotalGradePoints4 = 2.75 * CreditHour4
            Case "C+"
                TotalGradePoints4 = 2.5 * CreditHour4
            Case "C"
                TotalGradePoints4 = 2.0 * CreditHour4
            Case "C-"
                TotalGradePoints4 = 1.75 * CreditHour4
            Case "D"
                TotalGradePoints4 = 1 * CreditHour4
            Case "F"
                TotalGradePoints4 = 0 * CreditHour4
 
        End Select
 
        Select Case ComboBox5.Text
            Case "A"
                TotalGradePoints5 = 4 * CreditHour5
            Case "A-"
                TotalGradePoints5 = 3.75 * CreditHour5
            Case "B+"
                TotalGradePoints5 = 3.5 * CreditHour5
            Case "B"
                TotalGradePoints5 = 3.0 * CreditHour5
            Case "B-"
                TotalGradePoints5 = 2.75 * CreditHour5
            Case "C+"
                TotalGradePoints5 = 2.5 * CreditHour5
            Case "C"
                TotalGradePoints5 = 2.0 * CreditHour5
            Case "C-"
                TotalGradePoints5 = 1.75 * CreditHour5
            Case "D"
                TotalGradePoints5 = 1 * CreditHour5
            Case "F"
                TotalGradePoints5 = 0 * CreditHour5
 
        End Select
 
        Select Case ComboBox6.Text
            Case "A"
                TotalGradePoints6 = 4 * CreditHour6
            Case "A-"
                TotalGradePoints6 = 3.75 * CreditHour6
            Case "B+"
                TotalGradePoints6 = 3.5 * CreditHour6
            Case "B"
                TotalGradePoints6 = 3.0 * CreditHour6
            Case "B-"
                TotalGradePoints6 = 2.75 * CreditHour6
            Case "C+"
                TotalGradePoints6 = 2.5 * CreditHour6
            Case "C"
                TotalGradePoints6 = 2.0 * CreditHour6
            Case "C-"
                TotalGradePoints6 = 1.75 * CreditHour6
            Case "D"
                TotalGradePoints6 = 1 * CreditHour6
            Case "F"
                TotalGradePoints6 = 0 * CreditHour6
 
        End Select
 
        Select Case ComboBox7.Text
            Case "A"
                TotalGradePoints7 = 4 * CreditHour7
            Case "A-"
                TotalGradePoints7 = 3.75 * CreditHour7
            Case "B+"
                TotalGradePoints7 = 3.5 * CreditHour7
            Case "B"
                TotalGradePoints7 = 3.0 * CreditHour7
            Case "B-"
                TotalGradePoints7 = 2.75 * CreditHour7
            Case "C+"
                TotalGradePoints7 = 2.5 * CreditHour7
            Case "C"
                TotalGradePoints7 = 2.0 * CreditHour7
            Case "C-"
                TotalGradePoints7 = 1.75 * CreditHour7
            Case "D"
                TotalGradePoints7 = 1 * CreditHour7
            Case "F"
                TotalGradePoints7 = 0 * CreditHour7
 
        End Select
 
    End Sub
 
 
End Class

[iam_clint]

You don't do any verification on any of the text boxes.

TotalGradePoints1 = TextBoxTotalGradePoints1.Text
TotalGradePoints2 = TextBoxTotalGradePoints2.Text
TotalGradePoints3 = TextBoxTotalGradePoints3.Text
TotalGradePoints4 = TextBoxTotalGradePoints4.Text
TotalGradePoints5 = TextBoxTotalGradePoints5.Text
TotalGradePoints6 = TextBoxTotalGradePoints6.Text
TotalGradePoints7 = TextBoxTotalGradePoints7.Text
LabelTotalGradePoints2.Text = TotalTotalGradePoints
 
CreditHour1 = TextBoxCreditHour1.Text
CreditHour2 = TextBoxCreditHour2.Text
CreditHour3 = TextBoxCreditHour3.Text
CreditHour4 = TextBoxCreditHour4.Text
CreditHour5 = TextBoxCreditHour5.Text
CreditHour6 = TextBoxCreditHour6.Text
CreditHour7 = TextBoxCreditHour7.Text
 

if a textbox is blank it will give you a conversion error since you are trying to assign the text value of a text box to a double.

look at double.tryparse(), and double.parse

[silverkaito]

how do we do double.tryparse?

[Frinavale]

You use the Double.TryParse method like this:

Dim value As String = TextBoxCreditHour1.Text
Dim number As Double
 
If Double.TryParse(value, number) Then
  ' the value could successfully be converted into a Double
Else
 ' the value could NOT be converted into a Double
End If

-Frinny