GPA calculator textbox issues

[jmsutton334]

I have programmed a GPA calculator to where their are 8 textboxes to enter the course names, 8 textboxes for the number of hours each course is worth, and lastly 8 textboxes for the letter grade you made in those courses. I am having trouble with the 8 textboxes that take the credit hours. If I don't feel each one in with a number it gives me the error "Input String was not in correct format". I wanted to program it to where if the user is only taking 5 classes they can leave the other 3 textboxes blank and a "0" will be put in the textboxes they did not fill in for the credit hours, and just calculate the GPA from the 5 classes entered. It work correctly if the user inputs into all 8 textboxes for credit hours but, that error code I mentioned above comes up if they don't fill in all 8....

Here is the code I have so far.

Public Class Form1

Dim QualityPoints As Double
Dim CourseName, hours As String

Private Sub btnTotalHours_Click(sender As Object, e As EventArgs) Handles btnTotalHours.Click
    Dim totalHours As String
    Dim Credit1, Credit2, Credit3, Credit4, Credit5, Credit6, Credit7, Credit8 As Integer
    Credit1 = Integer.Parse(txtBoxCredit1.Text)
    Credit2 = Integer.Parse(txtBoxCredit2.Text)
    Credit3 = Integer.Parse(txtBoxCredit3.Text)
    Credit4 = Integer.Parse(txtBoxCredit4.Text)
    Credit5 = Integer.Parse(txtBoxCredit5.Text)
    Credit6 = Integer.Parse(txtBoxCredit6.Text)
    Credit7 = Integer.Parse(txtBoxCredit7.Text)
    Credit8 = Integer.Parse(txtBoxCredit8.Text)

    totalHours = CStr(txtBoxTotalHours.Text)
    txtBoxTotalHours.Text = (Credit1 + Credit2 + Credit3 + Credit4 + Credit5 + Credit6 + Credit7 + Credit8)

End Sub

Private Sub btnTotalQualityPoints_Click(sender As Object, e As EventArgs) Handles btnTotalQualityPoints.Click

    Select Case txtBoxGrade1.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit1.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit1.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit1.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit1.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit1.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit1.Text)

    End Select

    Select Case txtBoxGrade2.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit2.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit2.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit2.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit2.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit2.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit2.Text)

    End Select

    Select Case txtBoxGrade3.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit3.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit3.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit3.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit3.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit3.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit3.Text)

    End Select

    Select Case txtBoxGrade4.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit4.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit4.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit4.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit4.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit4.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit4.Text)

    End Select

    Select Case txtBoxGrade5.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit5.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit5.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit5.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit5.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit5.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit5.Text)

    End Select

    Select Case txtBoxGrade6.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit6.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit6.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit6.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit6.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit6.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit6.Text)

    End Select

    Select Case txtBoxGrade7.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit7.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit7.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit7.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit7.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit7.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit7.Text)

    End Select

    Select Case txtBoxGrade8.Text
        Case "A"
            QualityPoints += (4 * txtBoxCredit8.Text)
        Case "B"
            QualityPoints += (3 * txtBoxCredit8.Text)
        Case "C"
            QualityPoints += (2 * txtBoxCredit8.Text)
        Case "D"
            QualityPoints += (1 * txtBoxCredit8.Text)
        Case "F"
            QualityPoints += (0 * txtBoxCredit8.Text)
        Case " "
            QualityPoints += (0 * txtBoxCredit8.Text)

    End Select

    txtBoxTotalQualityPoints.Text = QualityPoints

End Sub

Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click
    txtBoxCourse1.Clear()
    txtBoxCourse2.Clear()
    txtBoxCourse3.Clear()
    txtBoxCourse4.Clear()
    txtBoxCourse5.Clear()
    txtBoxCourse6.Clear()
    txtBoxCourse7.Clear()
    txtBoxCourse8.Clear()

    txtBoxCredit1.Clear()
    txtBoxCredit2.Clear()
    txtBoxCredit3.Clear()
    txtBoxCredit4.Clear()
    txtBoxCredit5.Clear()
    txtBoxCredit6.Clear()
    txtBoxCredit7.Clear()
    txtBoxCredit8.Clear()

    txtBoxGrade1.Clear()
    txtBoxGrade2.Clear()
    txtBoxGrade3.Clear()
    txtBoxGrade4.Clear()
    txtBoxGrade5.Clear()
    txtBoxGrade6.Clear()
    txtBoxGrade7.Clear()
    txtBoxGrade8.Clear()

    txtBoxTotalGPA.Clear()
    txtBoxTotalHours.Clear()
    txtBoxTotalQualityPoints.Clear()

End Sub

Thanks for any help that is provided!

[dunfiddlin]

Why does everybody make this difficult for themselves? Use the NumericUpDown control for number entry. It doesn't need validating or conversion. It always returns a number and it should be obvious to the user that if they haven't got anything to enter it should remain at 0.