Understanding data validation - vb.net

Question

I am working on an assignment for school where I have to validate a data entry. My first issue is that running the function does not produce an error message when I run the code and use any other letter as customer type.
My second issue is that the 40% discount for the Else clause that is commented out is applying even though it is commented out.
Private Sub GetDiscountPercent(customerType As String, subtotal As Decimal,
ByRef discountPercent As Decimal)
If customerType = "R" Then
If subtotal < 100 Then
discountPercent = 0
ElseIf subtotal >= 100 AndAlso subtotal < 250 Then
discountPercent = 0.1D
ElseIf subtotal >= 250 Then
discountPercent = 0.25D
End If
ElseIf customerType = "C" Then
If subtotal < 250 Then
discountPercent = 0.2D
Else
discountPercent = 0.3D
End If
'Else
'discountPercent = 0.4D
End If
End Sub
Private Sub btnExit_Click(sender As Object,
e As EventArgs) Handles btnExit.Click
Me.Close()
End Sub
Private Sub ClearResultBoxes(sender As Object,
e As EventArgs) _
Handles txtCustomerType.TextChanged, txtSubtotal.TextChanged
txtDiscountPercent.Text = ""
txtDiscountAmount.Text = ""
txtTotal.Text = ""
End Sub
Function IsValidCustomerType() _
As Boolean
If IsValidCustomerType = "R" Then
MessageBox.Show("Customer Type has to be R or C or T", "Entry Error")
txtCustomerType.Select()
Return False
Else
Return True
End If
If Not IsValidCustomerType = "C" Then
MessageBox.Show("Customer Type has to be R or C or T", "Entry Error")
txtCustomerType.Select()
Return False
Else
Return True
End If
End Function
End Class

Answer 1

I am working on an assignment for school where I have to validate a data entry. My first issue is that running the function does not produce an error message when I run the code and use any other letter as customer type.
Speculating that this is wrong:
Function IsValidCustomerType() _
As Boolean
If IsValidCustomerType = "R" Then
^^^^^^^^^^^^^^^^^^^^^^^^^
MessageBox.Show("Customer Type has to be R or C or T", "Entry Error")
txtCustomerType.Select()
Return False
Else
Return True
End If
If Not IsValidCustomerType = "C" Then
MessageBox.Show("Customer Type has to be R or C or T", "Entry Error")
txtCustomerType.Select()
Return False
Else
Return True
End If
End Function
End Class
You've declared that this function returns a boolean. You cannot compare this to a string in a meaningful way, and if option strict is on it won't even compile.
Even if you could compare a bool with a string the logic of this is wrong. If you managed to make Type = C, then the code will never reach that check because it either returns true or false (in this case false) because C is not R, so while C is valid, you'll never check for it being C, because the code will only ever consider if it's R or not
You need to stop coding and write out your algorithm using pen and paper
My second issue is that the 40% discount for the Else clause that is commented out is applying even though it is commented out.
Gonna go out on a limb here and say that because this code is wrong, or because of some other compiler error elsewhere, you're seeing a dialog that says "There were build errors. Whould you like to run the last successful build?" and youre saying "Yes", and you're running a version of the app where that code isn't commented out..