Syntax Error - Cant Calculate - vb.net

Question

I can't figure out whats wrong with my code. The code is supposed to calculate the total cost of an item using cost, quantity and promo code if u have one. It keeps crashing when I put in unused characters such as ! . Any help or improvements would be welcome.
Public Class Form1
Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
Dim decDisplayTotal As Decimal
Dim decPrice As Decimal = txtPrice.Text
Dim intQuantity As Integer = txtQuantity.Text
Dim strPromoCode As String = txtPromoCode.Text
decDisplayTotal = decPrice * intQuantity
lblDisplayTotal.Text = "$" & decDisplayTotal
If decPrice < 0 Then
lblDisplayTotal.Text = ("")
txtPrice.Text = Nothing
txtQuantity.Text = Nothing
txtPromoCode.Text = Nothing
MessageBox.Show("Please enter an appropriate price.", "Invalid Input")
End If
If intQuantity < 0 Then
lblDisplayTotal.Text = ("")
txtPrice.Text = Nothing
txtQuantity.Text = Nothing
txtPromoCode.Text = Nothing
MessageBox.Show("Please enter an approriate quantity.", "Invalid Input")
End If
If strPromoCode = ("132") Then
MessageBox.Show("You used a limited time, 10% off code! Watch your price drop 10%!", "10% off")
decDisplayTotal = 0.9 * (decPrice * intQuantity)
lblDisplayTotal.Text = "$" & decDisplayTotal
End If
If strPromoCode = ("129") Then
MessageBox.Show("You used a limited time, 20% off code! Watch your price drop 20%!", "20% off")
decDisplayTotal = 0.8 * (decPrice * intQuantity)
lblDisplayTotal.Text = "$" & decDisplayTotal
End If
If strPromoCode = ("136") Then
MessageBox.Show("You used a limited time, 30% off code! Watch your price drop 30%!", "30% off")
decDisplayTotal = 0.7 * (decPrice * intQuantity)
lblDisplayTotal.Text = "$" & decDisplayTotal
End If
If strPromoCode = ("264") Then
MessageBox.Show("You used a limited time, buy 1 get 1 free code, so watch your total cut in half!", "Buy 1 Get 1 Free")
decDisplayTotal = 0.5 * (decPrice * intQuantity)
lblDisplayTotal.Text = "$" & decDisplayTotal
End If
If strPromoCode = ("125") Then
decDisplayTotal = (decPrice * intQuantity)
lblDisplayTotal.Text = "$" & decDisplayTotal
End If
Try
decPrice = Convert.ToInt16(txtPrice.Text)
Catch ex As Exception
lblDisplayTotal.Text = Nothing
MessageBox.Show("Please enter an acceptable price.", "Invalid Input")
txtPrice.Text = Nothing
End Try
Try
intQuantity = Convert.ToInt16(txtQuantity.Text)
Catch ex As Exception
lblDisplayTotal.Text = Nothing
MessageBox.Show("Please enter an acceptable quanitity.", "Invalid Input")
txtQuantity.Text = Nothing
End Try
Try
strPromoCode = Convert.ToInt16(txtPromoCode.Text)
Catch ex As Exception
lblDisplayTotal.Text = Nothing
MessageBox.Show("Please enter a valid Promo Code.", "Invalid Input")
txtPromoCode.Text = Nothing
End Try
End Sub
Private Sub txtPrice_TextChanged(sender As Object, e As EventArgs) Handles txtPrice.TextChanged
lblDisplayTotal.Text = ("")
End Sub
Private Sub txtQuantity_TextChanged(sender As Object, e As EventArgs) Handles txtQuantity.TextChanged
lblDisplayTotal.Text = ("")
End Sub
Private Sub txtPromoCode_TextChanged(sender As Object, e As EventArgs) Handles txtPromoCode.TextChanged
lblDisplayTotal.Text = ("")
End Sub
End Class

Answer 1

The first thing to understand clearly is that a string is not a number. VB allows this laxity but it bites back in many subtle ways.
I presume that you put the single point in any of your textboxes and then try to use that TEXT as it was a string. Sometime it works sometime not.
The correct approach is to ask the Framework to attempt the conversion and if it fails inform your user of the problem.
So, instead of using the various Convert.ToXXXXx use SomeType.TryParse (I.E. Int32.TryParse)
Dim decPrice As Decimal
Dim intQuantity As Integer
Dim strPromoCode As String
if Not Decimal.TryParse(txtPrice.Text, decPrice) Then
MessageBox.Show("Please type a valid number for Price")
ClearInputs()
return
End if
if Not Int32.TryParse(txtPrice.Text, intQuantity) Then
MessageBox.Show("Please type a valid number for Quantity")
ClearInputs()
return
End if
Private Sub ClearInputs()
lblDisplayTotal.Text = ""
txtPrice.Text = ""
txtQuantity.Text = ""
txtPromoCode.Text = ""
End Sub
Now your typed values are stored in the correct datatype variables and you could proceed with the remainder of your code.....
....
decDisplayTotal = decPrice * intQuantity
lblDisplayTotal.Text = "$" & decDisplayTotal.ToString
....
An important configuration that you need to set for your projects is Option Strict On in your project properties. This configuration will disallow the implicit conversion between strings and numbers and force you to write a more correct code.
And by the way, after the checks on the strPromoCode, you don't need to repeat again the process to convert the strings in the textboxes to the corresponding variables