Why is skipping the else part of my code? - vb.net

Question

'REQUIREMENTS:Write a Visual Basic procedure called CalculateTotalCost that reads user-entered data from the txtQuantity and txtUnitCost TextBox controls. The CalculateTotalCost procedure should convert the text entered in the two TextBox controls into numbers. It should then multiple the two numbers together, apply the appropriate discount based on the quantity ordered, and display the result in the lblTotalCost Label control.
The following error-checking rules apply:
a. The text entered by the user in the txtQuantity TextBox control must represent a non-negative Integer. If it does not, the procedure should output the phrase “Invalid quantity!” in the lblTotalCost Label control and no further processing should take place.
b. The text entered by the user in the txtUnitCost TextBox control must represent a non-negative Double. If it does not, the procedure should output the phrase “Invalid unit cost!” in the lblTotalCost Label control and no further processing should take place.
Assuming no user input errors, the properly discounted total presented in the lblTotalCost Label control should be displayed in currently format. The display should contain a leading currency symbol (depending on how the computer was set up, this will probably be a dollar sign) and exactly two trailing digits after the included decimal point.
Public Class Form1
Private Sub lblTotalCost_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lblTotalCost.Click
'Author: Eric Konga_ 14200694 _BCIT/3_ The Papaua New Guinea University of Technology
' this program will read read user entered data from the two text boxes on the form and
' will calcualte (Multiply) the two numbers together and will then apply the appropriate discount
'based on the quantity ordered, and display the result(Total Cost) in the Label control.
'Declaring Variables as strings. This sets will out put to the screen the appropriate percentages
'based the quantity ordered.
Dim strDiscount As String
Dim strDiscount1 As String
Dim strDiscount2 As String
Dim strDiscount3 As String
Dim strDiscount4 As String
'declaring variables as integer, double and long. this sets of program will output to the screen
'
Dim intQuantity As Integer
Dim dblUnitCost As Double
Dim dblTotalCost As Double
'Assigning Variables
strDiscount = "0%"
strDiscount1 = "20%"
strDiscount2 = "30%"
strDiscount3 = "40%"
strDiscount4 = "50%"
' This is a mathematical calculator that calculates the TotalCost (TC).
intQuantity = txtQuantity.Text
dblUnitCost = txtUnitCost.Text
dblTotalCost = intQuantity * dblUnitCost
If intQuantity <= 9 Then
lblTotalCost.Text = "The Total Cost is: $" & String.Format("{0:n2}", dblTotalCost) & " and it's " & strDiscount & _
" Discount."
ElseIf intQuantity <= 19 Then
lblTotalCost.Text = "The Total Cost is: $" & String.Format("{0:n2}", dblTotalCost) & " and it's " & strDiscount1 & _
" Discount."
ElseIf intQuantity <= 49 Then
lblTotalCost.Text = "The Total Cost is: $" & String.Format("{0:n2}", dblTotalCost) & " and it's " & strDiscount2 & _
" Discount."
ElseIf intQuantity <= 99 Then
lblTotalCost.Text = "The Total Cost is: $" & String.Format("{0:n2}", dblTotalCost) & " and it's " & strDiscount3 & _
" Discount."
ElseIf intQuantity >= 100 Then
lblTotalCost.Text = "The Total Cost is: $" & String.Format("{0:n2}", dblTotalCost) & " and it's " & strDiscount4 & _
" Discount."
' under this condition, it will only execute if the integer(QTY) is negative or
'the unser entered float(UC) is negative.
Else
lblTotalCost.Text = (" Invalid Quantity!" & " or Ivalid Unit Cost!")
End If
End Sub
End Class

Answer 1

Because your first if condition is <= 9. This includes all negative integers.