所以我想知道为什么当我点击“下一个项目”按钮时它告诉我＆＃34;请输入有效数量＆＃34;即使我在文本框中输入了1。当我点击计算时，没有错误。
为什么它会告诉我单击“下一个项目”按钮后我还没有输入有效数量？
Public Class Form1
'Modular Variable Declaration Section
Dim mintOrdersPlacedToday As Integer
Dim msngTotalOfOrdersToday As Single
Dim msngShippingCost As Single = -1
Const csngSalesTaxRate As Single = 0.0625
Dim msngItemPrice As Single = -1
Dim msngSubtotal As Single
Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
'Local Variable Declaration Section
Dim sngSalesTax As Single
Dim sngOrderTotal As Single
Dim intQuantity As Integer
Dim sngItemTotal As Single
'Data Input Section
If msngShippingCost = -1 Then
MessageBox.Show("Please choose a shipping method", "Error", MessageBoxButtons.OK) 'Displays an error messsage when no shipping method is chosen
Exit Sub 'Terminates the click event to allow shipping method to be chosen first
End If
Try 'Checks to see if the price is a valid number.
'If it is, then it is assigned to the quantity variable, if not, error message and the event is halted.
intQuantity = CInt(txtQuantity.Text)
Catch ex As Exception
MessageBox.Show("Please enter a valid quantity.", "Error", MessageBoxButtons.OK) 'Displays an error messsage when there is an invalid number
txtQuantity.SelectAll() 'Selects the text
txtQuantity.Focus() 'Puts the cursor in the quantity textbox
Exit Sub 'Terminates the click event to allow valid input.
End Try
'Calculation Section
sngItemTotal = msngItemPrice * intQuantity
msngSubtotal += sngItemTotal 'Calculates the subtotal
sngSalesTax = msngSubtotal * csngSalesTaxRate 'Calculates Sales Tax based on the sales tax rate constant
sngOrderTotal = msngSubtotal + sngSalesTax + msngShippingCost 'Calculates total for the sale
mintOrdersPlacedToday = mintOrdersPlacedToday + 1 'Calculates the number of orders placed today, adds one to the previous number
msngTotalOfOrdersToday = msngTotalOfOrdersToday + sngOrderTotal 'Calculates the Total of all the orders placed today
'Output section
lblShowSubTotal.Text = FormatCurrency(msngSubtotal) 'Displays the Sub Total
lblShowSalesTax.Text = FormatCurrency(sngSalesTax) 'Displays the Sales Tax
lblShowOrderTotal.Text = FormatCurrency(sngOrderTotal) 'Displays the Order Total
lblShowOrdersPlacedToday.Text = mintOrdersPlacedToday 'Displays the Orders placed today
lblShowTotalOfOrders.Text = FormatCurrency(msngTotalOfOrdersToday) 'Displays the Total of the Orders placed today
lblShowShippingCost.Text = FormatCurrency(msngShippingCost) 'Displays the total of the shipping cost
End Sub
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
Dim result = MessageBox.Show(" Are you sure you want to exit?", "Are you sure?", MessageBoxButtons.YesNo) 'Shows a messagebox for the user asking if they want to exit the program and gives them options.
If result = DialogResult.Yes Then 'States that if the user clicks Yes, the program will close
Me.Close() 'Exits the program
End If
End Sub
Private Sub btnClearCurentSale_Click(sender As Object, e As EventArgs) Handles btnClearCurentSale.Click, btnNextItem.Click, btnNextSale.Click
'Clears the information from current sale and resets the form for the next sale
radPickup.Checked = True 'Checks the Pickup radio button
radPickup.Checked = False 'Unchecks the Pickup radio button
btnCalculate.Enabled = True 'Enables the Calculate button
msngShippingCost = -1 'Sets Shipping Cost to -1
lblShowShippingCost.Text = "" 'Clears the Shipping Cost label
lblShowOrderTotal.Text = "" 'Clears the Order Total label
lblShowSalesTax.Text = "" 'Clears the Sales Tax label
lblShowSubTotal.Text = "" 'Clears the Sub Total label
txtQuantity.Text = "" 'Clears the Quantity text box
txtQuantity.Focus() 'Puts the cursor in the Quantity text box
End Sub
Private Sub radPickup_CheckedChanged(sender As Object, e As EventArgs) Handles radPickup.CheckedChanged
msngShippingCost = 0 'Sets shipping cost as $0
lblShowShippingCost.Text = "Free" 'Sets Shipping Cost label to show $0
lblShowOrderTotal.Text = "" 'Clears the Order Total label
End Sub
Private Sub radGround_CheckedChanged(sender As Object, e As EventArgs) Handles radGround.CheckedChanged
msngShippingCost = 6.75 'Sets shipping cost as $6.75
lblShowShippingCost.Text = FormatCurrency(6.75, 2) 'Sets Shipping Cost label to show $6.75
lblShowOrderTotal.Text = "" 'Clears the Order Total label
End Sub
Private Sub radTwoDay_CheckedChanged(sender As Object, e As EventArgs) Handles radTwoDay.CheckedChanged
msngShippingCost = 12 'Sets shipping cost as $12
lblShowShippingCost.Text = FormatCurrency(12, 2) 'Sets Shipping Cost label to show $12
lblShowOrderTotal.Text = "" 'Clears the Order Total label
End Sub
Private Sub btnReset_Click(sender As Object, e As EventArgs) Handles btnReset.Click
'Clears the information for everything on the form
txtQuantity.Text = "" 'Clears the Quantity text box
txtQuantity.Focus() 'Puts the cursor in the Quantity text box
lblShowSubTotal.Text = "" 'Clears the Sub Total label
lblShowSalesTax.Text = "" 'Clears the Sales Tax label
lblShowShippingCost.Text = "" 'Clears the Shipping Cost label
lblShowOrderTotal.Text = "" 'CLears the Order Total label
lblShowOrdersPlacedToday.Text = "" 'Clears the Orders Placed Today label
lblShowTotalOfOrders.Text = "" 'Clears the Total of Orders Today label
mintOrdersPlacedToday = 0 'Resets the counter
msngTotalOfOrdersToday = 0 'Resets the accumulator
End Sub
Private Sub btnNextSale_Click(sender As Object, e As EventArgs) Handles btnNextSale.Click
radShirts.Checked = True
radShirts.Checked = False
radGround.Checked = True
radGround.Checked = False
msngSubtotal = 0
msngItemPrice = -1
msngShippingCost = -1
lblShowSubTotal.Text = ""
lblShowShippingCost.Text = ""
lblShowOrderTotal.Text = ""
txtQuantity.Text = ""
txtQuantity.Focus()
End Sub
Private Sub radShirts_CheckedChanged(sender As Object, e As EventArgs) Handles radShirts.CheckedChanged
msngItemPrice = 10
End Sub
Private Sub radPremiumShirts_CheckedChanged(sender As Object, e As EventArgs) Handles radPremiumShirts.CheckedChanged
msngItemPrice = 20
End Sub
Private Sub radHats_CheckedChanged(sender As Object, e As EventArgs) Handles radHats.CheckedChanged
msngItemPrice = 15
End Sub
Private Sub radStickers_CheckedChanged(sender As Object, e As EventArgs) Handles radStickers.CheckedChanged
msngItemPrice = 5
End Sub
Private Sub btnNextItem_Click(Sender As Object, e As EventArgs) Handles btnNextItem.Click
'Local Variable Declaration Section
Dim sngSalesTax As Single
Dim sngOrderTotal As Single
Dim intQuantity As Integer
Dim sngItemTotal As Single
'Data Input Section
Try 'Checks to see if the quantity is a valid number
intQuantity = CInt(txtQuantity.Text)
Catch ex As Exception
MessageBox.Show("Please enter a valid quantity.", "Error", MessageBoxButtons.OK) 'Displays an error messsage when there is an invalid number
txtQuantity.SelectAll() 'Selects the text
txtQuantity.Focus() 'Puts the cursor in the quantity textbox
Exit Sub 'Terminates the click event to allow valid input.
End Try
'Calculation Section
sngItemTotal = msngItemPrice * intQuantity 'Calculates item total
msngSubtotal += sngItemTotal 'Calculates the subtotal
sngSalesTax = msngSubtotal * csngSalesTaxRate 'Calculates Sales Tax based on the sales tax rate constant
sngOrderTotal = msngSubtotal + sngSalesTax + msngShippingCost 'Calculates total for the sale
mintOrdersPlacedToday = mintOrdersPlacedToday + 1 'Calculates the number of orders placed today, adds one to the previous number
msngTotalOfOrdersToday = msngTotalOfOrdersToday + sngOrderTotal 'Calculates the Total of all the orders placed today
'Output section
lblShowSubTotal.Text = FormatCurrency(msngSubtotal) 'Displays the Sub Total
lblShowSalesTax.Text = FormatCurrency(sngSalesTax) 'Displays the Sales Tax
lblShowOrderTotal.Text = FormatCurrency(sngOrderTotal) 'Displays the Order Total
lblShowOrdersPlacedToday.Text = mintOrdersPlacedToday 'Displays the Orders placed today
lblShowTotalOfOrders.Text = FormatCurrency(msngTotalOfOrdersToday) 'Displays the Total of the Orders placed today
End Sub
End Class
答案 0 :(得分：1)
btnNextItem.Click有两个事件处理程序：
Private Sub btnClearCurentSale_Click(sender As Object, e As EventArgs) Handles btnClearCurentSale.Click, btnNextItem.Click, btnNextSale.Click
和
Private Sub btnNextItem_Click(Sender As Object, e As EventArgs) Handles btnNextItem.Click
在btnClearCurrentSale_Click中，将txtQuantity设置为＆＃34;＆＃34; 。这是在CInt之前执行的，因此当您尝试将其转换为整数时，txtQuantity.text始终为空。
你应该从btnClearCurentSale的句柄部分删除btnNextItem.click。