无效转换 - 转换异常：'System.InvalidCastException'

Question

我编写了一个代码，只要用户正确输入信息，它就可以完美运行。如果用户确实在文本框中输入了数字，我会收到一条错误消息，告诉用户必须输入数据。发生这种情况时，我得到一个转换异常：

Exception thrown: 'System.InvalidCastException' in Microsoft.VisualBasic.dll

Additional information: Conversion from string "" to type 'Integer' is not valid.

我是新手，如果有任何建议，我将不胜感激。

异常发生在这一行：

intDetermineSubTotalCost(intSiteSelected, CInt(txtNumberOfNights.Text))

如果我删除 strict on 选项，它将与预期的对话一起工作，但项目要求它打开。

Dim intSiteSelected As Double
Dim intNumberOfNights As Integer
Dim blnNightsStayingIsVaild As Boolean = False
Dim blnDiscountIsSelected As Boolean = False
Dim intDiscountChoice As Integer
Dim strDiscountSelected As String = ""

'   Call a function to confirm if the Number of Nights staying is valid.
blnNightsStayingIsVaild = ValidateNightsStaying()

'   call a function to confirm a discount has been seected.
intDiscountChoice = ValidateSelectedDiscount(blnDiscountIsSelected, strDiscountSelected)

'   If number of nights staying and available discount has been selected,
'   calculate the subtotal, tax, and final cost.
If (blnNightsStayingIsVaild And blnDiscountIsSelected) Then
    intNumberOfNights = Convert.ToInt32(txtNumberOfNights.Text)
    intSiteSelected = Me.cmbSelectASite.SelectedIndex
End If


If cmbSelectASite.SelectedIndex = 0 Then
    intSiteSelected = 20D
ElseIf cmbSelectASite.SelectedIndex = 1 Then
    intSiteSelected = 35D
ElseIf cmbSelectASite.SelectedIndex = 2 Then
    intSiteSelected = 55D
End If

intDetermineSubTotalCost(intSiteSelected, CInt(txtNumberOfNights.Text))


Private Function ValidateNightsStaying() As Boolean

'   This function validate the value entered for the number of nights staying in campground.
Dim intNightsStaying As Integer
Dim blnValidityCheck As Boolean = False
Dim StrNumberOfNightsErrorMessage As String =
    "Please Enter The Number Of Nights You Are Staying (1-99)"
Dim strMessageBoxTitle As String = " You Must Choose Number Of Nights"

Try
    intNightsStaying = Convert.ToInt32(txtNumberOfNights.Text)
    If intNightsStaying > 0 And intNightsStaying < 100 Then
        blnValidityCheck = True
    Else
        MsgBox(StrNumberOfNightsErrorMessage, , strMessageBoxTitle)
        blnValidityCheck = True
    End If
Catch Exception As FormatException
    MsgBox(StrNumberOfNightsErrorMessage, , strMessageBoxTitle)
    txtNumberOfNights.Focus()
    txtNumberOfNights.Clear()
Catch execption As OverflowException
    MsgBox(StrNumberOfNightsErrorMessage, , strMessageBoxTitle)
    txtNumberOfNights.Focus()
    txtNumberOfNights.Clear()
Catch execption As SystemException
    MsgBox(StrNumberOfNightsErrorMessage, , strMessageBoxTitle)
    txtNumberOfNights.Focus()
    txtNumberOfNights.Clear()
End Try
Return blnValidityCheck

End Function

Answer 1

有几种方法可以解决这个问题。

您可以检查字符串是否为空并对其进行验证。
将转换更改为 IIRC 将 return 0 为空的 TryParse
如果可能，将输入从文本框更改为数字 up/down 控件，最小值和最大值分别为 1 和 99。

Answer 2

你可以看看这样的东西，它会限制他们输入非数字按键。

Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress

'97 - 122 = Ascii codes for simple letters
'65 - 90  = Ascii codes for capital letters
'48 - 57  = Ascii codes for numbers

If Asc(e.KeyChar) <> 8 Then
    If Asc(e.KeyChar) < 48 Or Asc(e.KeyChar) > 57 Then
        e.Handled = True
    End If
End If

End Sub

Answer 3

只需在执行计算之前进行一些输入检查。例如需要输入的文本框：

        If txtInput.text.Contains("whatever you don't want there") Then
           MessageBox.Show("Please Enter The Correct Info")
           txtInput.Focus()
           Return
        End if

或

        If Integer.TryParse(txtInput.Text, input) Then
            MessageBox.Show("Please Enter Some Text.")
            txtInput.Focus()
            Return
        End If

这会阻止代码在出现错误时崩溃，并让用户有机会修复他们的输入。检查完所有输入后，执行计算。

Answer 4

将您的 TextBox 更改为 Masked TextBox。掩码中只允许数字。

https://msdn.microsoft.com/en-us/library/system.windows.forms.maskedtextbox%28v=vs.110%29.aspx

无效转换 - 转换异常：'System.InvalidCastException'

Invalid Conversion - Cast Exception: 'System.InvalidCastException'

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