Question

我正在研究一个vb程序，我遇到了错误（呃）。我试图在消息框中显示用户输入（文本框）的内容，但是它抛出了一个`System.InvalidCastException＆＃39;。这是我的代码：

Private Sub ThirteenButton1_Click(sender As Object, e As EventArgs) Handles ThirteenButton1.Click
    ' withdrawBtn
    If IsNumeric(withdrawTxtBox.Text) Then
        If Val(withdrawTxtBox.Text) > Val(My.Settings.accountBalanceBA) Then
            MessageBox.Show("Error: You can not withdraw " + withdrawTxtBox.Text + ", you only have " + My.Settings.accountBalanceBA.ToString + ".")
        ElseIf Val(withdrawTxtBox.Text) < Val(My.Settings.accountBalanceBA) Then
            MessageBox.Show("Successfully withdrawn $" + Val(withdrawTxtBox.Text) + ". Your remaining balance is $" + Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA.ToString)
        End If
    End If
End Sub

输入数字＆gt;帐户balanceBA将无错误地工作，但一旦我输入一个数字＆lt;平衡，它会引发错误。

非常感谢任何帮助。

Answer 1

该行

   MessageBox.Show("Successfully withdrawn $" + Val(withdrawTxtBox.Text) + ". Your remaining balance is $" + Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA.ToString)

特别是

Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA.ToString

正试图从数字中减去一个字符串。使用括号来获得正确的操作顺序。

(Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA).ToString

Answer 2

一点建议..在执行连接时，避免使用加号（+），特别是当你有数学条件连接字符串时。你可以使用“＆amp;”而不是连接。

将“加号”替换为“＆amp;”最佳实践。

要更正您的代码..您可以在关闭括号后移除.ToString或将.ToString放在减法之后。 ).ToString您必须小心使用括号以避免意外结果，否则最糟糕的情况会遇到错误。

    MessageBox.Show("Successfully withdrawn $" & Val(withdrawTxtBox.Text) & ". Your remaining balance is $" & Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA)

    MessageBox.Show("Successfully withdrawn $" & Val(withdrawTxtBox.Text) & ". Your remaining balance is $" & (Val(withdrawTxtBox.Text) - My.Settings.accountBalanceBA).ToString

Visual Basic无法显示文本框信息？

2 个答案: