乘法应用程序，可视基础。无效的强制转换例外

Question

Public Class Form1
Dim randomNumberOne As New Random
Dim randomNumberTwo As New Random

Dim ChildGrade As Integer
Dim strChildGrade As Integer
Dim strcorrectCounter As Integer
Dim Correctcounter As Integer = 0
Dim WrongCounter As Integer = 0
Dim totalQuestions As Integer = 0
Dim percent As Integer = 0



Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    ChildGrade = InputBox("Enter your age ", "age")
    grade()

    FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
    SecondNumberlbl.Text = randomNumberOne.Next(0, 12)

End Sub

Private Sub grade()
    If ChildGrade <= 4 Then
        Gradelbl.Text = "Preschool"
    ElseIf ChildGrade = 5 Then
        Gradelbl.Text = "Kindergarden"
    ElseIf ChildGrade <= 10 Then
        Gradelbl.Text = "Elementary"
    ElseIf ChildGrade <= 13 Then
        Gradelbl.Text = "Middle School"
    ElseIf ChildGrade <= 18 Then
        Gradelbl.Text = "High School"
    ElseIf ChildGrade > 18 Then
        Gradelbl.Text = "NA"
    End If
End Sub



Private Sub RandomNumberToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles RandomNumberToolStripMenuItem.Click
    NumbersGroupBox.Enabled = False
    FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
End Sub

Private Sub SingleTableToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles SingleTableToolStripMenuItem.Click
    NumbersGroupBox.Enabled = True
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    FirstNumberlbl.Text = "1"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    FirstNumberlbl.Text = "2"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
    FirstNumberlbl.Text = "3"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
    FirstNumberlbl.Text = "4"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button5_Click(sender As Object, e As EventArgs) Handles Button5.Click
    FirstNumberlbl.Text = "5"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
    FirstNumberlbl.Text = "6"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button7_Click(sender As Object, e As EventArgs) Handles Button7.Click
    FirstNumberlbl.Text = "7"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button8_Click(sender As Object, e As EventArgs) Handles Button8.Click
    FirstNumberlbl.Text = "8"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button9_Click(sender As Object, e As EventArgs) Handles Button9.Click
    FirstNumberlbl.Text = "9"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button10_Click(sender As Object, e As EventArgs) Handles Button10.Click
    FirstNumberlbl.Text = "10"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
    FirstNumberlbl.Text = "11"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub Button12_Click(sender As Object, e As EventArgs) Handles Button12.Click
    FirstNumberlbl.Text = "12"
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

Private Sub CheckAnswer()
    If TextBox1.Text <> (FirstNumberlbl.Text * SecondNumberlbl.Text) Then
        MsgBox("Please try again", MsgBoxStyle.Information, "Answer")
        WrongCounter = WrongCounter + 1
        If WrongCounter = 2 Then
            MsgBox("The correct answer is " & FirstNumberlbl.Text * SecondNumberlbl.Text, MsgBoxStyle.Information, "Correct answer")
            WrongCounter = 0
            FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
            SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
        End If
        TextBox1.Text = ""
    End If
End Sub

Private Sub rightAnswer()

    If FirstNumberlbl.Text * SecondNumberlbl.Text = TextBox1.Text Then
        Correctcounter = Correctcounter + 1
        NumberCorrectlbl.Text = Correctcounter
        TextBox1.Text = ""
        FirstNumberlbl.Text = randomNumberOne.Next(0, 12)
        SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    End If
    totalQuestions = totalQuestions + 1

End Sub

Private Sub Enterbtn_Click(sender As Object, e As EventArgs) Handles Enterbtn.Click
    CheckAnswer()
    rightAnswer()
    TotalCountlbl.Text = totalQuestions
    PercentCorrectlbl.Text = ((Correctcounter * 100) / (totalQuestions * 100)) * 100
End Sub

结束班级

我正在VB中创建一个乘法应用程序，该应用程序统计用户正确的问题数和错误的问题数。当我运行该程序时，它可以正确地计算正确的数量并累加百分比，但是当我测试它们何时出错时，我会收到此错误。 enter image description here

如果图像不可见，则错误提示， Microsoft.VisualBasic.dll中发生了'System.InvalidCastException'类型的未处理异常

其他信息：从字符串“”到类型“ Double”的转换无效

程序指出了问题所在，位于rightAnswer（）子类的if语句中，但是我不确定它到底出了什么问题。

Answer 1

问题是您正在尝试使用字符串进行计算。字符串是文本，而不是数字。

这不是一个完整的答案，但足以使您了解问题并更改代码以解决问题。

请勿对文本框和/或标签的内容进行任何计算。对此类任务使用正确的变量。使用控件可以简单地检索和显示。从长远来看，这将为您节省很多心痛。

例如，不要：

If TextBox1.Text <> (FirstNumberlbl.Text * SecondNumberlbl.Text) Then

您正在检查文本框中的数字以根据两个标签进行计算，它们都是文本！

因此，请按照适合目的的变量（例如整数）进行计算：

示例：

Dim FirstNumber As Integer
Dim SecondNumber As Integer

将它们用于计算，然后将结果显示为文本。

如果期望浮点数（十进制），请根据需要更改变量类型。

在“ CheckAnswer”子例程中，您应该按照以下思路开始思考：

    If Not String.IsNullOrWhiteSpace(TextBox1.Text) Then
        'We have something in the TextBox (you can check to allow only numbers to be entered in the TextBox.KeyDown/Press event)
        If Not Integer.Parse(TextBox1.Text) = (FirstNumber * SecondNumber) Then
            'Do what you need ...
        End If
    End If

还有一些其他事情可以简化您的代码。

您不需要12个按钮事件都在做同一件事。

在您的Form1_Load事件中输入以下内容：

    Dim bTags() As Button = {Button1, Button2, Button3, Button4, Button5, Button6, Button7, Button8, Button9, Button10, Button11, Button12}
    For t As Integer = 0 To bTags.Count - 1
        bTags(t).Tag = t + 1
    Next

然后删除所有12个Button Click事件并将其替换为：

Private Sub Buttons_Click(sender As Object, e As EventArgs) Handles Button9.Click, Button8.Click, Button7.Click, Button6.Click, Button5.Click, Button4.Click, Button3.Click, Button2.Click, Button12.Click, Button11.Click, Button10.Click, Button1.Click
    Dim selectedButton As Button = CType(sender, Button)
    FirstNumberlbl.Text = selectedButton.Tag.ToString
    SecondNumberlbl.Text = randomNumberTwo.Next(0, 12)
    TextBox1.Focus()
End Sub

工作完成...

此外，对子程序进行评分最好将其作为Select Case函数：

在您的Form1_Load中，将grade()替换为Gradelbl.Text = Grade()，并将您的grade()子例程替换为：

Private Function Grade() As String
    Select Case ChildGrade
        Case < 5 : Return "Preschool"
        Case = 5 : Return "Kindergarden"
        Case < 11 : Return "Elementary"
        Case < 14 : Return "Middle School"
        Case < 19 : Return "High School"
        Case Else : Return "N/A"
    End Select
End Function

我希望其中的一些建议会有所帮助。

Answer 2

您需要执行的是强制转换文本框的值。例如：

Convert.toInt32(FirstNumberlbl.Text)

或

Dim firstNumber as Integer
Integer.TryParse(FirstNumberlbl.Text, firstNumber)

或将一个变量分配给文本框，然后将其转换为文本框。