我试图有错误消息显示，如果用户不插入任何数字，在字母或符号类型，如果他们在一个数字的类型不是在1-100之间。如果您添加值，GUI工作正常，但尝试触发消息时会使GUI崩溃。
这是我到目前为止所拥有的：

Private Sub btnCalcaulate_Click(sender As Object, e As EventArgs) Handles btnCalcaulate.Click

    Dim int_grade1 As Integer = TxtboxNum1.Text
    Dim int_grade2 As Integer = txtBoxNum2.Text
    Dim int_grade3 As Integer = txtBoxNum3.Text

    Dim int_avgGrade As Integer = (int_grade1 + int_grade2 + int_grade3) / 3

    lblAverageNum.Text = int_avgGrade



    If (TxtboxNum1.Text = "") Then
        MsgBox("Please Enter a Value for Number 1")
        TxtboxNum1.Focus()
        LblNumber1.ForeColor = Color.Red

    ElseIf IsNumeric(TxtboxNum1.Text) = False Then
        MsgBox("Number 1 value must be a number")
        TxtboxNum1.Focus()
        LblNumber1.ForeColor = Color.Red

    ElseIf TxtboxNum1.Text > 100 Or TxtboxNum1.Text < -1 Then
        MsgBox("Number 1 value must be between 1-100")
        TxtboxNum1.Focus()
        LblNumber1.ForeColor = Color.Red

    ElseIf (txtBoxNum2.Text = "") Then
        MsgBox("Please Enter a Value for Number 2")
        txtBoxNum2.Focus()
        lblNumber2.ForeColor = Color.Red

    ElseIf IsNumeric(txtBoxNum2.Text) = False Then
        MsgBox("Number 2 value must be a number")
        txtBoxNum2.Focus()

    ElseIf txtBoxNum2.Text > 100 Or txtBoxNum2.Text < -1 Then
        MsgBox("Number 2 value must be between 1-100")
        txtBoxNum2.Focus()
        lblNumber2.ForeColor = Color.Red

    ElseIf (txtBoxNum3.Text = "") Then
        MsgBox("Please Enter a Value for Number 3")
        txtBoxNum3.Focus()
        lblNumber3.ForeColor = Color.Red

    ElseIf IsNumeric(txtBoxNum3.Text) = False Then
        MsgBox("Number 3 value must be a number")
        txtBoxNum3.Focus()

    ElseIf txtBoxNum3.Text > 100 Or txtBoxNum3.Text < -1 Then
        MsgBox("Number 3 value must be between 1-100")
        txtBoxNum3.Focus()
        lblNumber3.ForeColor = Color.Red

    ElseIf lblAverageNum.Text >= 60 Then
        lblAverageNum.ForeColor = Color.Green

    ElseIf lblAverageNum.Text < 60 Then
        lblAverageNum.ForeColor = Color.Red
    Else
        int_grade1 = CInt(TxtboxNum1.Text)
        int_grade2 = CInt(txtBoxNum2.Text)
        int_grade3 = CInt(txtBoxNum3.Text)
        int_avgGrade = CInt(lblAverageNum.Text)



    End If
End Sub

虽然计算工作，当我试图激活错误消息（空格，输入字母或非数字，或数字不之间（0-100））但它崩溃我的程序，而是显示一个错误，在我的程序说“从字符串转换“”类型''是无效的。