在子过程中使用消息框来阻止VB中的用户错误

Question

我一直试图阻止用户在文本框中输入字母。我尝试使用If语句在出现用户错误时显示一个消息框，但程序仍然崩溃。

崩溃的代码行是：

hours = CDec(txtHoursWorked.Text)
payPerHour = CDec(txtHourlyPay.Text)

我试过了：

小时= CDec（IsNumeric（txtHoursWorked.Text））

payPerHour = CDec（IsNumeric（txtHourlyPay.Text））

但这导致我的计算不正确

错误是：

  

发生了System.InvalidCastException

我的完整代码是：

Option Strict On
Option Explicit On
Public Class frmPay

    Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
        Dim hours, payPerHour, overtimeHours, pay As Decimal
        InputData(hours, payPerHour)
        CalculateValues(hours, payPerHour, overtimeHours, pay)
        DisplayData(overtimeHours, pay)
    End Sub
    Sub InputData(ByRef hours As Decimal, ByRef payPerHour As Decimal)
        If IsNumeric(txtHoursWorked.Text) = False Then
            MessageBox.Show("Please Enter A Value")
        ElseIf IsNumeric(txtHourlyPay.Text) = False Then
            MessageBox.Show("Please Enter A Value")
        End If
        If (txtHoursWorked.Text) = "" Then
            MessageBox.Show("Please Enter A Value")
        ElseIf (txtHourlyPay.Text) = "" Then
            MessageBox.Show("Please Enter A Value")
        End If
        hours = CDec(txtHoursWorked.Text)
        payPerHour = CDec(txtHourlyPay.Text)
    End Sub
    Sub CalculateValues(hours As Decimal, payPerHour As Double, ByRef overtimeHours As Decimal, ByRef pay As Decimal)
        overtimeHours = Math.Abs(40 - hours)
        If hours <= 40 Then
            pay = CDec((payPerHour * hours))
        ElseIf hours > 40 Then
            pay = CDec((CDec(payPerHour * hours) + (payPerHour * 1.5) * (overtimeHours)))
        End If
    End Sub

    Sub DisplayData(overtimeHours As Decimal, pay As Decimal)
        txtOvertime.Text = CStr(overtimeHours)
        txtWeeksPay.Text = pay.ToString("C")
    End Sub
End Class

我的代码中缺少什么或做错了什么？