[RESOLVED] [02/03] Need help with data validation function

[Confused.Net]

I am taking VB.NET programming and am at wits end with trying to figure out a function for both numeric and range validation. I was hoping someone could explain it better than my book. The exercise question says to add a function that will validate numeric and range using order total as the argument and return a value of 0 if it's valid data, 1 if it is not numeric, and 2 if it's out of the range. The calling procedure should process the data or display an error message based on the value returned by the function. It also says to move the range checking variables to the module level so they're available to both the function and calling procedure. I can get one validation to work but can't figure out how to get both to work together. I am so frustrated I want to give up. If someone can help I would really appreciate it. Thanks.

Private Sub btnCalculate_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles btnCalculate.Click
 
        Const dMinOrderTotal As Decimal = 10
        Const dMaxOrderTotal As Decimal = 10000
 
        Dim dOrderTotal As Decimal
        Dim dDiscountPct As Decimal
        Dim dDiscountAmount As Decimal
        Dim dInvoiceTotal As Decimal
 
        If IsNumeric(txtOrderTotal.Text) Then 
            If txtOrderTotal.Text > dMinOrderTotal And _
            txtOrderTotal.Text < dMaxOrderTotal Then 
                dOrderTotal = txtOrderTotal.Text
                dOrderTotal = Math.Round(dOrderTotal, 2) 
 
                dDiscountAmount = DiscountAmount(dOrderTotal) 
 
                dInvoiceTotal = dOrderTotal - dDiscountAmount
                lblDiscountAmount.Text = FormatNumber(dDiscountAmount)
                lblInvoiceTotal.Text = FormatNumber(dInvoiceTotal)
            Else ' Out of range
                MessageBox.Show("Order total must be between " & _
                dMinOrderTotal & " and " & _
                dMaxOrderTotal & ".", "Entry error")
            End If
        Else 
            MessageBox.Show("You must enter a numeric value.", "Entry error")
        End If
        txtOrderTotal.Focus()
 
    End Sub 
 
    
    Public Function DiscountAmount(ByVal totalval As Decimal) As Decimal
        Dim discount As Decimal
        If totalval >= 500 Then
            discount = 0.2
        ElseIf totalval >= 250 Then
            discount = 0.15
        ElseIf totalval >= 100 Then
            discount = 0.1
        Else 
            discount = 0
        End If
        Dim DiscountAmt As Decimal = totalval * discount
        Return DiscountAmt
    End Function
 
 
    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click
        Me.Close()
    End Sub