vb.net - Visual Basic はテキストボックスから数字を追加しませんか?

Question

だから私はこのプログラムを持っています。アイテムの販売価格を計算し、税金、小計、割引率を表示します。これらはすべて正常に機能します。私がやろうとしているのは、小計、割引額、平均割引を累積する 3 つのテキスト ボックスを表示することです。

何かを入力するたびに、蓄積されるはずのテキストボックスは、他のボックスの内容を表示するだけで、基本的にボックスを複製します

Dim numberOfInvoices As Integer
Dim totalOfInvoices As Decimal
Dim invoiceAverage As Decimal

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim subtotal As Decimal = CDec(txtEnterSubtotal.Text)
    Dim discountPercent As Decimal = 0.25D
    Dim discountAmount As Decimal = Math.Round(subtotal * discountPercent, 2)
    Dim invoiceTotal As Decimal = subtotal - discountAmount
    Dim enterSubtotal As Decimal = Val(txtEnterSubtotal.Text)
    Dim accumDiscount As Decimal = Val(txtAccumDiscAmount.Text)
    Dim avgDiscount As Decimal = Val(txtAccumDiscAmount.Text)
    Dim accumSubTotal As Decimal = Val(txtAccumSubtotal.Text)

    txtSubtotal.Text = FormatCurrency(subtotal)
    txtDiscountPercent.Text = FormatPercent(discountPercent, 1)
    txtDiscountAmount.Text = FormatCurrency(discountAmount)
    txtTotal.Text = FormatCurrency(invoiceTotal)

    numberOfInvoices += 1
    totalOfInvoices += invoiceTotal
    invoiceAverage = totalOfInvoices / numberOfInvoices

    txtNumberOfInvoices.Text = numberOfInvoices.ToString
    txtTotalOfInvoices.Text = FormatCurrency(totalOfInvoices)
    txtInvoiceAverage.Text = FormatCurrency(invoiceAverage)

    'below is where I'm trying to accumulate everything entered....
    txtAccumSubtotal.Text = FormatCurrency(accumSubTotal + subtotal)
    txtAccumDiscAmount.Text = FormatCurrency(accumDiscount + discountAmount)

    If avgDiscount = 0 Then
        txtAvgDiscAmount.Text = FormatCurrency(discountAmount)
    ElseIf avgDiscount > 0 Then
        txtAvgDiscAmount.Text = FormatCurrency(avgDiscount / numberOfInvoices)
    End If

    txtEnterSubtotal.Text = ""
    txtEnterSubtotal.Select()
    'This is a comment
End Sub