Input string was not in a correct format in event textbox changed with VB.NET [closed]

Question

I'm Trying to use event textbox change but I have an error "Input string was not in a correct format "

Is there something wrong with my code ?

or

Is there something wrong with my code implementation ?

Please Guide me

Thanks

Public Class Payment
    Private salesservices As New SalesService()
    Private Sub Payment_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim sales = salesservices.GetSales(LblInvono.Text)
        LabelTotalvalue.Text = sales.Totalinv.ToString("N0")
        txtDiscount.Text = sales.DiscountTotal.ToString("N0")
        txtPayment.Text = (sales.CashPayment + sales.NCashPayment).ToString("N0")
        LabelChange.Text = ((sales.CashPayment + sales.NCashPayment + sales.DiscountTotal) - sales.Totalinv).ToString("N0")
    End Sub
Private Sub OnTextChanged(sender As Object, e As EventArgs) Handles txtPayment.TextChanged, txtDiscount.TextChanged
        Dim payment As Double = 0
        Dim discount As Double = 0
        If Not String.IsNullOrEmpty(txtPayment.Text) OrElse Not String.IsNullOrEmpty(txtDiscount.Text) Then
           'Error code below this line  
           payment = Convert.ToDouble(txtPayment.Text)
            discount = Convert.ToDouble(txtDiscount.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - payment).ToString("N0")
        Else
            LabelChange.Text = "-" & Convert.ToDouble(LabelTotalvalue.Text).ToString("N0")
        End If
    End Sub
    Private Sub OnText_KeyPress(sender As Object, e As KeyPressEventArgs) Handles txtPayment.KeyPress, txtDiscount.KeyPress
        If Not Char.IsControl(e.KeyChar) AndAlso Not Char.IsDigit(e.KeyChar) Then
            e.Handled = True
        End If
    End Sub
    Private Sub txtPayment_KeyUp(sender As Object, e As KeyEventArgs) Handles txtPayment.KeyUp
        txtPayment.Text = txtPayment.Text.Replace(",", "")
        Dim discount As Double = 0
        Dim payment As Double = 0
        If Not String.IsNullOrEmpty(txtPayment.Text) Then
            payment = Convert.ToDouble(txtPayment.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - payment).ToString("N0")
            txtPayment.Text = Convert.ToDouble(txtPayment.Text).ToString("N0")
            txtPayment.[Select](txtPayment.Text.Length, 0)
        End If
    End Sub
    Private Sub txtDiscount_KeyUp(sender As Object, e As KeyEventArgs) Handles txtDiscount.KeyUp
        txtDiscount.Text = txtDiscount.Text.Replace(",", "")
        Dim discount As Double = 0
        Dim payment As Double = 0
        If Not String.IsNullOrEmpty(txtDiscount.Text) Then
            discount = Convert.ToDouble(txtDiscount.Text)
            LabelChange.Text = (Convert.ToDouble(LabelTotalvalue.Text) - discount - Payment).ToString("N0")
            txtDiscount.Text = Convert.ToDouble(txtDiscount.Text).ToString("N0")
            txtDiscount.[Select](txtDiscount.Text.Length, 0)
        End If
    End Sub
End Class

Answer 1

Method Convert.ToDouble(...) works only if you are sure that the string contains a numerical value. Otherwise, it's better to use Double.TryParse method, which, as the documentation says, 'does not throw an exception if the conversion fails'.

Here's an example of how you can rewrite the lines that causes the error. Instead of:

payment = Convert.ToDouble(txtPayment.Text)

write

Double.TryParse(txtPayment.Text, payment)

Answer 2

You need AndAlso, not OrElse.

If Not String.IsNullOrEmpty(txtPayment.Text) AndAlso Not String.IsNullOrEmpty(txtDiscount.Text) Then

Or this:

If Not (String.IsNullOrEmpty(txtPayment.Text) OrElse String.IsNullOrEmpty(txtDiscount.Text)) Then

Your current code is explicitly requiring one to be Nothing (or Null).