[2005] Newbie Help String & Double Trouble

I just started taking VB 2005 in school and decided to make my own program.

Right now you enter Cost, Rebate and Sold into text boxes respectively. When you get to the Sold textbox their is a textChanged Event which displays the profit while your typing it in. When you’re done hit the "Add" button and it will calculate the profit again and display it in the list box.

This is the error I get: "Conversion from string "" to type 'Double' is not valid."

VB Code:

Public Class business
 
    Dim fmtStrg As String = "{0, -19}{1, 12:C}{2, 12:C}{3, 12:C}{4, 12:C}"
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        lstBox.Items.Add(String.Format(fmtStrg, "Item", "Cost", "Rebate", "Sold", "Profit"))
        lstBox.Items.Add(String.Format(fmtStrg, "-------------------", "-----------", "-----------", "-----------", "-----------"))
    End Sub
 
    Private Sub txtAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
        Dim Item As String
        Dim Cost, Rebate, Sold, Profit As Double
 
        If txtItem.Text = "" Then
            MsgBox("Cannot have empty item field.", 0, "Error")
        End If
 
        Item = CStr(txtItem.Text)
        Cost = CDbl(txtCost.Text)
        Sold = CDbl(txtSold.Text)
        Rebate = CDbl(txtRebate.Text)
 
        Profit = (Sold + Rebate) - Cost
 
        lstBox.Items.Add(String.Format(fmtStrg, Item, Cost, Rebate, Sold, Profit))
        txtTotalProfit.Text = FormatCurrency(Profit + CDbl(txtTotalProfit.Text))
 
        txtCost.Clear()
        txtRebate.Text = 0
        txtSold.Clear()
        txtItem.Clear()
 
        txtItem.Focus()
    End Sub
    Private Sub txtSold_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtSold.TextChanged
        Dim Cost, Rebate, Sold As Double
 
        Cost = CDbl(txtCost.Text)
        Sold = CDbl(txtSold.Text)
        Rebate = CDbl(txtRebate.Text)
 
        txtProfit.Text = CStr((Sold + Rebate) - Cost)
    End Sub
End Class

Thank You.

your error occures when one of the textboxes is empty.

when you call CDble() it converts that text to a number, but if the text passed to it is not numeric, the call will fail.

You need to implement some input validation in your application to check the values entered to ensure they will pass through the rest of code without a problem.

If you're not going to validate input as it's typed then the appropriate way to convert user input to numbers is with the TryParse method of the appropriate type. If the number is an integer then use Integer.TryParse. If it's a double then use Double.TryParse. Note also that Decimal objects are usually used for financial calculations to avoid the round-off errors that can occur with Doubles, although there's unlikely to be an issue with the figures you're dealing with.

Also, the Text property of a control is a String object so the use of CStr is redundant. CStr should only be used to cast a non-string reference to a String abject as String. In English that means that if you have a variable of type Object that refers to a String object then you should use CStr to get a reference to it of type String, e.g.

VB Code:

Dim obj As Object = "Hello World"
Dim str As String = CStr(obj)

If you want to convert an object that is not a String, like a number, to a String then you should call its ToString method, e.g.

VB Code:

txtProfit.Text = (Sold + Rebate - Cost).ToString()