[2005] Newbie Help String & Double Trouble

[brosta]

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."

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.