I can't find the reason for InvalidCastException

Question

I am trying to make an app like an online shop for my project. The code is fine at first(at least for me since there's no errors indications) But when I run it and press the "Add to Cart" button, it says InvalidCastException and says that I'm trying to convert a string to double even though I am not converting anything

This is what I have so far

Public Class Form1

    Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
        Dim prditem As ListViewItem
        Dim price, cartprice As Integer
        Dim product As String
        If clbParts.SelectedItem = 0 Then
            product = "Power Supply"
            price = 1
        End If
        If clbParts.SelectedItem = 1 Then
            product = "CPU"
            price = 2
        End If
       ....
        If rdo512gb.Checked = True Then
            product = "Hard Drive (512GB)"
            price = 11
        End If
        If rdo1tb.Checked = True Then
            product = "Hard Drice (1TB)"
            price = 12
        End If
        cartprice = Price * numQuantity.Text
        prditem = lvCart.Items.Add(product)
        With prditem
            .SubItems.Add(numQuantity.Text)
            .SubItems.Add(cartprice)
        End With
        If numQuantity.Text = "0" Then
            MessageBox.Show("Please put the number of items you want to add", "Cart Error")
        End If
        MessageBox.Show("Item/s is/are added to your cart", "Cart")
        numQuantity.Text = "0"
    End Sub

    Private Sub btnTotal_Click(sender As Object, e As EventArgs) Handles btnTotal.Click
        Dim total As Integer = 0
        For i As Integer = 0 To lvCart.Items.Count - 1
            Dim quantity = CInt(lvCart.Items(i).SubItems(1).Text)
            Dim price = CInt(lvCart.Items(i).SubItems(2).Text)
            total += quantity + price
        Next
        txtTotal.Text = total.ToString("N2")
    End Sub

    Private Sub cboPayment_SelectedIndexChanged(sender As Object, e As EventArgs) Handles cboPayment.SelectedIndexChanged
        If cboPayment.SelectedItem = 0 Then
            Label10.Enabled = True
            cboOnline.Enabled = True
        Else
            Label10.Enabled = False
            cboOnline.Enabled = False
        End If
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        MessageBox.Show("Your items are being delivered in " + txtHomeAddress.Text + ", " + txtAddress.Text + "\n Please wait for the text confirmation", "Cart Error")
    End Sub
End Class

Answer 1

Do yourself a favor and watch a YouTube video on how to debug in Visual Studio!

What you need to do is put a Breakpoint on this line:

cartprice = Price * numQuantity.Text

convert a string to double

You're multplying a number by a String! You want to do something like this:

cartprice = Price * Convert.ToDouble(numQuantity.Text)

I'd personally do a double.TryParse to see if its valid and use the output value when performing the operation.

PS: As a challenge to yourself, try and move all the code logic to a Business Logic class, then use Bingings/BindingControls so the Presentation layer is separated from the logic. That way you can Unit Test the business logic layer!