[RESOLVED] conversion from string to type 'double' is not valid

[c0deNinja]

Imports System.Net

Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        lstboxoption.Items.Add("IP Address")
        lstboxoption.Items.Add("Source Code")
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        If lstboxoption.SelectedItem = 0 Then
            Form2.Show()
        End If
    End Sub
End Class

I want to make it so that when a user clicks on the first item which is "IP Address" to show Form2, I'm not sure if I'm using the SelectedItem correctly. I keep getting a "conversion from string to type double is not valid" error.

[jmcilhinney]

Look at this code:

lstboxoption.Items.Add("IP Address")
lstboxoption.Items.Add("Source Code")

You are adding to the Items collection of the ListBox the values "IP Address" and "Source Code". Those are the only items in the ListBox so those are the only items the user can select. If that's the case, when the user makes a selection, how can the SelectedItem possibly be 0? If the user selected the item "IP Address" then the SelectedItem will be "IP Address". That is the first item so its index is 0, thus the SelectedIndex will be 0.

I have two pieces of advice that will help you avoid situations like this in future. Firstly, ALWAYS read the documentation. The VS Help menu will open the documentation either locally or online. Either way, you can open the topic for the ListBox class and read what each member means and does, and often see code examples. That won't always answer all your questions but it should still be the first option when you need help. The more often you do it, the better you get at it and the more issues you'll be able to fix doing it. You'll also come across information that you weren't looking for that will help later quite regularly. I speak from personal experience, as someone who has used the MSDN documentation as my primary source of information since the first day I used VB.NET.

Secondly, learn to debug properly. That doesn't mean just reading the code or even just running it. It means running it, setting breakpoints and stepping through the code line by line, examining the state as you go. In this case, doing that would have shown you exactly what SelectedItem and SelectedIndex contained. If you don't know how to debug, start learning here:

https://msdn.microsoft.com/en-us/lib...or=-2147217396

[c0deNinja]

Look at this code:

lstboxoption.Items.Add("IP Address")
lstboxoption.Items.Add("Source Code")

You are adding to the Items collection of the ListBox the values "IP Address" and "Source Code". Those are the only items in the ListBox so those are the only items the user can select. If that's the case, when the user makes a selection, how can the SelectedItem possibly be 0? If the user selected the item "IP Address" then the SelectedItem will be "IP Address". That is the first item so its index is 0, thus the SelectedIndex will be 0.

I have two pieces of advice that will help you avoid situations like this in future. Firstly, ALWAYS read the documentation. The VS Help menu will open the documentation either locally or online. Either way, you can open the topic for the ListBox class and read what each member means and does, and often see code examples. That won't always answer all your questions but it should still be the first option when you need help. The more often you do it, the better you get at it and the more issues you'll be able to fix doing it. You'll also come across information that you weren't looking for that will help later quite regularly. I speak from personal experience, as someone who has used the MSDN documentation as my primary source of information since the first day I used VB.NET.

Secondly, learn to debug properly. That doesn't mean just reading the code or even just running it. It means running it, setting breakpoints and stepping through the code line by line, examining the state as you go. In this case, doing that would have shown you exactly what SelectedItem and SelectedIndex contained. If you don't know how to debug, start learning here:

https://msdn.microsoft.com/en-us/lib...or=-2147217396

Thank you for your help sir, and I will definitely follow your advice,