Error that can't be fixed?

Question

I'm getting an error in my VB.NET application that connects to my SQL database. It connects fine, but for some reason I can't fix this error. When I try to fix it, it moves from one part of my script to another part of my script (both of which were working yesterday). The error details are:

Unfortunately, it's difficult for me to describe how I produced this result, because it has happened in multiple parts of my code, and the only thing that these parts have in common is their interaction with Listbox1.

The first part of code to get this error was:

Dim sqlpage As MySqlCommand = New MySqlCommand("SELECT * FROM [" & frmMain.ListBox1.SelectedItem.value & "]", con)

Then I got the same exact error for:

Private Sub ListBox1_SelectedValueChanged( _
    ByVal sender As Object, ByVal e As System.EventArgs) _
    Handles ListBox1.SelectedValueChanged

    Try
        Form1.Label1.Text = ListBox1.SelectedItem
        Form1.Show()
    Catch myerror As MySqlException
        MessageBox.Show("Error Setting Up Project Page: " & myerror.Message)
    End Try
End Sub

More specifically:

Form1.Label1.Text = ListBox1.SelectedItem

And then I got it a few more times, but I think the examples above will suffice.

Since there are no "With Block Variables" in the examples above then the only other option is that it's object related. I've tried different methods of defining and redefining the object variables related to the error. However, the results are the same.

In response to Juxtaposition's answer, my original problem has been solved however two new problems have come up specifically because I turned Option Strict on.

• The first is:

Error1: Option Strict On disallows late binding.

The code in question is:

Try
    ' Retrieving the projects list.
    con.Open()
    DataAdapter2.SelectCommand = sqlprojects
    DataAdapter2.Fill(ds2, "projects")
    ListBox1.Items.Clear()

    For Each DataRow In ds2.Tables("projects").Rows

        ' Error occurs on the line below
        ListBox1.Items.Add(DataRow("project_name"))
    Next
    con.Close()

Catch myerror As MySqlException
    MessageBox.Show("Error Retrieving Projects List: " & myerror.Message)
End Try

• The second is:

Error 2: Option Strict On disallows implicit conversions from 'Object' to 'String'.

The code in question is:

Private Sub ListBox1_SelectedValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedValueChanged

    Try
        If ListBox1.SelectedItem IsNot Nothing Then

            ' Error occurs on the line below
            Form1.Label1.Text = ListBox1.SelectedItem
        End If

        Form1.Show()
    Catch myerror As MySqlException
        MessageBox.Show("Error Setting Up Project Page: " & myerror.Message)
    End Try
End Sub

---

It worked out... so I thank all of you for your time and patience.

Answer 1

You should always (99.999999% of the time) write VB.NET code with Option Strict On, unless you are writing interop code or interfacing with an esoteric database provider.

Simply place the words "Option Strict On" at the top of your file.

This will allow you to catch errors like the one you are dealing with.

Without Option Strict On you are allowed to write code like you have written:

Form1.Label1.Text = ListBox1.SelectedItem

The issue with that code is that is implicity tries to convert an object (ListBox1.SelectedItem) to a string (Form1.Label1.Text).

Turn option strict on and the compiler will give you an error up front.

You will then be forced to rewrite your code as such:

If ListBox1.SelectItem IsNot Nothing then
    Form1.Label1.Text = ListBox1.SelectedItem
End If

Answer 2

Focus on this line for the moment:

Form1.Label1.Text = ListBox1.SelectedItem

If you're getting a NullReferenceException on this line, then one of the following has to be true:

• Form1 is null
• Form1.Label1 is null
• ListBox1 is null

You can try to determine this by adding lines like these just before the above line:

Console.Writeline("Form1: " & (Form1 Is Nothing))
Console.Writeline("Form1.Label1: " & (Form1.Label1 Is Nothing))
Console.Writeline("ListBox1:" & (ListBox1 Is Nothing))

You should see a line that outputs true; that's the first clue. But then the next question is, why is it null? From what you've shown so far, I can't say.

Answer 3

Make sure ListBox1.SelectedItem is not Nothing in both of those circumstances.

Answer 4

Your original errors can be fixed without having to use Option Explicit On. You need to ensure that the Listbox.SelectedItem has a value before using it. The code should be written as:

If frmMain.ListBox1.SelectedItem IsNot Nothing Then
    Dim sqlpage As MySqlCommand = New MySqlCommand("SELECT * FROM [" & frmMain.ListBox1.SelectedItem.value & "]", con)
End If 

and

Try
    If ListBox1.SelectedItem IsNot Nothing Then
        Form1.Label1.Text = ListBox1.SelectedItem
    End If

    Form1.Show()
Catch myerror As MySqlException
    MessageBox.Show("Error Setting Up Project Page: " & myerror.Message)
End Try

Update #2
Your second error should be fixed by changing the code to:

If ListBox1.SelectedItem IsNot Nothing Then
    Form1.Label1.Text = ListBox1.SelectedItem.ToString
End If

Option Explicit On means that you have to explicitly convert data types.