Error: System.NullReferenceException at combo_company.SelectedIndexChanged event - vb.net

Question

VS 2012, VB.net, Winforms (Desktop App)
When I change item of combobox (combo_company. SelectedIndexChanged event), I m getting error System.NullReferenceException.
I am trying to get value of combo_company.SelectedItem and combo_company.SelectedValue. I tried in two way to bind combobox, but what the problem is:
case 1 output:
combo_company.SelectedItem= output OK
combo_company.SelectedValue= System.NullReferenceException
case 2 output:
combo_company.SelectedItem= System.Data.DataRowview
combo_company.SelectedValue= output OK
common code for both case:
Dim da As New SqlDataAdapter
Dim cmd As New SqlCommand
Dim dt As New DataTable
cmd.Connection = con
Dim sql As String = "select company_name, company_id from tbl_company"
cmd = New SqlCommand(sql, con)
If con.State = ConnectionState.Open Then con.Close()
con.Open()
da = New SqlDataAdapter(cmd)
da.Fill(dt)
case 1 code:
combo_company.DisplayMember = "company_name"
combo_company.ValueMember = "company_id"
If dt.Rows.Count >= 1 Then
For i = 0 To dt.Rows.Count - 1
combo_company.Items.Add(dt.Rows(i).Item(0).ToString)
Next
End If
case 1 output:
combo_company.SelectedItem= output OK
combo_company.SelectedValue= System.NullReferenceException
case 2 code:
combo_company.DisplayMember = "company_name"
combo_company.ValueMember = "company_id"
combo_company.DataSource = dt
case 2 output:
combo_company.SelectedItem= System.Data.DataRowview
combo_company.SelectedValue= output OK

Answer 1

Your case 2 is what you should be doing and it is working exactly as it should. When you bind a DataTable to a WinForms control, the data actually comes from its DefaultView, which is a DataView. Each item in a DataView is a DataRowView, so the SelectedItem of the bound control will be a DataRowView, which is exactly what you're seeing.
You say that you want the SelectedItem but I don't think that's actually the case, because the SelectedItem is the whole row. I suspect that what you actually want is the company_name value from that row, i.e. the text displayed in the ComboBox. To get that you use the Text property of the ComboBox. The SelectedValue will give you the value from the column specified in the ValueMember and the Text will give you the value (as a String) of the column specified in the DisplayMember.
To elaborate on the first paragraph, complex data-binding in WinForms requires an object that implements either the IList interface or the IListSource interface. The IListSource interface has one method: GetList, which returns an IList. If you bind an IList then its items are used. If you bind an IListSource then its GetList method is called and the items of the IList returned are used. The DataTable class implements the IListSource interface and its GetList method returns its DefaultView property value, which is a DataView, which implements IList and contains DataRowView objects. When you bind a DataTable, the data you see is from its DefaultView, which is how you're able to filter and sort the bound data.