Adding associated double values from database when checkedlistbox item is selected - vb.net

Question

Edited
Hi I have this program where I need to display the sum of the values when read from the database. I tried something like this:
Dim selec As String
Dim con As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\rose&mike\Desktop\DbSysDel3\salondbaccess.accdb")
Dim dt2 As New DataTable
selec = ""
con.Open()
For Each incheck In chcklstbx1.CheckedIndices
Dim valName As String
valName = chcklstbx1.Items.Item(incheck).ToString
Dim sqlstr2 As String = "SELECT Service_Fee FROM Service_Types WHERE [Service_Name] = '" & valName & "'"
Dim cmd As New OleDbCommand(sqlstr2, con)
Dim sum As Double = 0
Dim rdr As OleDbDataReader = cmd.ExecuteReader
If rdr.HasRows Then
While rdr.Read
selec += "P" + rdr("Service_Fee").ToString & ControlChars.NewLine
sum = sum + rdr("Service_Fee")
End While
End If
lblFees.Text = selec
lblTotal.Text = sum.ToString
rdr.Close()
Next
con.Close()
But all it does is display the values I want to add one by one when checked.

Answer 1

Here's an example of how this whole thing should be handled:
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Using connection As New OleDbConnection("connection string here"),
adapter As New OleDbDataAdapter("SELECT Name, Amount FROM MyTable", connection)
Dim table As New DataTable
adapter.Fill(table)
'Bind the DataTable to the CheckedListBox.
'Normally the DataSource should be set last but, in my experience, that can cause problems with a CheckedListBox.
With CheckedListBox1
.DataSource = table
.DisplayMember = "Name"
.ValueMember = "Amount"
End With
End Using
End Sub
Private Sub CheckedListBox1_ItemCheck(sender As Object, e As ItemCheckEventArgs) Handles CheckedListBox1.ItemCheck
'Get all the currently checked items.
Dim checkedItems = CheckedListBox1.CheckedItems.Cast(Of DataRowView)().ToList()
'Get the item that is being checked or unchecked.
Dim currentItem = DirectCast(CheckedListBox1.Items(e.Index), DataRowView)
If e.NewValue = CheckState.Checked Then
'The current item is being checked so add it to the list.
checkedItems.Add(currentItem)
Else
'The current item is being unchecked so remove it from the list.
checkedItems.Remove(currentItem)
End If
'Sum the Amounts from each checked item.
Dim sum = checkedItems.Sum(Function(drv) CDec(drv("Amount")))
'Use sum here.
'...
End Sub
You get all the data up front, including all the text to display and all the associated numeric values. That data can be bound to the CheckedListBox as a DataTable, keeping it all packaged together.
Each time the user checks or unchecks an item in the list, you perform the calculation based on the data you already have. No need to go back to the database.
The important thing to note about the ItemCheck event is that it is raised BEFORE the item is checked or unchecked. That's why the code needs to ad the current item, or remove it from, the list of checked items.
When you bind a DataTable, each bound item is a DataRowView, i.e. an item from the table's DefaultView. You get all the checked DataRowView items, get their numeric value and sum them. Note that I've used a LINQ query to calculate the sum succinctly but you could use a loop if you wanted to, e.g.
Dim sum As Decimal = 0D
For Each drv In checkedItems
sum += CDec(drv("Amount"))
Next