8.1 Work with Data-Bound Multi-Select List Boxes Using Windows Forms It is common to have to assign products to categories, which is a one-to-many relationship Sometimes you want to be able to this in a somewhat bulk fashion One of the methods that works well is using the ListBox control Using the ListBox control for single selections is no big deal, but when it comes to using it in a multi-select method, it starts getting trickier This How-To shows you how to create an intuitive interface for assigning products to categories using a couple of multi-select list boxes on a Windows Form You can assign a category to each product, but you would like to have a method of maintaining all the products for a category at one time How you take advantage of a multi-select list box to perform this task? Technique Using the ListBox control in regular single selection mode is as straightforward in NET as in prior versions of Visual Basic The same can be said in the case of using the multiselect mode of list boxes It is as confusing in NET as it was in prior versions Using the ListBox control in single entry mode is pretty straightforward You just need to use the SelectedItem property with the index of However, if you want to use the ListBox control in multi-select mode, then you must perform some more work and access some other properties (see Table 8.1) Table 8.1 Properties Having to Do with Multi-Selection on ListBox Controls (In Order of Use in This How-To's Steps) Property/Object Description SelectionMode Property of the ListBox control The settings for this are None, One, MultiSimple, or MultiExtended SelectedIndices (index) A collection of the ListBox control, this returns the indices (location in the list) of all the selected items SelectedIndices.Count Property of the ListBox control A count of the number of items selected in the list box DataRowView Object type that the data provider provides Items (index) A collection of the ListBox control Returns a DataRowView type object If you have multiple columns, they are also returned in the DataRowView object Steps Open and run the VB.NET -Chapter solution From the main Windows Form, click on the command button with the caption How-To 8.1 You will then see the form displayed in Figure 8.2 Figure 8.2 This form uses controls bound at runtime and takes advantage of multiselect list boxes When the form loads, you will see the Beverages category chosen in the top combo box The Selected and Unselected Products ListBox controls are filled in with the appropriate products If you click on a product in the Unselected Products list box and then click on the arrow button pointing to the right (>), then the item is moved to the Selected Products list box If you select items in the Selected Products list box and click on the arrow button pointing to the left ( Name btnUnSelect Text < Name chkUnAssignedOnly Text UnAssigned Products Only Command Button Command Button CheckBox As with some of the other chapters' projects, before creating the code that will be attached to the Load event of the form, you need to create a support routine to create the Connection string Called BuildCnnStr, the function can been seen in Listing 8.1 This function takes a server and database names passed to it and creates a connection string Listing 8.1 modGeneralRoutines.vb: Creating a Connection String Function BuildCnnStr(ByVal strServer As String, ByVal strDatabase As String) As String Dim strTemp As String strTemp = "Provider=SQLOleDB; Data Source=" & strServer & ";" strTemp &= "Initial Catalog=" & strDatabase & ";" strTemp &= "Integrated Security=SSPI" Return strTemp End Function Although you could create a routine that would pass back a Connection object, a more versatile method would be to pass back a string Some objects ask you for a Connection object, but others just ask for a string You will see BuildCnnStr called in the next step On the form, add the code in Listing 8.2 to the Load event In this code, you will start off by creating a data adapter called odaCategories and loading the category's SQL Statement into it The dtCategories data table is then filled and set as the DataSource property of cboCategories The DisplayMember and ValueMember of cboCategories are then set Finally, two new subroutines called LoadUnSelectedProducts and LoadSelectedProducts are called to populate the appropriate list boxes These routines are discussed in the next two steps Listing 8.2 frmHowTo8_1.vb: Loading Categories into the List Box Private Sub frmHowTo8_1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim odaCategories As OleDb.OleDbDataAdapter Dim dtCategories As New data table() ' Load the Categories combo box up first odaCategories = New _ OleDb.OleDbDataAdapter( _ "Select CategoryID, CategoryName From Categories", (BuildCnnStr("(local)", "Northwind"))) odaCategories.Fill(dtCategories) Me.cboCategories.DataSource = dtCategories Me.cboCategories.DisplayMember = "CategoryName" Me.cboCategories.ValueMember = "CategoryID" ' Load each of the product list boxes based on the selected category LoadUnSelectedProducts() LoadSelectedProducts() End Sub Create the LoadUnSelectedProducts routine by entering the code shown in Listing 8.3 into the form you created for this example This routine starts off by testing the check box called chkUnAssignedOnly Based on that value, a SQL string is created that grabs the products that are not assigned to any product, if chkUnAssignedOnly = True All products that are not assigned to the chosen category are retrieved The SQL String is stored in the variable called strSQL Next, the DataAdapter object called odaUnselected is set to strSQL and the SQL Server connection string The DataTable object called dtUnSelected is then filled and assigned to the list box called lstUnSelected The DisplayMember and ValueMember properties are then set Last, the ClearSelected method is called to make sure no entries remain selected Listing 8.3 frmHowTo8_1.vb: Populating the List Box Displaying Unselected Products Sub LoadUnSelectedProducts() Dim odaUnSelected As OleDb.OleDbDataAdapter Dim dtUnSelected As New DataTable() Dim strSQL As String ' If the check box for Unassigned Only is checked, then ' grab the product items where the category is null; otherwise, load ' it up with those products not assigned to the current category If chkUnAssignedOnly.Checked Then strSQL = "Select ProductID, ProductName From Products " & _ " Where CategoryID IS NULL Order By ProductName" Else strSQL = "Select ProductID, ProductName From Products " & _ "Where CategoryID " & _ Me.cboCategories.SelectedItem(0) & " Or CategoryID IS NULL Order By ProductName" End If ' Pretty well same old, same old here Create a data adapter ' and fill the dataset ' Next, bind it to the list box odaUnSelected = New OleDb.OleDbDataAdapter(strSQL, (BuildCnnStr("(local)", "Northwind"))) odaUnSelected.Fill(dtUnSelected) Me.lstUnSelected.DataSource = dtUnSelected Me.lstUnSelected.DisplayMember = "ProductName" Me.lstUnSelected.ValueMember = "ProductID" Me.lstUnSelected.ClearSelected() End Sub Create the LoadSelectedProducts routine by entering the code in Listing 8.4 into the form you created for this tutorial This routine performs basically the same tasks that the routine listed in the previous step does, except that it performs the tasks using the lstSelected ListBox control It also doesn't need to test the CheckBox control Listing 8.4 frmHowTo8_1.vb: Populating the List Box Displaying Selected Products Sub LoadSelectedProducts() Dim odaSelected As OleDb.OleDbDataAdapter Dim dtSelected As New DataTable() Dim strSQL As String ' Load the products assigned to this category strSQL = _ "Select ProductID, ProductName From Products Where CategoryID = " & Me.cboCategories.SelectedItem(0) & " Order By ProductName" odaSelected = New _ OleDb.OleDbDataAdapter(strSQL, (BuildCnnStr("(local)", "Northwind"))) odaSelected.Fill(dtSelected) Me.lstSelected.DataSource = dtSelected Me.lstSelected.DisplayMember = "ProductName" Me.lstSelected.ValueMember = "ProductID" Me.lstSelected.ClearSelected() End Sub Add the code in Listing 8.5 to the SelectedIndexChanged event of the cboCategories combo box Listing 8.5 frmHowTo8_1.vb: Repopulating the List Boxes Based on the Current Category That Is Selected Private Sub cboCategories_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboCategories.SelectedIndexChanged ' Load each of the product list boxes based on the selected category LoadUnSelectedProducts() LoadSelectedProducts() End Sub Add the code in Listing 8.6 to the CheckChanged event of the chkUnAssignedOnly check box Listing 8.6 frmHowTo8_1.vb: Call the Routine to Reload the lstUnSelected List Box If This Check Box Is Changed Private Sub chkUnAssignedOnly_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles chkUnAssignedOnly.CheckedChanged LoadUnSelectedProducts() End Sub Add the code in Listing 8.7 to the Click event of the btnSelect command button This and the next step contain the most code as well as some new objects and properties The first thing that happens is that the number of highlighted items (SelectedIndices.Count) is stored to an Integer variable called intItemsNum One is subtracted from the figure because the collections in NET are zero based Next, the code iterates through the SelectedItems collection of the lstUnSelected list box, and using the indices in that collection, the code accesses selected items The type of object derived from the Items collection is the DataRowView object, mentioned in the "Techniques" section of this example These items are then added to a string variable called strItems, which is then used to create the criteria for an IN clause of a SQL Update statement This statement is passed to the Command object called ocmdSelect This Command object is then executed, and the selected products are updated to reflect the category chosen Last, the list boxes are reloaded to reflect the changes Listing 8.7 frmHowTo8_1.vb: Updating the Server with Products Selected for the Given Category Private Sub btnSelect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSelect.Click Dim intItemsNum As Integer Dim intCurr As Integer Dim strItems As String Dim drv As DataRowView ' Grab the number of selected items for the products ' unselected list box intItemsNum = Me.lstUnSelected.SelectedIndices.Count - ' Iterate through each of the items and create a string For intCurr = To intItemsNum If Len(strItems) > Then strItems = strItems & ", " End If drv = Me.lstUnSelected.Items(Me.lstUnSelected _ SelectedIndices(intCurr)) strItems = strItems & CType(drv(0), String) Next ' Run an update query to assign the category to the desired products ' using an IN clause in the SQL statement Try Dim ocnn As New OleDb.OleDbConnection(BuildCnnStr("(local)", "Northwind")) Dim ocmdSelect As New _ OleDb.OleDbCommand("Update Products Set CategoryID = " & Me.cboCategories.SelectedItem(0) & _ " Where ProductID IN (" & strItems & ")", ocnn) ocmdSelect.CommandType = CommandType.Text ocnn.Open() ocmdSelect.ExecuteNonQuery() Catch excpCommand As Exception MessageBox.Show(excpCommand.Message) End Try LoadUnSelectedProducts() LoadSelectedProducts() End Sub Add the code in Listing 8.8 to the Click event of the btnUnSelect command button Again, this code is similar to the previous step, but it is used to set the CategoryID column to null if the product was highlighted in the lstSelected list box and btnUnSelect was clicked Listing 8.8 frmHowTo8_1.vb: Updating the Server with Products That Are Unselected for the Given Category Private Sub btnUnSelect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnUnSelect.Click Dim intItemsNum As Integer Dim intCurr As Integer Dim strItems As String Dim drv As DataRowView ' Grab the number of selected items for the products ' selected list box intItemsNum = Me.lstSelected.SelectedIndices.Count - ' Iterate through each of the items and create a string For intCurr = To intItemsNum If Len(strItems) > Then strItems = strItems & ", " End If drv = Me.lstSelected.Items(Me.lstSelected.SelectedIndices(intCurr)) strItems = strItems & CType(drv(0), String) Next Try Dim ocnn As New OleDb.OleDbConnection(BuildCnnStr("(local)", _ "Northwind")) Dim ocmdUnSelect As New OleDb.OleDbCommand( "Update Products Set CategoryID = Null Where ProductID IN (" & strItems & ")", ocnn) ocmdUnSelect.CommandType = CommandType.Text ocnn.Open() ocmdUnSelect.ExecuteNonQuery() Catch excpCommand As Exception MessageBox.Show(excpCommand.Message) End Try LoadUnSelectedProducts() LoadSelectedProducts() End Sub How It Works When the user chooses a category, the appropriate items are loaded into the two list boxes; unselected items are placed in the list box on the left, and selected items are placed in the list box on the right If the check box is selected, then only those items that are not currently assigned to a category are displayed in the list box on the left, which is titled Unselected Products When the btnSelect button is clicked, any items highlighted in the lstUnSelected list box are used in a query that updates the server with the new category they now belong to Similarly, when the btnUnSelect is clicked, items in the lstSelected list box are used in a query that updates the CategoryID of the products to null Comments This example is not the smartest to create in real life because you want products to be assigned to a category However, this example does a good job of showing the properties and methods you can use to work with the multi-select features of the ListBox control For examples of using this same basic technique in a Web Form, check out example 8.5  ... Products list box and then click on the arrow button pointing to the right (>), then the item is moved to the Selected Products list box If you select items in the Selected Products list box and... box for Unassigned Only is checked, then '' grab the product items where the category is null; otherwise, load '' it up with those products not assigned to the current category If chkUnAssignedOnly.Checked... Add the code in Listing 8.8 to the Click event of the btnUnSelect command button Again, this code is similar to the previous step, but it is used to set the CategoryID column to null if the product