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Run the ViewEditCustomers application, browse all the customers in the DataSet, and edit a few. You can commit the changes to the database at any time by clicking the Update Table button. Or you can discard the edits and reload the data into the DataSet by clicking the Load Table button.

In Chapter 22, you’ll learn how to validate the data and how to commit the changes to the database in a more robust manner. A sure way to crash this application is to load the data, remove one of the customers from the underlying table (use the Enterprise Manager to delete a row of the Customers table in the Northwind database), and then attempt to update the database. The operation will fail, because the Update method will attempt to update a row that no longer exists in the database.

There are two more problems with this application. First, we’re downloading the entire Customers table to the client. Northwind is a small database, but an actual table with customer information might be quite large. It’s recommended that you move only as many rows as you need from the database (customers from a specific country, or customers that placed an order in the last few weeks). You can even move a single row. If the user wants to change a customer’s phone number, your application should give the user a chance to specify the desired row and then retrieve only that row. You should always keep in mind the fact that the DataSet resides on the client, and you shouldn’t move too much information too frequently from the database to the client.

The other problem of this application is the user interface. If the number of customers were in the thousands, moving to the next or previous row would clearly be out of the question. We’re going to look at a more functional interface in the following section, where we’ll also discuss the process of data-binding the ListBox and ComboBox controls.

Binding Complex Controls

The process of binding the ListBox and ComboBox controls is different than binding simple controls, or even the DataGrid control. These two controls are commonly used as lookup and navigational devices. They can display one field and keep track of another field. For example, you can display the customers’ names and keep track of the customer ID. In effect, the user sees names, but your program sees IDs. The application we’ll use in the following section does exactly that: it displays the company names in a ComboBox control and lets the user select a customer by clicking a name. The selected customer’s fields will appear in the data-bound controls, and the user can either edit a row, or move to a different row by clicking another item on the ListBox control. The new interface is far more functional that the previous one, and you will use it in many situations, especially if you want to present many rows to the user.

An even more common use of the complex data-bound controls is as lookup devices. Each row in the Products table has a CategoryID field and a SupplierID field, which link each product to a category and a supplier. The actual names of the categories and suppliers are stored in separate tables, related to the Products table through a pair of primary/foreign keys. You’ll see how you can create a form with data-bound controls for the Products table and use two ComboBox controls populated with the names of the categories and suppliers. As you move through the products, the proper items will be selected in the two ComboBox controls.

VB.NET at Work: The LookupCustomers Project

The LookupCustomers project is very similar to the ViewEditCustomers project—the only difference is in the navigational model. Copy the ViewEditCustomers project’s folder to a new location

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CREATING A DATASET 947

and rename it to LookupCustomers. Then open the new project and rename it to LookupCustomers. (Or use the project provided on the companion CD.) The main form of the new project is shown in Figure 21.13.

Figure 21.13

Using a data-bound ComboBox control as navigational tool

Delete all the navigational controls at the bottom of the form, and place a ComboBox control to the left side of the form (you will have to move the other controls to make room for the ComboBox control). The ComboBox control must be populated with the names of all customers. Set the control’s DataSource property to DSCustomers1.Customers and its DisplayMember property to CompanyName. The ComboBox control will get its data from the Customers table and will display the CompanyName field of each row.

Bind the TextBox controls on the form to the appropriate fields in the Customers table. The last step is to populate the DataSet by calling the Fill method of the associated DataAdapter. Enter the following statements in the Load button’s Click event handler:

Private Sub LoadData(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles bttnLoad.Click

DSCustomers1.Clear()

DACustomers.Fill(DSCustomers1, “Customers”)

End Sub

If you run the application now, you’ll be able to populate the DataSet, but nothing will happen as you click its items. You must add a few lines of code to set the current row in the DataSet every time the user selects another customer in the ComboBox control. Since the order of the items on the ComboBox control is the same as the order of rows in the DataSet, you can set the Position property to the index of the selected item. This is done with a single statement:

Me.BindingContext(DsCustomers1, “Customers”).Position = ComboBox1.SelectedIndex

If you allow users to edit the current row, however, and that row contains errors, you may not be able to move to another row. One such error would be to clear the current row’s CompanyName field. This field can’t be empty (this column’s AllowNull property is False), and any attempt to set it to Null will cause an exception when you attempt to leave the current row. You must insert an exception handler to reject the changes if the current row contains errors. Listing 21.5 shows the event handler of the ComboBox control’s SelectedIndexChanged event handler.

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948 Chapter 21 BUILDING DATABASE APPLICATIONS WITH ADO.NET

Listing 21.5: Moving to a Row in the DataSet

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged

Try

Me.BindingContext(DsCustomers1, “Customers”).EndCurrentEdit() Me.BindingContext(DsCustomers1, “Customers”).Position = _

ComboBox1.SelectedIndex Catch updateException As Exception

MsgBox(updateException.Message)

Me.BindingContext(DsCustomers1, “Customers”).CancelCurrentEdit() Me.BindingContext(DsCustomers1, “Customers”).Position = _

ComboBox1.SelectedIndex End Try

End Sub

The code in the Catch clause of the exception handler displays the error message that prevented the DataSet from moving to another row. Then it cancels the current edit action (the fields are restored to their initial values) and, finally, moves to the new row. This isn’t the most elegant method of handling update errors, but we’ll discuss more robust techniques in the following chapter.

VB.NET at Work: The Products Project

In this section, you’ll learn how to use a data-bound ComboBox as a lookup mechanism in your applications. The main form of the application, shown in Figure 21.14 and available in this chapter’s folder on the CD, allows you to navigate through the rows of the Products table with the help of a ListBox control, a technique you’re already familiar with. Most of the fields of each row are displayed on TextBox controls. If you bind the CategoryID and SupplierID fields to two TextBox controls, you will see two numbers, which are the foreign keys to the other two tables. By using data-bound ComboBox controls, users see the actual names of the related fields.

Figure 21.14

Browsing and editing the Products table

Start a new project and design a form like the one shown in Figure 21.14. Then you must create a DataSet with three tables that store product-related information: the Products, Categories, and Suppliers tables. Drop the three tables on the form, configure each DataAdapter, and create the Products

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CREATING A DATASET 949

DataSet. You can select a few fields from each table, as you aren’t going to display many fields. The SELECT statements for the three DataAdapters are as follows:

DAProducts:

SELECT ProductID, ProductName, SupplierID, CategoryID,

QuantityPerUnit, UnitPrice, UnitsInStock

FROM dbo.Products

DACategories:

SELECT CategoryID, CategoryName FROM dbo.Categories

DASuppliers:

SELECT SupplierID, CompanyName FROM dbo.SuppliersASuppliers

Then create the DSProducts DataSet and place all three tables in it. Since we want the current row of the Categories and Suppliers tables to change each time another row of the Products table is selected, we must establish the appropriate relationships between the tables. Double-click the DSProducts.xsd file in the Solution Explorer window to open the DataSet in design view. Then establish a relationship between the Products and Categories table based on the CategoryID field and another relationship between the Products and Suppliers tables based on the SupplierID field. Preview the DataSet to see that all tables and relations are in place. If you view one of the Categories or Suppliers tables, you will see that each row leads to a set of related rows.

Now you must bind all the controls on the form. Bind the Text property of each of the TextBox controls to the appropriate field of the Products table of the DataSet. The top TextBox is bound to the expression

DsProducts1 - Products.ProductName

and it will display the ProductName field of the Products table. Do the same for the controls that display the Price and Stock fields.

Select the ListBox control and bind it as follows:

Property	Value
DataSource	DSProducts1.Products
DataMember	ProductName

These two properties will cause the ListBox control to be populated with the ProductName field of the Products DataTable. Then set the control’s ValueMember property to the ProductID field. This property connects the selected item on the control to one of the rows of the Products table in the DSProducts1 DataSet.

The last step is to bind the two ComboBox controls to the related tables. Select the first ComboBox control on the form and bind it to the DataSets as follows:

Property	Value
DataSource	DSProducts1.Categories
DataMember	CategoryName
ValueMember	CategoryID
SelectedValue	DSProducts1 – Products.CategoryID

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950 Chapter 21 BUILDING DATABASE APPLICATIONS WITH ADO.NET

The ComboBox control will be populated with the names of the categories and will be connected to the Products table through the CategoryID field. In effect, the current selection on the control will be the name of the category of the row whose CategoryID field matches the CategoryID field of the Products DataTable.

Binding the second ComboBox control to the Suppliers table is quite similar, so I will only list the data-binding properties and their settings:

Property

Value

DataSource DSProducts1.Suppliers

DataMember CompanyName

ValueMember SupplierID

SelectedValue

DSProducts1 – Products.SupplierID

You should be very familiar with the code that populates the DataSet and updates the database by now. The code behind the two buttons on the form is shown in Listings 21.6 and 21.7.

Listing 21.6: Populating the DSCustomers DataSet

Private Sub Button1_Click(ByVal sender As System.Object,_

ByVal e As System.EventArgs) Handles Button1.Click

DACategories.Fill(DsProducts1, “Categories”)

DASuppliers.Fill(DsProducts1, “Suppliers”)

DAProducts.Fill(DsProducts1)

End Sub

Listing 21.7: Updating the Database

Private Sub Button2_Click(ByVal sender As System.Object,_

ByVal e As System.EventArgs) Handles Button2.Click

DAProducts.Update(DsProducts1)

End Sub

The code for updating the underlying table is quite primitive. You will learn how to properly update the underlying table in the following chapter.

The last step is to add some code to the ListBox control to turn it into a navigational tool. We want to move to the row of the product selected on the control. We can take advantage of the fact that the items on the ListBox appear in the same order as in the DataSet: we’ll use the control’s SelectedIndex property to move to the appropriate row in the Products DataTable. Listing 21.8 shows how to move to the correct row of Products when the user selects another item on the ListBox.

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