USING VISUAL DATABASE AND FORM DESIGN TOOLS in Font

Generator Code 39 Full ASCII in Font USING VISUAL DATABASE AND FORM DESIGN TOOLS

Figure 13-22. Creating basic parameter queries for a DataGridView control is easy and powerful with the TableAdapter object.

You can readily extend the preceding example for parameter queries to all kinds of conditional, interactive displays. This section shows a simple extension that removes the need for a user to type in a number indicating the ProductCategoryID value for the list of product categories to show. Instead of typing in a number, a user can select from a list of product category names. After the selection of the name in one ListBox control, the application populates a second ListBox control with the product subcategory names from the currently selected category in the first ListBox control. In addition, the Click event procedure for a Button control changes the Items collection for the second ListBox to include all product subcategory names. Form9 in the WinCh13 project implements this solution.

Figure 13-23 shows side by side the Design view and Form view of Form9 in the WinCh13 project. The Load event procedure for Form9 formats the controls by assigning Text property values to the Label and Button controls as well as changing the position and size of the Button control. The Load event procedure also populates the first ListBox control on the left (ListBox1), which, in turn, populates the remaining ListBox control (ListBox2). Clicking items in ListBox1 repopulates ListBox2 with items from the most recently selected product category. Clicking the Button control (Button1) above ListBox2 repopulates ListBox2 to show all product subcategory names.

Figure 13-23. While it is OK to allow a user to enter a value to specify a parameter query, it is generally easier for users to select an item from a ListBox control. It is preferable to drag the ListBox controls to a form from the Toolbox. This avoids having the Windows Form Designer generate a BindingNavigator control for each ListBox control (the BindingNavigator controls do not participate in the solution anyway). You can still use the smart tag window to set the bound data for each ListBox control. Figure 13-24 shows the smart tag settings for ListBox1. This control binds to ProductCategory DataTable through the ProductCategoryBindingSource. The settings in the smart tag ListBox Tasks window cause ListBox1 to display Name column values from the ProductCategory DataTable and return the ProductCategoryID value as its SelectedValue property. The particular ProductCategoryID column value corresponds to the most recently selected item from the ListBox1 control. The smart tag ListBox Tasks settings for ListBox2 appear in Figure 13-25. ListBox2 binds to the ProductSubcategory DataTable through the ProductSubcategoryBindingSource. You only need to set the Display Member for ListBox2 because its sole purpose is to show column values corresponding to the selected item in ListBox1.

A BindingSource class interfaces a Windows Form control with a source of data. The BindingSource can work with all types of databases (SQL Server, OLE DB, and ODBC) and other types of sources as well (web services and business objects). As with the TableAdapter class, particular instances of the class take the name of the source with which they interface. For example, the ProductCategoryBindingSource is a BindingSource class instance that interfaces with the ProductCategory DataTable.

Figure 13-24. When specifying a parameter through a ListBox selection, it is common to designate the parameter value with the Value Member setting.

Figure 13-25. There is no need to specify a Value Member setting when a ListBox control is just meant to show a collection of values. The graphical tools for displaying data are so rich that they drastically reduce the amount of code that is necessary. These tools include the Windows Form Designer that writes a substantial amount of code in a partial class that is out of view most of the time (except when Exception objects are thrown to halt normal execution). In addition, the Windows Form designer even inserts some code in the Load event procedure. You can modify or comment out this code as your needs dictate. The code inside the Load event procedure for Form9 appears as the next code segment. The first two statements are generated by the Windows Form Designer. The statement to populate the ProductSubcategory DataTable is commented out because the procedure uses another approach for populating this DataTable. The alternative approach is necessary because the values in the DataTable need to have the option to be conditional. The statement to populate the ProductCategory DataTable is also supplied by the Windows Form Designer. This code is acceptable as supplied for use in the demonstration. The call to the UpdateListBox2 procedure populates the ProductSubcategory DataTable based on the currently selected item in ListBox1. When the form loads, the currently selected item in ListBox1 is the first Name column value for the first row in the ProductCategory DataTable. The remaining lines of code in the Load event procedure format the controls in Form9. 'TODO: This line of code loads data into the 'CatSubCatAdventureWorks_DataDataSet.ProductSubcategory' table. 'You can move, or remove it, as needed. 'Me.ProductSubcategoryTableAdapter.Fill( _ 'Me.CatSubCatAdventureWorks_DataDataSet.ProductSubcategory) 'TODO: This line of code loads data into the 'CatSubCatAdventureWorks_DataDataSet.ProductCategory' table. 'You can move, or remove it, as needed. Me.ProductCategoryTableAdapter.Fill( _ Me.CatSubCatAdventureWorks_DataDataSet.ProductCategory)