Design Time Binding
In Visual Basic 6, you can use an ADO Data control (ADODC) to select records from a database. You can then bind the ADODC to other controls that display the data.
As in so many other cases, Visual Basic .NET does things the same way, only different. You can still bind controls to data at design time or at runtime. The objects and methods you use are different, however.
The basic plan is to do the following:
- Make a database connection.
- Make a data adapter to move data in and out of the database.
- Make a DataSet to hold the data.
- Bind the DataSet to controls.
- Write code to load and save the data.
The next several sections explain how to perform these steps at design time. The sections that follow explain how to perform the same steps at runtime.
The examples described here use OLE DB objects to connect to an Access database. You can just as easily use SQL Server objects to connect to a SQL Server database by replacing the OLE DB objects with the corresponding SQL Server objects. For example, instead of using an OleDbConnection object to connect to the database, the program would use a SqlConnection object.
Make a Data Connection
Start a new form. In the Toolbox, select the Data tab. This tab contains data-related objects that you can put on the form. These objects are similar to the Timer and dialog controls in previous version of Visual Basic in the sense that they are invisible to the user at runtime.
Double click the OleDbConnection tool to place a new OleDbConnection object on the form. In Visual Basic .NET, invisible controls such as this one appear below the form's design surface (see Figure 1).
Figure 1 Invisible controls appear off the form's main design area.
Click on the new OleDbConnection object to select it. In the Properties window, click ConnectionString. Click the drop-down arrow to the right and select the <New Connection...> item. This starts the Data Link Properties dialog box shown in Figure 2.
Figure 2 The Data Link Properties dialog box.
Initially, the dialog box assumes that you will use a SQL Server database, even if you started the dialog box from an OleDbConnection object. To use an OLE DB database, click the Provider tab, and select the Microsoft Jet 4.0 OLE DB Provider (see Figure 3).
Figure 3 Select the Microsoft Jet 4.0 OLE DB Provider.
Return to the Connection tab, which now looks like the one shown in Figure 4. Enter or select the name of the Access database in the first text box. The example available for download uses the database Books.mdb.
Figure 4 The Connection tab for an OLE DB connection.
Click the Test Connection button to make sure that the program can connect to the database. If the dialog box connects successfully, click the OK button.
At this point, you have created an OleDbConnection object that knows how to connect to the database. You still need other objects to do something with the connection.
Make a Data Adapter
A data adapter moves data back and forth between a program and a database. To create a new data adapter, double-click the OleDbDataAdapter tool on the Toolbox's Data tab. This adds a new data adapter to the form and automatically starts the Data Adapter Configuration Wizard. Click Next to move past the introduction and see the screen shown in Figure 5.
Figure 5 The Data Adapter Configuration Wizard.
Use the drop-down list to select the connection you created in the previous section. If you want, you can create a new data connection by clicking the New Connection button and following the same steps described in the previous section.
After you select or create a database connection, click Next to see the form shown in Figure 6. This version of the form automatically selects the Use SQL statements option, so the data adapter will use SQL statements to manipulate the database. If the connection used a SQL Server database connection, you could use stored procedures to interact with the data instead.
Figure 6 This data adapter will use SQL statements to interact with the database.
Click Next to see the screen shown in Figure 7. Enter the SQL SELECT statement that the data adapter should use to pull data from the database. If you prefer, you can click the Query Builder button to design the query graphically.
Figure 7 Enter the SQL SELECT statement.
When you click Next, the Data Adapter Configuration Wizard uses the SQL SELECT statement you entered to generate other SQL statements that insert, update, and delete records. Figure 8 shows the Wizard's results screen.
Figure 8 The Data Adapter Configuration Wizard's results screen.
Note that the Wizard uses the selected table's primary key to identify the records it needs to update or delete. If the table used in the SELECT statement does not have a primary key, the Wizard cannot generate commands to perform those operations. In that case, you'll see a results screen more like the one shown in Figure 9. In this case, the data adapter can select data and insert records, but it cannot update or delete existing records.
Figure 9 The Wizard cannot generate UPDATE and DELETE statements for a table with no primary key.
After you have read the Wizard's summary, click Finish.
When the Data Adapter Configuration Wizard generates SQL statements to insert, update, and delete records, it stores them in new database objects. The four objects it creates are named OleDbSelectCommand1, OleDbInsertCommand1, OleDbUpdateCommand1, and OleDbDeleteCommand1.
To see these objects, use the drop-down list at the top of the Properties window. After you select an object, you can examine its properties. For example, you will see that all of these objects have their Connection properties set to the OleDbConnection1 object you created earlier.
These objects' CommandText properties are particularly interesting. For example, the CommandText for OleDbSelectCommand1 is
SELECT Title, URL FROM Books ORDER BY Title
The CommandText property for OleDbUpdateCommand1 is
UPDATE Books SET Title = ?, URL = ? WHERE (Title = ?) AND (URL = ?)
The question marks in this statement are placeholders for values inserted when the data adapter uses the commands.
Make a DataSet
A data adapter object moves data between a database and some program data structure. The next step is to build that data structure.
Click the new OleDbDataAdapter you created in the previous step to select it. Open the Data menu, and select Generate Dataset to see the dialog box shown in Figure 10.
Figure 10 Generating a DataSet for a data adapter.
Give the DataSet a meaningful name such as DsBooks. Make sure that the OleDbDataAdapter you created in the previous section is selected, and check the "Add this dataset to the designer" box. Then click OK.
The dialog box creates an XML schema file named DsBooks.xsd to define the DataSet. You can double-click this file if you want to see the schema code, but it won't mean much to you unless you are familiar with XML schemas. See my book, Visual Basic .NET and XML (Wiley, 2002) for more information on using XML and schemas in Visual Basic .NET.
The dialog box also creates an instance of the DataSet named DsBooks1, and adds it to the form. The DataSet is attached to the data adapter, which in turn is attached to the database connection. This gives the path you need to move data between the database and the DataSet.
Bind Controls to the DataSet
You are finally ready to bind controls to the data. On the Toolbox's Windows Forms tab, double-click the DataGrid tool to add a DataGrid control to the form. Set the control's Dock property to Fill (click the middle box in the property's drop-down list).
In the Properties window, select the control's DataSource property and click the drop-down list on the right. Then select the DsBooks1.Books entry. This selects the Books table from the DsBooks1 DataSet.
If the DataSet contained more than one table, you could select one of its other tables or the DataSet itself. If you select the DataSet, the DataGrid control provides access to all of the DataSet's tables, not just the Books table.
Load and Save Data
At this point, you have created all of the objects you need to manipulate the data, but the form doesn't contain any code to use those objects. The objects are ready and waiting but the program doesn't tell them to do anything.
Open the form's code view, and enter the following code. Not counting the subroutine declarations and comments, it's just two lines.
Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load ' Load the data. OleDbDataAdapter1.Fill(DsBooks1) End Sub Private Sub Form1_Closing(ByVal sender As Object, _ ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing ' Save the data. OleDbDataAdapter1.Update(DsBooks1) End Sub
The call to the data adapter's Fill method loads data from the database into the DataSet. The DataGrid automatically displays the data.
The call to the Update method saves any changes made to the data back into the database.
If you run the program and resize the form and the DataGrid's columns a bit, you'll see a result similar to the one shown in Figure 11.
Figure 11 The DataGrid automatically displays the data.