Tài liệu Retrieve Results from SQL Server by Using the DataTable Object docx

3.2 Retrieve Results from SQL Server by Using the DataTable Object The data reader is great when you just want to load data into a ListBox or ComboBox control manually, but you can save

3.2 Retrieve Results from SQL Server by Using the DataTable Object

The data reader is great when you just want to load data into a ListBox or ComboBox control manually, but you can save some coding by binding the ListBox control to a data table at runtime, as well as providing the ability to get not only the displayed value, but the key column as well This How-To demonstrates how to bind a limited ListBox

control to a data table

Although getting the quick information is great, you need to be able to refer back to the table of information, and you don't want to have to open another connection to get there You know that the DataTable object should allow you to perform this task How do you get results from SQL Server by using the DataTable object?

Technique

Using the Windows Forms controls that were introduced in How-To 3.1, you will use a familiar object from Chapter 1 called the DataAdapter object This time, instead of using the OleDbDataAdapter data control, you will use the OleDbDataAdapter class from the System.Data.OleDb Namespace

Using a similar technique that was used when filling a DataSet, you will instantiate the data adapter by assigning the SQL string and connection object Then, instead of filling a DataSet object, you will fill a DataTable object Because you will only be dealing with a table's worth of data, you just need to use a data table That way, you will be able to perform lookups more conveniently, as shown in the next How-To

For now, the next step will be to assign the following properties of the list box:

• DataSource This will be set to the DataTable object-in this case, dtCust

• DisplayMember This specifies which column from the data table to use for

display in the list box

• ValueMember Here, you will specify which column you want to use for the value

that is retrieved when an item is selected from the list box

By programming the ListBox control using this technique, you can access the

ValueMember column in the SelectItem property of the list box

Steps

Open and run the VB.NET-Chapter 3 solution From the main form, click on the

command button with the caption How-To 3.2 When the form loads, click on the Load List command button You will see the list below fill with all the company names that start with A

1 To save time, you can make a copy of the form that was created in the first

How-To in this chapter

2 Replace the btnLoadList Click event with the following code listed here in Listing 3.3 That's it After creating the SQL string that will be used and storing it in strSQL, the data adapter called odaCust is created The odtCust data table is then filled using odaCust Last, the DataSource, DisplayMember, and ValueMember properties are set for the lstCustomers list box This was all accomplished with a Try-Catch-End-Try block of code

Listing 3.3 frmHowTo3_2.vb: Loading a List Box By Using the DataTable Object

Private Sub btnLoadList_Click(ByVal sender As System.Object,

ByVal e As System.EventArgs) Handles btnLoadList.Click

Dim odaCust As OleDb.OleDbDataAdapter

Dim dtCust As DataTable = New DataTable()

Dim strSQL As String

' Create the SQL String

strSQL = "Select CustomerID, CompanyName From Customers " & _

"Where CustomerID Like '" &

Me.txtCustLimit.Text & "%'"

' Set up the exception catch

Try

' Create an instance of the data adapter,

' and then fill the data table

odaCust = New OleDb.OleDbDataAdapter(strSQL, _

BuildCnnStr("(local)", "Northwind"))

odaCust.Fill(dtCust)

' Bind the data to the list box

lstCustomers.DataSource = dtCust

lstCustomers.DisplayMember = "CompanyName"

lstCustomers.ValueMember = "CustomerID"

Catch oexpData As OleDb.OleDbException

MsgBox(oexpData.Message)

End Try

End Sub

How It Works

When the user clicks on the btnLoadList button, the data adapter called odaCust is instantiated The data adapter is passed strSQL and the connection string that is created

by the function called BuildCnnStr, which was introduced in the first How-To in this chapter The data table is then filled, and then the DataSource, DisplayMember, and ValueMember properties of the ListBox control are assigned

Comments

Using the data table sets up the scene for using the list box in retrieving data in the next How-To Remember: By using the DataTable object, you can assign both the display value and the data item to be tracked