Create a Dialog Box to Connect to a New Database, Including Listing Available SQL Servers and Databases

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7,1 Tạo một Dialog Box để kết nối đến một cơ sở dữ liệu mới, bao gồm danh mục Available SQL Server và cơ sở dữ liệu người dùng đôi khi cần phải kết nối với cơ sở dữ liệu khác nhau.

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7.1 Create a Dialog Box to Connect to a New Database, Including Listing Available SQL Servers and Databases Users sometimes need to connect to various databases. An example of this is a large company that might keep its site information in separate databases in the same-or even different-SQL Servers. Management might need to point the application to different site databases, depending on which one it needs to work with. This How-To shows you how to create a dialog box to let the user pick the SQL Server and database and then create a new connection based on the selections. Within a database application, it is necessary to allow users to select a SQL Server back end to which to connect. If you have created my application with the necessary flexibility, you should not have hard-coded SQL Server or database names within my application. How do you create a dialog box that lists available SQL Servers and databases and that the user can utilize to connect to a new database? Technique For this How-To, you will be using the base object in SQL-DMO, which is the Application object, and then the SQLServer object. You can use many objects off this object, the first layer of which is shown in Figure 7.5. Figure 7.5. These are just the tip of the iceberg as far as collections and objects used in SQL-DMO.
Table 7.1 presents the objects, properties, and methods that will be used for this -To. Table 7.1. SQL-DMO Objects Used for Listing SQL Servers, Databases, and Connecting to a Database Object Property/Method Description Application NameList Collection used to hold the list of available servers
ListAllAvailableServers Method used to retrieve available servers on the network SQLServer Connect Connection string that connects you to the SQL Server, allowing you access to the databases LoginSecure Flag that specifies that you want to connect to the SQL Server using a trusted connection Databases Collection of databases for the specified SQL Server You will also be using the OleDbConnection object. Steps Open and run the VB.NET -Chapter 7 solution. From the main Windows form, click on the command button with the caption How-To 7.1. When the form loads, you will see the SQL Servers that are available on your network. If you are not on the network, you will only see a SQL Server called "local." If you click on a SQL Server and you are in fact using the Windows NT Integrated Security, then you will see the list of databases located in the chosen SQL Server. You can then select a database and then click the Connection button. If you are successful, you will see the connection string displayed in the text box at the bottom of the form. The form will then look like the form displayed in Figure 7.6. 1. Create a Windows Form. Then place the controls shown in Figure 7.6 with the following properties set. Table 7.2. Label, ComboBox, ListBox, and Command Button Control Property Settings Object Property Setting Label Name Label1 Text SQL Servers ListBox Name lstSQLServers Label Name Label2
Text Databases ListBox Name lstDatabases Label Name Label3 Text Connection String TextBox Name txtConnectionString Text Not Connected Command Button Name btnConnect Text Connect 2. 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 be seen in Listing 7.1. This function takes a server and database names passed to it and returns a connection string. You will want to create a basic module in which to keep the routine. Listing 7.1 modSQLDMORoutines.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. The reason for this is that some objects ask for a Connection object, but others just want a string. 3. On the form, add the code in Listing 7.2 to the Load event. Listing 7.2 frmHowTo7_1.vb: Loading SQL Servers into a List Box Private Sub frmHowTo7_1_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load '-- Load up the SQL Servers LoadSQLServers(Me.lstSQLServers) End Sub 4. In the same module as step 2, create the routine called LoadSQLServers. After establishing an instance of the SQL-DMO application, the code calls the ListAvailableSQLServer method. If no names are loaded into the oNames namelist object (meaning that they were not available or you weren't on the network), then the (local) server is added to the list box. Otherwise, the list returned is added to the list box. The first item in the list box is then selected, causing the event described in the next step to be executed. Listing 7.3 modSQLDMORoutines.vb: Loading SQL Servers into a List Box Sub LoadSQLServers(ByRef lstSQLServers As ListBox) Dim intCurrSQL As Integer Dim oNames As SQLDMO.NameList Dim oSQLApp As New SQLDMO.Application() '-- Load available SQL Servers into a NameList ' (those that are able to be seen by the network) oNames = oSQLApp.ListAvailableSQLServers '-- Load the local instance as first in the list, if not on the network. If oNames.Count = 0 Then lstSQLServers.Items.Add("(local)") End If '-- Load the namelist into the list box For intCurrSQL = 1 To oNames.Count lstSQLServers.Items.Add(oNames.Item(intCurrSQL)) Next intCurrSQL '-- Choose the first instance lstSQLServers.SelectedIndex = 0 End Sub Note
At this point, if you try to run your sample application, you might get an error that includes the text QueryInterface for interface SQLDMO.NameList failed. on the ListAvailableSQLServer method. If this is the case, then you need to install Service Release 2 for SQL Server 2000. This will take care of this problem. 5. Add the code listed in Listing 7.4 to the SelectedIndexChanged event of the list box called lstSQLServers. This routine starts by testing to see whether a SQL Server and database have been selected. If so, the routine enables the btnConnect button; otherwise, the button is disabled. The routine called GetSQLDatabases is then called, passing selected SQL Server and the lstDatabases list box. You can see the GetSQLDatabases routine listed in the next step. Listing 7.4 frmHowTo7_1.vb: Enabling or Disabling the btnConnect Button and Calling the Routine to Load the Database List Box Private Sub lstSQLServers_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) _ Handles lstSQLServers.SelectedIndexChanged '-- If both the SQL Server and database are chosen, enable ' the Connect button. If lstSQLServers.SelectedItems.Count > 0 And _ lstDatabases.SelectedItems.Count > 0 Then Me.btnConnect.Enabled = True Else Me.btnConnect.Enabled = False End If GetSQLDatabases(Me.lstSQLServers.SelectedItem, Me.lstDatabases) End Sub 6. Create the GetSQLDatabases routine by entering the code in Listing 7.5 to the new module that you created in step 2. The first task that this routine attempts is to connect to the server that is selected. If the routine can't connect, then the function is exited; otherwise, the names of the database that are within the SQL Server are loaded into the list box called lstDatabases. Listing 7.5 modSQLDMORoutines.vb: Retrieving Database Names for a Given SQL Server
Function GetSQLDatabases(ByVal strSQLServer As String, _ ByRef lstTemp As ListBox) Dim intDBs As Integer Dim db As Object Dim strDBs As String '-- Establish a connection to the server. If not, then exit the function. Dim osvr As SQLDMO.SQLServer osvr = New SQLDMO.SQLServer() osvr.LoginSecure = True Try osvr.Connect(strSQLServer) Catch excp As Exception MessageBox.Show("There is a problem retrieving " & _ "databases for this server. " & "Please check the name and try again. ", vbInformation, "Error with Server") Exit Function End Try '-- Clear the current list lstTemp.Items.Clear() '-- Load up the database names for the selected server into the list box. For Each db In osvr.Databases lstTemp.Items.Add(db.Name) Next End Function 7. Add the code from Listing 7.6 to the SelectedIndexChanged event of the lstDatabases list box. This code is similar to the listing in step 4 in that it enables the btnConnect button if an item is selected in both the lstSQLServers and lstDatabases list boxes.
Listing 7.6 frmHowTo7_1.vb: Repopulating the List Boxes Based on the Current Category Selected Private Sub lstDatabases_SelectedIndexChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles lstDatabases.SelectedIndexChanged '-- If both the SQL Server and database are chosen, ' enable the Connect button. If lstSQLServers.SelectedItems.Count > 0 And _ lstDatabases.SelectedItems.Count > 0 Then Me.btnConnect.Enabled = True Else Me.btnConnect.Enabled = False End If End Sub 8. Add the code in Listing 7.7 to the Click event of the btnConnect button. In this routine, a Connection object is instantiated and its Connection string is set to the chosen SQL Server and database. If the connection has a problem opening, then the Text property of txtConnectionString is set to an error message. Otherwise, the Text property is set to the new connection string. Listing 7.7 frmHowTo7_1.vb: Calling the Routine to Reload the lstUnSelected List Box If This Check Box Is Changed Private Sub btnConnect_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnConnect.Click Dim ocnn As New OleDb.OleDbConnection() Try '-- Create the connection string and open the connection ocnn.ConnectionString = BuildCnnStr(Me.lstSQLServers.SelectedItem, _ Me.lstDatabases.SelectedItem) ocnn.Open() Catch excp As Exception Me.txtConnectString.Text = "Error with Connection" Exit Sub
End Try Me.txtConnectString.Text = ocnn.ConnectionString End Sub Figure 7.6. This form works great for logging users into a database using Windows NT Integrated Security. How It Works When the form opens, the SQL Servers that are available on the network are loaded into the first list box on the form. When the user clicks on a specific SQL Server, then the databases from that SQL Server are loaded into the Databases list box. The user can then select a database from the list. When the user does this, the Connect button is enabled. If the user then clicks on the Connect button, the connect string is loaded into the text box on the bottom of the form. Comments Like the others, this How-To is set up to use the Windows NT Integrated Security. If you wanted to use the SQL Server security, you could add text boxes for login name and password and then supply them when calling certain methods such as the Connect method in the code listed in step 6. You would also then set the LoginSecure property used in the same listing to False. This How-To is great for exactly what it does: letting the user (or administrator) pick a new SQL Server and database to connect to if necessary. You can then save this
connection string to a Public variable and use it with all your connection objects. You will also be using similar controls and code for the rest of the How-Tos in this chapter.