zxing qr code encoder example c# Part 6: Excel and the Outside World: Collaborating Made Easy in Visual C#

Encoding QR Code ISO/IEC18004 in Visual C# Part 6: Excel and the Outside World: Collaborating Made Easy

Part 6: Excel and the Outside World: Collaborating Made Easy
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Microsoft Office Excel 2003 Programming Inside Out
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Getting the Query
Printing Quick Response Code In Visual Basic .NET
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Getting the query string from the combo box on the command bar follows the same basic approach discussed elsewhere in this chapter. After locating the appropriate command bar, the FindControl method is used to search for the specific control containing the combo box. The following function is also located in the ThisWorkbook module:
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Function GetDBQuery() As String
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Dim c As CommandBar
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Dim cc As CommandBarComboBox
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On Error Resume Next
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Set c = Application.CommandBars("Excel2k3 VBA Query")
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Set cc = c.FindControl(, , "Excel2k3 VBA Query Statement") If Not cc Is Nothing Then GetDBQuery = cc.Text ElseIf cc.Text = "<enter a query>" Then GetDBQuery = "" Else GetDBQuery = "" End If End Function
The main benefit of this routine is that the query string is compared to the default value that was loaded when the program began. If it is, the function will return an empty string, indi cating that the user should be prompted to enter a query.
Running a Query
Once the user has supplied values for the connection string and the query string, which have passed some simple edit checks, the RunQuery routine, located in the ThisWorkbook module, is called to get the information from the database.
Sub RunQuery(c As String, q As String)
Dim cn As ADODB.Connection
Dim cmd As ADODB.Command
Dim rs As ADODB.Recordset
On Error Resume Next
Set cn = New ADODB.Connection
cn.ConnectionString = c
cn.Open
If Err.Number <> 0 Then
Part 6: Excel and the Outside World: Collaborating Made Easy
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Excel Query Program
MsgBox "Connection error: " & Err.Description Exit Sub End If Set cmd = New ADODB.Command Set cmd.ActiveConnection = cn cmd.CommandText = q cmd.CommandType = adCmdText Set rs = New ADODB.Recordset Set rs.Source = cmd cn.Errors.Clear rs.Open If Err.Number = 0 Then CopyRows rs Else MsgBox "Query error: " & Err.Description End If rs.Close cn.Close End Sub
To execute the query, this routine uses three ActiveX Data Object (ADO) database objects: a Connection object, a Command object, and a Recordset object. Each is declared at the start of this routine. Next, error checking is disabled by using the On Error Resume Next statement because the routine checks for errors after any critical statement. The first step in retrieving rows from a database is to establish a connection to the database. A new instance of the ADODB.Connection object is created, and the connection string value that s passed to this routine is assigned to the Connection object s ConnectionString property. After instantiating the Connection object, the Open method is used to open a connection to the database. Tip
Out with the New Never use the New keyword when defining an object using a Dim, a Private, or a Public state ment. Visual Basic for Applications (VBA) includes extra code around each object reference to determine whether the object has been instantiated. If the object hasn t been instanti ated, the code will automatically create a new instance of the object for you. Although this extra code doesn t add a lot of overhead, you will be better off controlling exactly when a new object is instantiated.
If there was a problem opening the connection, an error message is displayed to the user and the Exit Sub statement is used to leave the subroutine with any further processing. 513
Part 6: Excel and the Outside World: Collaborating Made Easy
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Microsoft Office Excel 2003 Programming Inside Out Next a new instance of the ADODB.Command object is created. The newly opened Connection object is assigned to the ActiveConnection property, and the query is assigned to the CommandText property. Finally, the CommandType property is set to adCmdText, meaning that the Command object contains an SQL statement. A new ADODB.Recordset object is created, and the Source property is set to the Command object that was just initialized. The Connection object s Errors collection is explicitly cleared, and then the Recordset object s Open method is called. Any errors that occur while opening the Recordset are trapped, and an error message is dis played to the user. Otherwise, the CopyRows subroutine is called to copy the rows from the Recordset object to the current worksheet. Finally, the Recordset object and the Connection object are closed (in that order). These steps release any resources held by those objects back to the operating system. Tip Close Your Connections For the best database server performance, always minimize the amount of time that a connection is open. Closing unneeded connections reduces the resources required to run the database server, which in turn lets the database server handle more connections and perform more work.
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