Data Source: Managing Connections (ODBC)

Artigo
05/05/2015

This topic applies to the MFC ODBC classes.

This topic explains:

How to configure a data source.
How a multiuser environment affects a data source and its recordsets.
Why you generalize a connection string to a data source.
How to connect to a data source.
How to disconnect from a data source.
How to reuse a CDatabase object.

Connecting to a data source means establishing communications with a DBMS to access the data. When you connect to a data source from an application through an ODBC driver, the driver makes the connection for you, either locally or across a network.

You can connect to any data source for which you have an ODBC driver. Users of your application must also have the same ODBC driver for their data source. For more information about redistributing ODBC drivers, see Redistributing ODBC Components to Your Customers.

Configuring a Data Source

ODBC Administrator is used to configure your data sources. You can also use ODBC Administrator after installation to add or remove data sources. When you create applications, you can either direct your users to the ODBC Administrator to let them add data sources or you can build this functionality into your application by making direct ODBC installation calls. For more information, see ODBC Administrator.

You can use an Excel file as a data source, and you need to configure the file so that it is registered and appears in the Select Data Source dialog box.

To use an Excel file as a data source

Configure the file with the ODBC Data Source Administrator.
On the File DSN tab, click Add.
In the Create New Data Source dialog box, select an Excel driver, and then click Next.
Click Browse, and select the name of the file to be used as a date source.

Note

You might need to select All Files in the drop-down menu to view the .xls files.

Click Next, and then click Finish.
In the ODBC Microsoft Excel Setup dialog box, select the database Version and Workbook.

Working in a Multiuser Environment

If multiple users are connected to a data source, they can change data while you are manipulating it in your recordsets. Similarly, your changes might affect other users' recordsets. For more information, see Recordset: How Recordsets Update Records (ODBC) and Transaction (ODBC).

Generalizing the Connection String

The wizards use a default connection string to establish a connection to a data source. You use this connection to view tables and columns while you are developing your application. However, this default connection string might not be appropriate for your users' connections to the data source through your application. For example, their data source and the path to its location might be different from the one used in developing your application. In that case, you should reimplement the CRecordset::GetDefaultConnect member function in a more generic fashion and discard the wizard implementation. For example, use one of the following approaches:

Register and manage the connection strings using ODBC Administrator.
Edit the connection string and remove the data source name. The framework supplies ODBC as the data source; at run time, ODBC displays a dialog box asking for the data source name and any other required connection information.
Supply the data source name only. ODBC asks for the user ID and password, if required. For example, before generalizing, the connection string looks like this:
```
CString CApp1Set::GetDefaultConnect()
{
   return "ODBC;DSN=afx;Trusted_Connection=Yes;";
}
```
This connection string specifies a trusted connection, which uses Windows NT integrated security. You should avoid hard-coding a password or specifying a blank password, because doing so creates a major security weakness. Instead, you can give GetDefaultConnect a new connection string so that it queries for a user ID and password.
```
// User must select data source and supply user ID and password:
    return "ODBC;";
// User ID and password required:
    return "ODBC;DSN=mydb;";
// Password required (myuserid must be replaced with a valid user ID):
    return "ODBC;DSN=mydb;UID=myuserid;";
// Hard-coded user ID and password (SECURITY WEAKNESS--AVOID):
    return "ODBC;DSN=mydb;UID=sa;PWD=777;";
```

Connecting to a Specific Data Source

To connect to a specific data source, your data source must already have been configured with ODBC Administrator.

To connect to a specific data source

Construct a CDatabase object.
Call its OpenEx or Open member function.

For more information about how to specify the data source if it is something other than the one you specified with a wizard, see CDatabase::OpenEx or CDatabase::Open in the MFC Reference.

Disconnecting from a Data Source

You must close any open recordsets before calling the Close member function of CDatabase. In recordsets associated with the CDatabase object you want to close, any pending AddNew or Edit statements are canceled and all pending transactions are rolled back.

To disconnect from a data source

Call the CDatabase object's Close member function.
Destroy the object unless you want to reuse it.

Reusing a CDatabase Object

You can reuse a CDatabase object after disconnecting from it, whether you use it to reconnect to the same data source or to connect to a different data source.

To reuse a CDatabase object

Close the object's original connection.
Instead of destroying the object, call its OpenEx or Open member function again.

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