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(local) is an alternative to the . (dot) to specify the local machine, so .\sqlexpress can be replaced with (local)\sqlexpress.
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The next clause indicates that you should use Windows Authentication (i.e., any valid logged-on Windows user can log on to SSE): integrated security = true You could alternatively have used sspi instead of true, as they both have the same effect. Other parameters are available. You ll use one later to specify the database to which you want to connect. Next you create a connection (a SqlConnection object), passing it the connection string. This doesn t create a database session. It simply creates the object you ll use later to open a session: 'Create connection Dim conn As SqlConnection = New SqlConnection(connstring)
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CHAPTER 10 s MAKING CONNECTIONS
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Now you have a connection, but you still need to establish a session with the database by calling the Open method on the connection. If the attempt to open a session fails, an exception will be thrown, so you use a try statement to enable exception handling. You display a message after calling Open, but this line will be executed only if the connection was successfully opened. Try ' Open connection conn.Open() Console.WriteLine("Connection opened") At this stage in the code, you d normally issue a query or perform some other database operation over the open connection. However, we ll save that for later chapters and concentrate here on just connecting. Next comes an exception handler in case Open() fails: Catch e As SqlException Console.WriteLine("Error: " &
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Each data provider has a specific exception class for its error handling; SqlException is the class for the SQL Server data provider. Specific information about database errors is available from the exception, but here you re just displaying its raw contents. When you ve finished with the database, you call Close() to terminate the session and then print a message to show that Close() was called: Finally ' Close connection conn.Close() Console.WriteLine("Connection closed.") End Try You call Close() within the finally block to ensure it always gets called.
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s Note Establishing connections (database sessions) is relatively expensive. They use resources on both
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the client and the server. Although connections may eventually get closed through garbage collection or by timing out, leaving one open when it s no longer needed is a bad practice. Too many open connections can slow a server down or prevent new connections from being made.
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Note that you can call Close() on a closed connection, and no exception will be thrown. So, your message would have been displayed if the connection had been closed earlier or even if it had never been opened. See Figure 10-2, where the connection failed but the close message is still displayed. In one typical case, multiple calls to both Open() and Close() make sense. ADO.NET supports disconnected processing of data, even when the connection to the data provider has been closed. The pattern looks like this:
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CHAPTER 10 s MAKING CONNECTIONS
Try ' Open connection conn.Open() 'online processing (e.g., queries) here 'close connection conn.Close() 'offline processing here 'reopen connection conn.Open() 'online processing(e.g., INSERT/UPDATE/DELETE) here 'reclose connection conn.Close() Finally ' Close connection conn.Close() End Try The finally block still calls Close(), calling it unnecessarily if no exceptions are encountered, but this isn t a problem or expensive, and it ensures the connection will be closed. Although many programmers hold connections open until program termination, this is usually wasteful in terms of server resources. With connection pooling, opening and closing a connection as needed is actually more efficient than opening it once and for all. That s it! You ve finished with the first connection example. However, since you saw a possible error, let s look at typical causes of connection errors.
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