Querying Information in a Database in Visual Studio .NET

Creator PDF417 in Visual Studio .NET Querying Information in a Database

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Querying Information in a Database
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2. After the catch block, add the following nally block:
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catch(SqlException e) { ... } finally { dataConnection.Close(); }
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Database connections are scarce resources. You need to ensure that they are closed when you have nished with them. Putting this statement in a nally block guarantees that the SqlConnection will be closed, even if an exception occurs; remember that the code in the nally block will be executed after the catch handler has nished. Tip An alternative approach to using a nally block is to wrap the code that creates the
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SqlDataConnection object in a using statement, as shown in the following code. At the end of the block de ned by the using statement, the SqlConnection object is closed automatically, even if an exception occurs:
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using (SqlConnection dataConnection = new SqlConnection()) { try { dataConnection.ConnectionString = ... ; ... } catch (SqlException e) { Console.WriteLine( Error accessing the database: {0} , e.Message); } }
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3. On the Debug menu, click Start Without Debugging to build and run the application. 4. At the customer ID prompt, type the customer ID VINET, and press Enter. The SQL SELECT statement appears, followed by the orders for this customer, as shown in the following image:
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Part V Managing Data
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You can scroll back through the console window to view all the data. Press the Enter key to close the console window when you have nished. 5. Run the application again, and then type BONAP when prompted for the customer ID. Some rows appear, but then an error occurs. If you are using Windows Vista, a message box appears with the message ReportOrders has stopped working. Click Close program (or Close the program if you are using Visual C# Express). If you are using Windows XP, a message box appears with the message ReportOrders has encountered a problem and needs to close. We are sorry for the inconvenience. Click Don t Send. An error message containing the text Data is Null. This method or property cannot be called on Null values appears in the console window. The problem is that relational databases allow some columns to contain null values. A null value is a bit like a null variable in C#: It doesn t have a value, but if you try to read it, you get an error. In the Orders table, the ShippedDate column can contain a null value if the order has not yet been shipped. You should also note that this is a SqlNullValueException and consequently is not caught by the SqlException handler. 6. Press Enter to close the console window and return to Visual Studio 2008.
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Closing Connections
In many older applications, you might notice a tendency to open a connection when the application starts and not close the connection until the application terminates. The rationale behind this strategy was that opening and closing database connections were expensive and time-consuming operations. This strategy had an impact on the scalability of applications because each user running the application had a connection to the database open while the application was running, even if the user went to lunch for a few hours. Most databases limit the number of concurrent connections that they allow. (Sometimes this is because of licensing, but usually it s because each connection consumes resources on the database server that are not in nite.) Eventually, the database would hit a limit on the number of users that could operate concurrently. Most .NET Framework data providers (including the SQL Server provider) implement connection pooling. Database connections are created and held in a pool. When an application requires a connection, the data access provider extracts the next available connection from the pool. When the application closes the connection, it is returned to the pool and made available for the next application that wants a connection. This means that opening and closing database connections are no longer expensive operations. Closing a connection does not disconnect from the database; it just returns the connection to the pool. Opening a connection is simply a matter of obtaining an already-open connection from the pool. Therefore, you should not hold on to connections longer than you need to open a connection when you need it, and close it as soon as you have nished with it.
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