Loading Data Sets from a Database in VS .NET

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Loading Data Sets from a Database
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You may occasionally load data into a data set from an XML file that has been cached on the client machine, and you may even (more rarely) have occasion to programmatically create a data set as shown in the previous section However, the vast majority of the time you will be using data sets in conjunction with some form of relational data store When that is the case, you will be loading the data into a data set using one of the managed providers introduced earlier, or you will be using typed data sets, where the schema is built into the type instead of needing to be determined dynamically I am going to focus on the use of the SQL Server managed provider, as it is probably the most common database used in NET applications If you need to use the OLE DB, Oracle, or ODBC providers, the coding patterns are virtually identical, thanks to the fact that they are all based on the same interfaces Keep in mind that the SQL Server managed provider can be used with either a full-up instance of SQL Server or with a SQL Server 2005 Express instance, which is just a scaled-down free version of the SQL Server engine I recommend that you use SQL Express for creating client-side or small application databases, rather than Microsoft Access databases The SQL Express engine is much more robust for client-server applications, and you can use many features of the SQL Server engine that aren't available in Access, such as stored procedures and triggers You can easily establish a connection to a SQL Express database just by specifying a file path to the MDF file that contains the database You have already seen one case of loading a data set from a SQL Server table earlier in the section Relational Data Access You can also use a SQL Server 2005 Express database connection, as shown in Listing D3
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Listing D3 Filling a DataSet Through a SQL 2005 Express Connection
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private DataSet GetCustomersFromDb() { // Create the DataSet DataSet ds = new DataSet(); // Create the connection to the SQL Express DB SqlConnection conn = new SqlConnection( @"server=\SQLEXPRESS; AttachDbFileName=|DataDirectory|\SimpleDatabasemdf; trusted_connection=true"); // Create the command and the adapter that uses it SqlCommand cmd = new SqlCommand("SELECT * FROM Customers", conn); SqlDataAdapter adapter = new SqlDataAdapter(cmd); // Opens the connection, executes the query, // puts the result in the DataSet, closes the connection adapterFill(ds, "Customers"); // Return the result return ds; }
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Let's dissect the code in Listing D3 First, to create a connection, you construct an instance of theSqlConnection class The constructor for this class takes a connection string that can contain any number of parameters, as defined by the SQL Server managed provider The set of allowable parameters in the connection string are determined by each managed provider and will be different for each There are a number of different parameter names for some of the typical parameters; consult the documentation for the ConnectionString property of the SqlConnection class for a comprehensive list The most common parameters to use for a SQL Server database are the server, database, TRusted_connection, user id, and password parameters There are other parameter names that accomplish the same thing as these (such as pwd instead of password and Integrated Security instead of TRusted_connection), so you can choose which ones you want to use for common connection scenarios You can see from this example that with SQL Express 2005, you can use the AttachDBFilename parameter to specify the database based on a file path The |DataDirectory| keyword is a special syntax that instructs the connection object to look in the working directory of the executing application The connection string also specifies the server instance to be the default SQLEXPRESS instance, and trusted_connection=true uses integrated security, which will log you into the database using the credentials of the Windows account under which the application is running You can also pass an explicit username and password in the connection string if you want to use a SQL Server login Normally, you wouldn't want to hard-code connection strings in your application code as shown in Listing D3 A better way is to place them into your configuration file, and both Visual Studio 2005 and NET 20 support a rich new model for placing connection strings and other custom user and application settings in your configuration file, while still allowing you to programmatically access those settings in a strongly typed way These features are discussed in detail in s 2 and 5 Once you have a connection to work with, you create a SqlCommand object, passing the SQL text query to execute and the connection to use to the constructor A command object can be provided with SQL statements or with the name of a stored procedure to use For real data access layers, I recommend you wrap your data access at the database level in stored procedures and only consume those stored procedures from your data layer Doing so decouples your application code from the specific schema of your tables, preventing small changes to the underlying schema from affecting application code Stored procedures also let you use SQL Server's own security mechanisms to prevent direct access to the tables, and stored procedures sometimes have higher performance than dynamic SQL statements executed from the code in Listing D3 (although the performance difference is insignificant for most queries in NET) I have broken this guidance in many of the code samples in this book to be able to use the Northwind database with minimal modification, and so that you could clearly see what is being retrieved by the queries
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