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There are several scenarios this block specifically sets out to simplify (see Table 11-6). These data access methods require many lines of code when you re using a Managed Provider directly, but only a few when you re using the helper methods exposed by the block. Table 11-6. Different Data Access Scenarios the Data Access Application Block Assists With
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A couple of methods geared towards helping with a command that prepares and returns tabular data exist. Results can be returned with a data reader or a data set. Simple queries can be executed, and results returned, with two lines of code. A number of commands to send to a database that do not return a result set exist. Helper methods simplify several of these. You may be doing inserts, updates, and deletes; doing work within a transaction; or may expect output parameters or a single value. After changes are applied in memory to data tables in a Dataset, the time comes to ship these back. The block can do this without a data adapter. The block will automatically transform result sets into XML.
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Executing commands that do not return result sets
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Sending Dataset updates back to the database Getting XML data
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These scenarios are all exposed via methods on the Database object. The snippet that follows connects to the pubs database (described as the default in the configuration file) and binds it to an instance of the GridView control:
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CHAPTER 11 DATA ACCESS LAYER SERVICES
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protected void Page_Load(object sender, EventArgs e) { Database db = DatabaseFactory.CreateDatabase(); GridView1.DataSource = db.ExecuteReader(CommandType.Text, "select * from authors"); GridView1.DataBind(); } You could do this just as easily by caching a DataSet: void Page_Load(object sender, EventArgs e) { GridView1.DataSource = GetAuthors(); GridView1.DataBind(); } private DataSet GetAuthors() { DataSet ds; ds = (DataSet)Cache["Authors"]; if (ds == null) { Database db = DatabaseFactory.CreateDatabase(); ds = db.ExecuteDataSet(CommandType.Text, "select * from authors"); Cache.Insert("Authors", ds); } return ds; } In both of these cases, the amount of code is reduced compared to what you d need if you used a Managed Provider directly. Changes to the configuration file can switch the type of database in use. And the connection lifetime is managed by the block, creating consistently in your application s data access code. Regardless of the data access method needed for a given result set, the block standardizes the code and the process that will be used to acquire resources, execute commands, and release those resources. Dynamic SQL generation works fine for limited scenarios. Typically statements are more complex, or data access is being done with stored procedures, which usually require parameters. When parameters are in use, whether they re built into a dynamic SQL statement, or they re input or output parameters to a stored procedure, it is time to use a command wrapper.
CHAPTER 11 DATA ACCESS LAYER SERVICES
The Command Wrapper Object
The command wrapper is a helper type for a Command object in a Managed Provider. It s created for you by a database object and automatically populates several properties of the wrapped Command. There are two main factory methods for retrieving an instance of a command wrapper (see Table 11-7). Table 11-7. Factory Methods for Command Wrapper Objects
Factory Method
GetStringCommandWrapper GetStoredProcCommandWrapper
Meaning in Life
Creates a wrapper configured for CommandType of Text. Creates a wrapper configured for stored procedure execution.
Once returned, the wrapper exposes AddXYZParameter methods to define the parameters using simple names, types, and values. Consider this stored procedure again. (Recall that we used it in 10 to demonstrate calling a stored procedure with Managed Provider code.) create procedure usp_GetPubDetails @pub_id char(4), @pub_name varchar(40) OUTPUT, @city varchar(20) OUTPUT, @state char(2) OUTPUT, @country varchar(30) OUTPUT as SELECT @pub_name = pub_name, @city = city, @state = state, @country = country FROM publishers WHERE (pub_id = @pub_id) The command wrapper code to execute this stored procedure is much simpler than in the Managed Provider code: private PublisherBO GetPublisher(string PubID) { PublisherBO boReturn = new PublisherBO(); Database db = DatabaseFactory.CreateDatabase(); DBCommandWrapper cw = db.GetStoredProcCommandWrapper("usp_GetPubDetails"); cw.AddInParameter("@pub_id", DbType.String, PubID); cw.AddOutParameter("@pub_name", DbType.String, 40); cw.AddOutParameter("@city", DbType.String, 20); cw.AddOutParameter("@state", DbType.String, 2); cw.AddOutParameter("@country", DbType.String, 30);
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