Figure 121 NET Data Architecture in VS .NET

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Figure 121 NET Data Architecture
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This chapter and 13 include a lot of code samples that depend on an instance of SQL Server running with the Northwind database installed If you don't have SQL Server running, you can install the Microsoft SQL Server Developer Edition (MSDE) that comes with the NET Framework SDK Follow the instructions in Start | Programs | Microsoft NET Framework SDK | Samples and QuickStartTutorials | Install the NET Framework Samples Database[2]
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If you're using SQL Server, whether it's MSDE or not, you'll want to watch out for the SQL Slammer, which is covered at http://wwwmicrosoftcom/security/slammerasp
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Retrieving Data
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Given this basic architecture, the following shows an example of filling a DataSet object using the classes from the SystemData namespace and the SQL Server data provider classes from
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the SystemDataSqlClient namespace: using SystemData; using SystemDataSqlClient; // Access to SQL Server // A data set for use by the form DataSet dataset = new DataSet(); void Form1_Load(object sender, EventArgs e) { // Configure the connection SqlConnection conn = new SqlConnection(@"Server=localhost;"); // Create the adapter from the connection SqlDataAdapter adapter = new SqlDataAdapter(connCreateCommand()); adapterSelectCommandCommandText = "select * from customers"; // Fill the data set with the Customers table adapterFill(dataset); // Populate list box PopulateListBox(); } void PopulateListBox() { // Clear the list box listBox1ItemsClear(); // Enumerate cached data foreach( DataRow row in datasetTables[0]Rows ) { string item = row["ContactTitle"] + ", " + row["ContactName"]; listBox1ItemsAdd(item); } } This code creates a connection using a data-provider-specific connection string , which tells the connection where to go to get the data It then creates an adapter with the appropriate command text to retrieve data over the connection The adapter is used to fill the data set, which produces one table The code then enumerates the table's rows, picking out columns by name that we happen to know that the table will contain Then it uses the data to populate items in a list box, as shown in Figure 122
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Figure 122 Showing Retrieved Data
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Notice that although the sample code creates a connection, it never opens or closes it Instead, the data adapter opens the connection for an operation in this case, retrieving the data and filling the data set and closes it when an operation is complete The data set itself never uses the connection, nor does it know about where the data comes from It's the data adapter's job to translate data in provider-specific format into the provider-neutral data set Because the data set has no concept of a connection to the provider, it is a cache of both data and operations on data Data can be updated and even removed from the data set, but those operations aren't reflected to the actual provider until you tell the data adapter to do so Before we discuss that, however, let's take a look at the rest of the common data operations: creating, updating, and deleting data
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Creating Data
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Creating a new row in a table is a matter of asking the table for an empty DataRow object and filling it with column data: void addRowMenuItem_Click(object sender, EventArgs e) { // Ask table for an empty DataRow DataRow row = datasetTables[0]NewRow(); // Fill DataRow with column data row["CustomerID"] = "SELLSB"; // Add DataRow to the table datasetTables[0]RowsAdd(row); // Update list box PopulateListBox(); }
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Updating Data
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You can update existing data by reaching into the data set, pulling out the row of interest, and updating the column data as appropriate: void updateSelectedRowMenuItem_Click(object sender, EventArgs e) { // Get selection index from list box int index = listBox1SelectedIndex; if( index == -1 ) return; // Get row from data set DataRow row = datasetTables[0]Rows[index]; // Update the row as appropriate row["ContactTitle"] = "CEO"; // Update list box PopulateListBox(); }
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