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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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column to an Orders table that uses a relationship to calculate the total from the OrderItem child table. This example adds an average price column to the DataSet used in the last example, and displays it at the top of the grid when a publisher is chosen. First you add a line of code to the GetSourceData method, just after the relation is created. ds.Tables["Publishers"].Columns.Add( new DataColumn( "AveragePrice", typeof(double), "Avg(Child.price)")); Here you re adding a new DataColumn to the collection of columns for the publisher table. The first two constructor parameters are the name and type of the column. The third is our column expression. This string tells the DataTable to use the relation to get to its child table, and compute an average of all the prices in its collection of child rows. Other simple aggregates are valid in these expressions as well. If there s more than one relation on the parent table, you can also build the relation name into the expression. Avg(Child(Pub_Title).price) You can now use this column to output the average when the user makes a selection from the publisher list. You ve added a label control named lblAvgPrice next to the drop-down list in the markup of the page. The output is then done from the SelectedIndexChanged event trap. if (dv[0]["AveragePrice"] != DBNull.Value) lblAvgPrice.Text = Convert.ToDouble(dv[0]["AveragePrice"]).ToString("C");
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The DataSet gets a nice set of enhancements in version 2.0 of the Framework. The performance of the DataSet has been optimized for large row counts in a DataTable. Performance here is typically twice as fast. The indexing engine is a complete rewrite, also to accommodate large row counts. For Remoting, the DataSet can now be marshaled across processes as a binary stream of information, greatly reducing the footprint of what gets passed on the wire. The DataTable itself also gets a major facelift, reducing many of the dependencies it has on the DataSet in version 1.x of the Framework. For example, with version 1.x of the Framework there s no way you can read an XML document from the file system and create a DataTable without using a DataSet, even if you know the DataSet will have only a single DataTable in it. And a new method on the DataTable allows you to get a reference to a data reader for reading the DataTable data using a cursor-based approach, without the aid of a DataAdapter. A very useful new feature of the DataTable allows you to read data from the database using a data reader and create an instance of a DataTable without using the services of a DataAdapter and without involving a DataSet. This would take a significant amount of coding to do in 1.x, but in 2.0, it s built into the DataTable via the Load method. Here you use the new Load method on the DataTable to load the author's data into memory (see DataTableIE.aspx in the Web10 project).
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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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public DataTable GetAuthorData() { string conn = ConfigurationManager.ConnectionStrings["localPubs"].ToString(); SqlConnection cn = new SqlConnection(conn); SqlCommand cm = new SqlCommand("select * from authors", cn); DataTable dt = new DataTable(); cn.Open(); dt.Load(cm.ExecuteReader()); cn.Close(); return dt; } The benefit here is twofold. First, you don t have to create a DataAdapter. The smarts of the adapter have been built into the DataTable, making the coding model much simpler. Second, you don t need to create a DataSet. Creating an instance of a DataSet is pure wasted overhead whenever you re expecting a single result set. What good is it with a single result set It can write out the data as XML, you may say. It can write out schema for the data, too! you may add. Well the DataTable can now do those things, too. The ReadXML and WriteXml methods from the DataSet have been added to the DataTable, along with the corresponding methods for reading and writing schema information, so you can do single table transformations from hierarchical to relational data and back again. Here you see a method that will write a DataTable out to disk (in DataTableIE.aspx from Web10). public void SaveDataTableToDisk(DataTable dt) { string schemaFile = string.Format("{0}.xsd",dt.TableName); string xmlFile = string.Format("{0}.xml",dt.TableName); schemaFile = Server.MapPath(schemaFile); xmlFile = Server.MapPath(xmlFile); dt.WriteXmlSchema(schemaFile); dt.WriteXml(xmlFile); } This code may seem familiar to you, because WriteXml and WriteXmlSchema have been available on the DataSet for years. This is their first appearance on the DataTable, though. You can read the DataTable back in from disk with other familiar methods adopted from the DataSet, as well (in DataTableIE.aspx from Web10). public DataTable ReadDataTableFromDisk(string TableName) { DataTable dt = new DataTable(TableName); string schemaFile = string.Format("{0}.xsd", TableName); string xmlFile = string.Format("{0}.xml", TableName); schemaFile = Server.MapPath(schemaFile); xmlFile = Server.MapPath(xmlFile);
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