Evjen c08.tex V2 - 01/28/2008 2:05pm Page 446 Chapter 8: Data Management with ADO.NET to a specific customer. The GridView control on the bottom shows the results of executing the second query that retrieved a list of all orders submitted by a specific customer. Figure 8-25 The code shown in Listing 8-33 reveals some of the elegance of using the wait approach. However, it is still not the most efficient code you can write with ADO.NET 2.0. The code should allow for a wait until both asynchronous processes finish running before the data binds the result sets to the respective GridView controls. You can change the code shown in Listing 8-33 just a little t o gain even more efficiency. Replace the WaitAll method with the WaitAny method. The WaitAny method enables you to handle the results of each of the asynchronous processes as soon as each is completed without waiting for other processing to finish. To use the WaitAny method and still manage the execution of all asynchronous processes, you can also add a loop that enables you to make sure that all asynchronous processes are handled after they are completed. The WaitAny method returns an Integer value that indicates an array index of the wait handle that has finished running. Using this return value, you can easily find the correct wait handle and process the result set retrieved from the query that was executed in that particular process, as shown in Listing 8-34. 446 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 447 Chapter 8: Data Management with ADO.NET Listing 8-34: Use of the WaitAny method to process multiple asynchronous processes VB < %@ Page Language="VB" % > < %@ Import Namespace="System.Data" % > < %@ Import Namespace="System.Data.SqlClient" % > < %@ Import Namespace="System.Configuration" % > < script runat="server" > Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Dim DBCon As SqlConnection Dim OrdersCommand As SqlCommand = New SqlCommand() Dim CustCommand As SqlCommand = New SqlCommand() Dim OrdersReader As SqlDataReader Dim CustReader As SqlDataReader Dim OrdersASyncResult As IAsyncResult Dim CustAsyncResult As IAsyncResult Dim WHIndex As Integer Dim WHandles(1) As Threading.WaitHandle Dim OrdersWHandle As Threading.WaitHandle Dim CustWHandle As Threading.WaitHandle DBCon = New SqlConnection() DBCon.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_NorthWind").ConnectionString CustCommand.CommandText = _ "SELECT * FROM Customers WHERE CompanyName = ’Alfreds Futterkiste’" CustCommand.CommandType = CommandType.Text CustCommand.Connection = DBCon OrdersCommand.CommandText = _ "SELECT Customers.CompanyName, Customers.ContactName, " & _ "Orders.OrderID, Orders.OrderDate, " & _ "Orders.RequiredDate, Orders.ShippedDate " & _ "FROM Orders, Customers " & _ "WHERE Orders.CustomerID = Customers.CustomerID " & _ "AND Customers.CompanyName = ’Alfreds Futterkiste’ " & _ "ORDER BY Customers.CompanyName, Customers.ContactName" OrdersCommand.CommandType = CommandType.Text OrdersCommand.Connection = DBCon ’ Opening the database connection DBCon.Open () Continued 447 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 448 Chapter 8: Data Management with ADO.NET ’ Retrieving customer information asynchronously CustAsyncResult = CustCommand.BeginExecuteReader() ’ Retrieving orders list asynchronously OrdersASyncResult = OrdersCommand.BeginExecuteReader() CustWHandle = CustAsyncResult.AsyncWaitHandle OrdersWHandle = OrdersASyncResult.AsyncWaitHandle ’ Filling Wait Handles array with the two wait handles we ’ are going to use in this code WHandles(0) = CustWHandle WHandles(1) = OrdersWHandle ’ Looping 2 times because there are 2 wait handles ’ in the array For Index As Integer = 0 To 1 ’ We are only waiting for any of the two ’ asynchronous process to finish running WHIndex = Threading.WaitHandle.WaitAny(WHandles) ’ The return value from the WaitAny method is ’ the array index of the Wait Handle that just ’ finsihed running Select Case WHIndex Case 0 CustReader = CustCommand.EndExecuteReader(CustAsyncResult) gvCustomers.DataSource = CustReader gvCustomers.DataBind() Case 1 OrdersReader = _ OrdersCommand.EndExecuteReader(OrdersASyncResult) gvOrders.DataSource = OrdersReader gvOrders.DataBind() End Select Next ’ Closing connection DBCon.Close() End Sub < /script > < html xmlns="http://www.w3.org/1999/xhtml" > < head id="Head1" runat="server" > < title > The Wait Any Approach < /title > < /head > < body > < form id="form1" runat="server" > < div > < asp:GridView ID="gvCustomers" Width="100%" runat="server" >< /asp:GridView > < br / >< br / > Continued 448 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 449 Chapter 8: Data Management with ADO.NET < asp:GridView ID="gvOrders" Width="100%" AutoGenerateColumns="False" runat="server" > < Columns > < asp:BoundField HeaderText="Company Name" DataField="CompanyName" >< /asp:BoundField > < asp:BoundField HeaderText="Contact Name" DataField="ContactName" >< /asp:BoundField > < asp:BoundField HeaderText="Order Date" DataField="orderdate" DataFormatString="{0:d}" >< /asp:BoundField > < asp:BoundField HeaderText="Required Date" DataField="requireddate" DataFormatString="{0:d}" >< /asp:BoundField > < asp:BoundField HeaderText="Shipped Date" DataField="shippeddate" DataFormatString="{0:d}" >< /asp:BoundField > < /Columns > < /asp:GridView > < /div > < /form > < /body > < /html > C# < %@ Page Language="C#" % > < %@ Import Namespace="System.Data" % > < %@ Import Namespace="System.Data.SqlClient" % > < %@ Import Namespace="System.Configuration" % > < script runat="server" > protected void Page_Load(object sender, EventArgs e) { SqlConnection DBCon; SqlCommand OrdersCommand = new SqlCommand(); SqlCommand CustCommand = new SqlCommand(); SqlDataReader OrdersReader; SqlDataReader CustReader; IAsyncResult OrdersASyncResult; IAsyncResult CustAsyncResult; int WHIndex; System.Threading.WaitHandle[] WHandles = new System.Threading.WaitHandle[1]; System.Threading.WaitHandle OrdersWHandle; System.Threading.WaitHandle CustWHandle; DBCon = new SqlConnection(); DBCon.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_NorthWind"].ConnectionString; CustCommand.CommandText = "SELECT * FROM Customers WHERE CompanyName = ’Alfreds Futterkiste’"; CustCommand.CommandType = CommandType.Text; CustCommand.Connection = DBCon; OrdersCommand.CommandText = Continued 449 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 450 Chapter 8: Data Management with ADO.NET "SELECT Customers.CompanyName, Customers.ContactName, " + "Orders.OrderID, Orders.OrderDate, " + "Orders.RequiredDate, Orders.ShippedDate " + "FROM Orders, Customers " + "WHERE Orders.CustomerID = Customers.CustomerID " + "AND Customers.CompanyName = ’Alfreds Futterkiste’ " + "ORDER BY Customers.CompanyName, Customers.ContactName"; OrdersCommand.CommandType = CommandType.Text; OrdersCommand.Connection = DBCon; // Opening the database connection DBCon.Open(); // Retrieving customer information asynchronously CustAsyncResult = CustCommand.BeginExecuteReader(); // Retrieving orders list asynchronously OrdersASyncResult = OrdersCommand.BeginExecuteReader(); CustWHandle = CustAsyncResult.AsyncWaitHandle; OrdersWHandle = OrdersASyncResult.AsyncWaitHandle; // Filling Wait Handles array with the two wait handles we // are going to use in this code WHandles[0] = CustWHandle; WHandles[1] = OrdersWHandle; // Looping 2 times because there are 2 wait handles // in the array for (int Index = 0; Index < 2; Index++ ) { // We are only waiting for any of the two // asynchronous process to finish running WHIndex = System.Threading.WaitHandle.WaitAny(WHandles); // The return value from the WaitAny method is // the array index of the Wait Handle that just // finsihed running switch (WHIndex) { case 0: CustReader = CustCommand.EndExecuteReader(CustAsyncResult); gvCustomers.DataSource = CustReader; gvCustomers.DataBind(); break; case 1: OrdersReader = OrdersCommand.EndExecuteReader(OrdersASyncResult); gvOrders.DataSource = OrdersReader; gvOrders.DataBind(); break; Continued 450 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 451 Chapter 8: Data Management with ADO.NET } } // Closing connection DBCon.Close(); } < /script > Next, look at the callback approach. Using this approach, you assign a callback method to the asyn- chronous process and use it to display the result returned by executing the SQL query. The Callback Approach Listing 8-35 creates an inline SQL statement that retrieves the top five records f rom the database. It starts the asynchronous process by calling the BeginExecuteReader method and passing it the callback delegate. No further processing is needed, and the method ends a fter the asynchronous process has started. After the callback method is fired, it retrieves the result and displays it on the screen. Listing 8-35: Asynchronous command processing using the callback approach VB < %@ Page Language="C#" % > < %@ Import Namespace="System.Data" % > < %@ Import Namespace="System.Data.SqlClient" % > < %@ Import Namespace="System.Configuration" % > < script runat="server" > Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Dim DBCon As SqlConnection Dim Command As SqlCommand = New SqlCommand() Dim ASyncResult As SqlAsyncResult DBCon = New SqlConnection() Command = New SqlCommand() DBCon.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_NorthWind").ConnectionString ’ Selecting top 5 records from the Orders table Command.CommandText = _ "SELECT TOP 5 Customers.CompanyName, Customers.ContactName, " & _ "Orders.OrderID, Orders.OrderDate, " & _ "Orders.RequiredDate, Orders.ShippedDate " & _ "FROM Orders, Customers " & _ "WHERE Orders.CustomerID = Customers.CustomerID " & _ "ORDER BY Customers.CompanyName, Customers.ContactName" Command.CommandType = CommandType.Text Command.Connection = DBCon DBCon.Open() ’ Starting the asynchronous processing AsyncResult = Command.BeginExecuteReader(New _ AsyncCallback(AddressOf CBMethod), CommandBehavior.CloseConnection) Continued 451 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 452 Chapter 8: Data Management with ADO.NET End Sub Public Sub CBMethod(ByVal ar As SQLAsyncResult) Dim OrdersReader As SqlDataReader ’ Retrieving result from the asynchronous process OrdersReader = ar.EndExecuteReader(ar) ’ Displaying result on the screen gvOrders.DataSource = OrdersReader gvOrders.DataBind() End Sub < /script > < html xmlns="http://www.w3.org/1999/xhtml" > < head id="Head1" runat="server" > < title > The Call Back Approach < /title > < /head > < body > < form id="form1" runat="server" > < div > < asp:GridView ID="gvOrders" Width="100%" AutoGenerateColumns="False" runat="server" > < Columns > < asp:BoundField HeaderText="Company Name" DataField="CompanyName" >< /asp:BoundField > < asp:BoundField HeaderText="Contact Name" DataField="ContactName" >< /asp:BoundField > < asp:BoundField HeaderText="Order Date" DataField="orderdate" DataFormatString="{0:d}" >< /asp:BoundField > < asp:BoundField HeaderText="Required Date" DataField="requireddate" DataFormatString="{0:d}" >< /asp:BoundField > < asp:BoundField HeaderText="Shipped Date" DataField="shippeddate" DataFormatString="{0:d}" >< /asp:BoundField > < /Columns > < /asp:GridView > < /div > < /form > < /body > < /html > C# < %@ Page Language="C#" % > < %@ Import Namespace="System.Data" % > < %@ Import Namespace="System.Data.SqlClient" % > < %@ Import Namespace="System.Configuration" % > < script runat="server" > protected void Page_Load(object sender, EventArgs e) { SqlConnection DBCon; Continued 452 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 453 Chapter 8: Data Management with ADO.NET SqlCommand Command = new SqlCommand(); SqlAsyncResult ASyncResult; DBCon = new SqlConnection(); Command = new SqlCommand(); DBCon.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_NorthWind"].ConnectionString; // Selecting top 5 records from the Orders table Command.CommandText = "SELECT TOP 5 Customers.CompanyName, Customers.ContactName, " + "Orders.OrderID, Orders.OrderDate, " + "Orders.RequiredDate, Orders.ShippedDate " + "FROM Orders, Customers " + "WHERE Orders.CustomerID = Customers.CustomerID " + "ORDER BY Customers.CompanyName, Customers.ContactName"; Command.CommandType = CommandType.Text; Command.Connection = DBCon; DBCon.Open(); // Starting the asynchronous processing AsyncResult = Command.BeginExecuteReader(new AsyncCallback(CBMethod), CommandBehavior.CloseConnection); } public void CBMethod(SQLAsyncResult ar) { SqlDataReader OrdersReader; // Retrieving result from the asynchronous process OrdersReader = ar.EndExecuteReader(ar); // Displaying result on the screen gvOrders.DataSource = OrdersReader; gvOrders.DataBind(); } < /script > The callback approach enables you to handle the result of a command execution at a different part of your code. This feature is useful in cases where the command execution takes longer than usual and you want to respond to the user without waiting for the command execution to finish. Canceling Asynchronous Processing The asynchronous process often takes longer than expected. To alleviate this problem, you can provide an option to the user to cancel the process without waiting for the result. Canceling an asynchronous process is as easy as calling the Cancel method on the appropriate Command object. This method doesn’t return any value. To roll back the work that was already completed by the Command object, you must provide a custom transaction to the Command object before executing the query. You can also handle the rollback or the commit process yourself. 453 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 454 Chapter 8: Data Management with ADO.NET Asynchronous Connections Now that you understand how to execute multiple database queries asynchronously using the Command object, take a quick look at how you can open database connections asynchronously, as well. The prin- ciples of working with asynchronous connections are the same as when you work with asynchronous commands. You can still use any of the three approaches you learned previously. In ADO.NET, the SqlConnection class exposes a couple of new properties needed when working asyn- chronously. These properties are shown in the following table. Property Description Asynchronous This read-only property returns a Boolean value indicating whether the connection has been opened asynchronously. State This property returns a value from System.Data.ConnectionState enumeration indicating the state of the connection. The possible values are as follows: ❑ Broken ❑ Closed ❑ Connecting ❑ Executing ❑ Fetching ❑ Open Summary In summary, ADO.NET is a powerful tool to incorporate within your ASP.NET applications. ADO.NET has a number of new technologies that provide you with data solutions that you could only dream of in the past. Visual Studio also makes ADO.NET programming quick and easy when you use the wizards that are available. In this chapter, you saw a number of the wizards. You do not have to use these wizards in order to work with ADO.NET. On the contrary, you can use some of the wizards and create the rest o f the code yourself, or you can use none of the wizards. In any case, you have complete and full access to everything that ADO.NET provides. This chapter covered a range of advanced features of ADO.NET as well. These features are designed to give you the flexibility to handle database processing in a manner never before possible with either of the previous versions of ADO.NET or ADO. This chapter also covered the features of Multiple Active Result Sets (MARS), which enables you to reuse a single open connection f or multiple accesses to the database, even if the connection is currently processing a result set. This feature becomes even more powerful when it is used in conjunction with the asynchronous command processing. 454 Evjen c09.tex V2 - 01/28/2008 2:09pm Page 455 Querying with LINQ .NET 3.5 introduces a new technology called Language Integrated Query, or LINQ (pronounced ‘‘link’’). LINQ is designed to fill the gap that exists between traditional .NET languages, which offer strong typing and full object-oriented development, and query languages such as SQL, with syntax specifically designed for query operations. With the introduction of LINQ into .NET, query becomes a first class concept in .NET, whether object, XML, or data queries. LINQ includes three basic types of queries, LINQ to Objects, LINQ to XML (or XLINQ), and LINQ to SQL (or DLINQ). Each type of query offers specific capabilities and is designed to query a specific source. In this chapter, we look at all three flavors of LINQ, and how each enables you to simplify query operations. We also look at some new language features of the .NET CLR that you will use to create LINQ queries, as well as the tooling support added to Visual Studio to support using LINQ. LINQ to Objects The first and most basic flavor of LINQ is LINQ to Objects. LINQ to Objects enables you to perform complex query operations against any enumerable object (any object that implements the IEnumer- able interface). While the notion of creating enumerable objects that can be queried or sorted is not new to .NET, doing this in versions prior to version 3.5 usually required a significant amount of code. Often that code would end up being so complex that it would be hard for other developers to read and understand, making it difficult to maintain. Traditional Query Methods In order to really understand how LINQ improves your ability to query collections, you really need to understand how this is done without it. To do this, let’s take a look at how you might create a simple query that includes a group and sort without using LINQ. Listing 9-1 shows a simple Movie class you can use as the basis of these examples. . command processing. 454 Evjen c09.tex V2 - 01/28/2008 2:09pm Page 455 Querying with LINQ .NET 3. 5 introduces a new technology called Language Integrated Query, or LINQ (pronounced ‘‘link’’). LINQ. are going to use in this code WHandles[0] = CustWHandle; WHandles[1] = OrdersWHandle; // Looping 2 times because there are 2 wait handles // in the array for (int Index = 0; Index < 2; Index++. CustAsyncResult; int WHIndex; System.Threading.WaitHandle[] WHandles = new System.Threading.WaitHandle[1]; System.Threading.WaitHandle OrdersWHandle; System.Threading.WaitHandle CustWHandle; DBCon