Evjen c08.tex V2 - 01/28/2008 2:05pm Page 436 Chapter 8: Data Management with ADO.NET Method Description WaitAll (waitHandles , milliseconds , exitContext) This overloaded method receives the time-out value in the form of milliseconds and a Boolean value specifying whether the method requires asynchronous context. It should be set to False for asynchronous processing. WaitAll (waitHandles , timeSpan, exitContext) This overloaded method receives the time-out value in the form of TimeSpan object. The second parameter receives a Boolean value specifying whether the method requires asynchronous context. It should be set to False for asynchronous processing. Close ( ) This method releases all wait handles and reclaims their resources. Now that you understand asynchronous methods added to the SqlCommand and how to properly interact with them, you can write some code to see the asynchronous processing in action. Approaches of Asynchronous Processing in ADO.NET You can process asynchronous commands in t hree distinct ways. One approach is to start t he asyn- chronous process and start polling the IAsyncResult object to see when the process has finished. The second approach is to provide a callback method while starting the asynchronous pro cess. This approach enables you to perform other tasks in parallel. When the asynchronous process finishes, it fires the callback method that cleans up after the process and notifies other parts of the program that the asyn- chronous process has finished. The third and most elegant method is to associate a wait handle with the asynchronous process. Using this approach, you can start all the asynchronous processing you want and then wait for all or any of them to finish so that you can process them accordingly. The Poll Approach The code shown in Listing 8-31 creates an inline SQL statement to retrieve the top five records from the Orders table from the Northwind database. It starts the asynchronous process by calling the Begin- ExecuteReader . After the asynchronous process has started, it uses a while loop to wait for the process to finish. While waiting, the main thread sleeps for 10 milliseconds after checking the status of the asyn- chronous process. After the process has finished, it retrieves the result using the EndExecuteReader method. Listing 8-31: The Poll approach to working with asynchronous commands 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 Command As SqlCommand = New SqlCommand() Dim OrdersReader As SqlDataReader Continued 436 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 437 Chapter 8: Data Management with ADO.NET Dim ASyncResult As IAsyncResult DBCon = New SqlConnection() DBCon.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_NorthWind").ConnectionString 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() ’ This loop with keep the main thread waiting until the ’ asynchronous process is finished While Not ASyncResult.IsCompleted ’ Sleeping current thread for 10 milliseconds System.Threading.Thread.Sleep(10) End While ’ Retrieving result from the asynchronous process OrdersReader = Command.EndExecuteReader(ASyncResult) ’ Displaying result on the screen gvOrders.DataSource = OrdersReader gvOrders.DataBind() ’ Closing connection DBCon.Close() End Sub < /script > < html xmlns="http://www.w3.org/1999/xhtml" > < head id="Head1" runat="server" > < title > The Poll Approach < /title > < /head > < body > < form id="form1" runat="server" > < div > < asp:GridView ID="gvOrders" runat="server" AutoGenerateColumns="False" Width="100%" > < Columns > < asp:BoundField HeaderText="Company Name" DataField="CompanyName" >< /asp:BoundField > < asp:BoundField HeaderText="Contact Name" DataField="ContactName" >< /asp:BoundField > Continued 437 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 438 Chapter 8: Data Management with ADO.NET < 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 Command = new SqlCommand(); SqlDataReader OrdersReader; IAsyncResult ASyncResult; DBCon = new SqlConnection(); DBCon.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_NorthWind"].ConnectionString; 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(); // This loop with keep the main thread waiting until the // asynchronous process is finished while (!ASyncResult.IsCompleted) { // Sleeping current thread for 10 milliseconds System.Threading.Thread.Sleep(10); } Continued 438 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 439 Chapter 8: Data Management with ADO.NET // Retrieving result from the asynchronous process OrdersReader = Command.EndExecuteReader(ASyncResult); // Displaying result on the screen gvOrders.DataSource = OrdersReader; gvOrders.DataBind(); // Closing connection DBCon.Close(); } < /script > Ifyousetabreakpointatthe while loop, you will be able to see that the code execution continues after calling the BeginExecuteReader method. The code then continues to loop until the asynchronous execution has finished. The Wait Approach The most elegant of the three approaches is neither the poll approach nor the callback approach. The approach that provides the highest level of flexibility, efficiency, and (admittedly) a bit more complexity is the wait approach. Using this approach, you can write code that starts multiple asynchronous processes and waits for any or all the processes to finish running. This approach allows you to wait for only those processes that are dependent on each other and to proceed with the ones that don’t. This approach, by its design, requires you to think about asynchronous processes in great detail. You must pick a good candidate for running in parallel and, most importantly, determine how different processes depend on each other. The complexity of this approach requires you to understand its details and design the code accordingly. The end result is, typically, a very elegant code design that makes the best use of synchronous and asynchronous processing models. The code shown in Listing 8-32 uses the WaitOne method of the WaitHandle class. This method causes the program execution to wait until the asynchronous process has finished running. Listing 8-32: The wait approach to handling a single asynchronous process 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 Command As SqlCommand = New SqlCommand() Dim OrdersReader As SqlDataReader Dim ASyncResult As IAsyncResult Dim WHandle As Threading.WaitHandle DBCon = New SqlConnection() DBCon.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_NorthWind").ConnectionString Continued 439 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 440 Chapter 8: Data Management with ADO.NET 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() WHandle = ASyncResult.AsyncWaitHandle If WHandle.WaitOne = True Then ’ Retrieving result from the asynchronous process OrdersReader = Command.EndExecuteReader(ASyncResult) ’ Displaying result on the screen gvOrders.DataSource = OrdersReader gvOrders.DataBind() ’ Closing connection DBCon.Close() Else ’ Asynchronous process has timed out. Handle this ’ situation here. End If End Sub < /script > < html xmlns="http://www.w3.org/1999/xhtml" > < head id="Head1" runat="server" > < title > The Wait Approach < /title > < /head > < body > < form id="form1" runat="server" > < div > < asp:GridView ID="gvOrders" runat="server" AutoGenerateColumns="False" Width="100%" > < 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 > Continued 440 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 441 Chapter 8: Data Management with ADO.NET < 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 Command = new SqlCommand(); SqlDataReader OrdersReader; IAsyncResult ASyncResult; System.Threading.WaitHandle WHandle; DBCon = new SqlConnection(); DBCon.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_NorthWind"].ConnectionString; 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(); WHandle = ASyncResult.AsyncWaitHandle; if (WHandle.WaitOne() == true) { // Retrieving result from the asynchronous process OrdersReader = Command.EndExecuteReader(ASyncResult); // Displaying result on the screen gvOrders.DataSource = OrdersReader; gvOrders.DataBind(); Continued 441 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 442 Chapter 8: Data Management with ADO.NET // Closing connection DBCon.Close(); } else { // Asynchronous process has timed out. Handle this // situation here. } } < /script > If you set a break point and step through this code, you will notice that the program execution stops at the WHandle.WaitOne method call. The program automatically resumes when the asynchronous commands finishes its execution. Using Multiple Wait Handles The real power of the wait approach doesn’t become apparent until you start multiple asynchronous processes. The code shown in Listing 8-33 starts two asynchronous processes. One process queries a database to get information about a specific customer and runs another query to retrieve all orders sub- mitted by that the same customer. The code example shown in this listing creates two separate Command objects, Data Reader objects, and wait handles. However, it uses the same connection object for both queries to demonstrate how well Multiple Active Result Set (MARS) supports work in conjunction with the asynchronous processing. Listing 8-33: Use of m ultiple wait handles in conjunction with MARS 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 WHandles(1) As System.Threading.WaitHandle Dim OrdersWHandle As System.Threading.WaitHandle Dim CustWHandle As System.Threading.WaitHandle DBCon = New SqlConnection() DBCon.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_NorthWind").ConnectionString Continued 442 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 443 Chapter 8: Data Management with ADO.NET CustCommand.CommandText = _ "SELECT * FROM Customers WHERE CompanyName = ’Alfreds Futterkiste’" CustCommand.CommandType = CommandType.Text CustCommand.Connection = DBCon ’ Selecting all orders for a specific customer 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 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 System.Threading.WaitHandle.WaitAll(WHandles) CustReader = CustCommand.EndExecuteReader(CustAsyncResult) OrdersReader = OrdersCommand.EndExecuteReader(OrdersASyncResult) gvCustomers.DataSource = CustReader gvCustomers.DataBind() gvOrders.DataSource = OrdersReader gvOrders.DataBind() DBCon.Close() End Sub < /script > < html xmlns="http://www.w3.org/1999/xhtml" > < head id="Head1" runat="server" > Continued 443 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 444 Chapter 8: Data Management with ADO.NET < title > Wait All Approach < /title > < /head > < body > < form id="form1" runat="server" > < div > < asp:GridView ID="gvCustomers" Width="100%" runat="server" >< /asp:GridView > < br / >< br / > < 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; 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; Continued 444 Evjen c08.tex V2 - 01/28/2008 2:05pm Page 445 Chapter 8: Data Management with ADO.NET CustCommand.CommandText = "SELECT * FROM Customers WHERE CompanyName = ’Alfreds Futterkiste’"; CustCommand.CommandType = CommandType.Text; CustCommand.Connection = DBCon; // Selecting all orders for a specific customer 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; 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; System.Threading.WaitHandle.WaitAll(WHandles); CustReader = CustCommand.EndExecuteReader(CustAsyncResult); OrdersReader = OrdersCommand.EndExecuteReader(OrdersASyncResult); gvCustomers.DataSource = CustReader; gvCustomers.DataBind(); gvOrders.DataSource = OrdersReader; gvOrders.DataBind(); DBCon.Close(); } < /script > When you compile and execute the code shown in Listing 8-33, you see the result o n the screen, as shown in Figure 8-25. This figure clearly shows two GridView controls that were used in the code example. The GridView control on the top shows the result of executing a query that retrieved all information related 445 . CustAsyncResult; System.Threading.WaitHandle[] WHandles = new System.Threading.WaitHandle[1]; System.Threading.WaitHandle OrdersWHandle; System.Threading.WaitHandle CustWHandle; DBCon = new SqlConnection(); DBCon.ConnectionString. Customers.ContactName" Command.CommandType = CommandType.Text Command.Connection = DBCon DBCon.Open() ’ Starting the asynchronous processing ASyncResult = Command.BeginExecuteReader() WHandle = ASyncResult.AsyncWaitHandle If. IAsyncResult Dim WHandles(1) As System.Threading.WaitHandle Dim OrdersWHandle As System.Threading.WaitHandle Dim CustWHandle As System.Threading.WaitHandle DBCon = New SqlConnection() DBCon.ConnectionString