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SqlCommand.BeginExecuteXmlReader Method

Initiates the asynchronous execution of the Transact-SQL statement or stored procedure that is described by this SqlCommand and returns results as an XmlReader object.

Namespace:  System.Data.SqlClient
Assembly:  System.Data (in System.Data.dll)

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public IAsyncResult BeginExecuteXmlReader()

Return Value

Type: System.IAsyncResult
An IAsyncResult that can be used to poll or wait for results, or both; this value is also needed when invoking EndExecuteXmlReader, which returns a single XML value.

ExceptionCondition
SqlException

Any error that occurred while executing the command text.

InvalidOperationException

The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

The BeginExecuteXmlReader method starts the process of asynchronously executing a Transact-SQL statement that returns rows as XML, so that other tasks can run concurrently while the statement is executing. When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the XML returned by the command. The BeginExecuteXmlReader method returns immediately, but until the code executes the corresponding EndExecuteXmlReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. However, CommandText can also specify a statement that returns ntext data that contains valid XML.

A typical BeginExecuteXmlReader query can be formatted as in the following C# example:

SqlCommand command = new SqlCommand("SELECT ContactID, FirstName, LastName FROM dbo.Contact FOR XML AUTO, XMLDATA", SqlConn);
NoteNote

This type of query only works with Microsoft SQL Server 2000 or later versions. For more information, see "Guidelines for Using the FOR XML Clause" in SQL Server Books Online.

When used with SQL Server 2005, this method can also be used to retrieve a single-row, single-column result set. In this case, if more than one row is returned, the EndExecuteXmlReader method attaches the XmlReader to the value on the first row, and discards the rest of the result set.

When you use versions of SQL Server earlier than SQL Server 2005, while the XmlReader is being used the associated SqlConnection is busy serving the XmlReader. While in this state, no other operations can be performed on the SqlConnection other than closing it. This is the case until the Close method of the XmlReader is called. Starting with SQL Server 2005, the multiple active result set (MARS) feature lets multiple actions use the same connection.

Note that the command text and parameters are sent to the server synchronously. If a large command or many parameters are sent, this method may block during writes. After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Although command execution is asynchronous, value fetching is still synchronous.

Because this overload does not support a callback procedure, developers need to either poll to determine whether the command has completed, using the IsCompleted property of the IAsyncResult returned by the BeginExecuteXmlReader method; or wait for the completion of one or more commands using the AsyncWaitHandle property of the returned IAsyncResult.

If you use ExecuteReader or BeginExecuteReader to access XML data, SQL Server returns any XML results greater than 2,033 characters in length in multiple rows of 2,033 characters each. To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries. For more information, see article Q310378, "PRB: XML Data Is Truncated When You Use SqlDataReader," in the Microsoft Knowledge Base at http://support.microsoft.com.

NoteNote

The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The following console application starts the process of retrieving XML data asynchronously. While waiting for the results, this simple application sits in a loop, investigating the IsCompleted property value. Once the process has completed, the code retrieves the XML and displays its contents.


using System.Data.SqlClient;
using System.Xml;

class Class1
{
    static void Main()
    {
        // This example is not terribly effective, but it proves a point.
        // The WAITFOR statement simply adds enough time to prove the 
        // asynchronous nature of the command.
        string commandText =
            "WAITFOR DELAY '00:00:03';" +
            "SELECT Name, ListPrice FROM Production.Product " +
            "WHERE ListPrice < 100 " +
            "FOR XML AUTO, XMLDATA";

        RunCommandAsynchronously(commandText, GetConnectionString());

        Console.WriteLine("Press ENTER to continue.");
        Console.ReadLine();
    }

    private static void RunCommandAsynchronously(
        string commandText, string connectionString)
    {
        // Given command text and connection string, asynchronously execute
        // the specified command against the connection. For this example,
        // the code displays an indicator as it is working, verifying the 
        // asynchronous behavior. 
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            SqlCommand command = new SqlCommand(commandText, connection);

            connection.Open();
            IAsyncResult result = command.BeginExecuteXmlReader();

            // Although it is not necessary, the following procedure
            // displays a counter in the console window, indicating that 
            // the main thread is not blocked while awaiting the command 
            // results.
            int count = 0;
            while (!result.IsCompleted)
            {
                Console.WriteLine("Waiting ({0})", count++);
                // Wait for 1/10 second, so the counter
                // does not consume all available resources 
                // on the main thread.
                System.Threading.Thread.Sleep(100);
            }

            XmlReader reader = command.EndExecuteXmlReader(result);
            DisplayProductInfo(reader);
        }
    }

    private static void DisplayProductInfo(XmlReader reader)
    {
        // Display the data within the reader.
        while (reader.Read())
        {
            // Skip past items that are not from the correct table.
            if (reader.LocalName.ToString() == "Production.Product")
            {
                Console.WriteLine("{0}: {1:C}",
                    reader["Name"], Convert.ToSingle(reader["ListPrice"]));
            }
        }
    }

    private static string GetConnectionString()
    {
        // To avoid storing the connection string in your code,            
        // you can retrieve it from a configuration file. 

        // If you have not included "Asynchronous Processing=true" in the
        // connection string, the command is not able
        // to execute asynchronously.
        return "Data Source=(local);Integrated Security=true;" +
            "Initial Catalog=AdventureWorks; Asynchronous Processing=true";
    }
}


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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