SqlCommand.BeginExecuteXmlReader SqlCommand.BeginExecuteXmlReader SqlCommand.BeginExecuteXmlReader SqlCommand.BeginExecuteXmlReader Method

Definición

Inicia la ejecución asincrónica de la instrucción de Transact-SQL o del procedimiento almacenado que describe SqlCommand y devuelve los resultados como un objeto XmlReader.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.

Sobrecargas

BeginExecuteXmlReader() BeginExecuteXmlReader() BeginExecuteXmlReader() BeginExecuteXmlReader()

Inicia la ejecución asincrónica de la instrucción de Transact-SQL o del procedimiento almacenado que describe SqlCommand y devuelve los resultados como un objeto XmlReader.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.

BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object)

Inicia la ejecución asincrónica de la instrucción de Transact-SQL o del procedimiento almacenado que describe SqlCommand y devuelve los resultados como un objeto XmlReader a través de un procedimiento de devolución de llamada.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, using a callback procedure.

BeginExecuteXmlReader() BeginExecuteXmlReader() BeginExecuteXmlReader() BeginExecuteXmlReader()

Inicia la ejecución asincrónica de la instrucción de Transact-SQL o del procedimiento almacenado que describe SqlCommand y devuelve los resultados como un objeto XmlReader.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.

public:
 IAsyncResult ^ BeginExecuteXmlReader();
public IAsyncResult BeginExecuteXmlReader ();
member this.BeginExecuteXmlReader : unit -> IAsyncResult
Public Function BeginExecuteXmlReader () As IAsyncResult

Devoluciones

IAsyncResult que se puede utilizar para sondear y/o esperar los resultados; este valor también es necesario al invocar a EndExecuteXmlReader, que devuelve un valor XML único.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.

Excepciones

Un valor SqlDbType distinto a Binary o VarBinary se usó cuando Value se estableció en Stream.A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

O bien-or-

Un valor SqlDbType distinto a Char, NChar, NVarChar, VarChar o Xml se usó cuando Value se estableció en TextReader.A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

o bien-or-

Se usó un SqlDbType distinto de Xml cuando Value se estableció en XmlReader.A SqlDbType other than Xml was used when Value was set to XmlReader.

Cualquier error que se produjo al ejecutar el texto del comando.Any error that occurred while executing the command text.

o bien-or-

Se agotó el tiempo de espera durante una operación de streaming.A timeout occurred during a streaming operation. Para más información sobre el streaming, vea Compatibilidad con el streaming de SqlClient.For more information about streaming, see SqlClient Streaming Support.

El par nombre/valor "Asynchronous Processing=true" no se incluyó dentro de la cadena de conexión que define la conexión para este SqlCommand.The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

O bien-or-

La SqlConnection se cerró o se interrumpió durante una operación de streaming.The SqlConnection closed or dropped during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

Se produjo un error en un objeto Stream, XmlReader o TextReader durante una operación de streaming.An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

El objeto Stream, XmlReader o TextReader se cerró durante una operación de streaming.The Stream, XmlReader or TextReader object was closed during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

Ejemplos

La siguiente aplicación de consola inicia el proceso de recuperación de datos XML de forma asincrónica.The following console application starts the process of retrieving XML data asynchronously. Mientras espera los resultados, esta aplicación simple se ubica en un bucle, investigando IsCompleted el valor de la propiedad.While waiting for the results, this simple application sits in a loop, investigating the IsCompleted property value. Una vez completado el proceso, el código recupera el XML y muestra su contenido.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";
    }
}
Imports System.Data.SqlClient
Imports System.Xml

Module Module1

    Sub 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.
        Dim commandText As String = _
         "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()
    End Sub

    Private Sub RunCommandAsynchronously( _
     ByVal commandText As String, ByVal connectionString As String)

        ' 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 connection As New SqlConnection(connectionString)
            Try
                Dim command As New SqlCommand(commandText, connection)
                connection.Open()
                Dim result As IAsyncResult = 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.
                Dim count As Integer = 0
                While Not result.IsCompleted
                    count += 1
                    Console.WriteLine("Waiting ({0})", count)
                    ' Wait for 1/10 second, so the counter
                    ' does not consume all available resources 
                    ' on the main thread.
                    Threading.Thread.Sleep(100)
                End While

                Using reader As XmlReader = command.EndExecuteXmlReader(result)
                    DisplayProductInfo(reader)
                End Using
            Catch ex As SqlException
                Console.WriteLine("Error ({0}): {1}", ex.Number, ex.Message)
            Catch ex As InvalidOperationException
                Console.WriteLine("Error: {0}", ex.Message)
            Catch ex As Exception
                ' You might want to pass these errors
                ' back out to the caller.
                Console.WriteLine("Error: {0}", ex.Message)
            End Try
        End Using
    End Sub

    Private Sub DisplayProductInfo(ByVal reader As XmlReader)
        ' 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" Then
                Console.WriteLine("{0}: {1:C}", _
                 reader("Name"), CSng(reader("ListPrice")))
            End If
        End While
    End Sub

    Private Function GetConnectionString() As String
        ' 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"
    End Function
End Module

Comentarios

El BeginExecuteXmlReader método inicia el proceso de ejecución asincrónica de una instrucción Transact-SQL que devuelve las filas como XML, de modo que se puedan ejecutar otras tareas simultáneamente mientras se ejecuta la instrucción.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. Cuando la instrucción se haya completado, los desarrolladores deben EndExecuteXmlReader llamar al método para finalizar la operación y recuperar el XML devuelto por el comando.When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the XML returned by the command. El BeginExecuteXmlReader método se devuelve inmediatamente, pero hasta que el código ejecuta la EndExecuteXmlReader llamada al método correspondiente, no debe ejecutar ninguna otra llamada que inicie una ejecución sincrónica o asincrónica en el mismo SqlCommand objeto.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. Llamar a SqlCommand antes de que se complete la ejecución del comando hace que el objeto se bloquee hasta que finalice la ejecución. EndExecuteXmlReaderCalling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

La CommandText propiedad normalmente especifica una instrucción Transact-SQL con una cláusula for XML válida.The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. Sin embargo CommandText , también puede especificar una instrucción que ntext devuelva datos que contengan XML válido.However, CommandText can also specify a statement that returns ntext data that contains valid XML.

Se puede BeginExecuteXmlReader dar formato A una consulta típica como en C# el ejemplo siguiente: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);  

Este método también se puede utilizar para recuperar un conjunto de resultados de una sola fila y una sola columna.This method can also be used to retrieve a single-row, single-column result set. En este caso, si se devuelve más de una fila, el EndExecuteXmlReader método asocia el XmlReader al valor de la primera fila y descarta el resto del conjunto de resultados.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.

La característica de conjunto de resultados activos múltiples (MARS) permite que varias acciones usen la misma conexión.The multiple active result set (MARS) feature lets multiple actions use the same connection.

Tenga en cuenta que el texto y los parámetros del comando se envían al servidor de forma sincrónica.Note that the command text and parameters are sent to the server synchronously. Si se envían comandos grandes o muchos parámetros, este método puede bloquearse durante las operaciones de escritura.If a large command or many parameters are sent, this method may block during writes. Después de enviar el comando, el método vuelve inmediatamente sin esperar una respuesta del servidor; es decir, las lecturas son asincrónicas.After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Aunque la ejecución del comando es asincrónica, la obtención de valores sigue siendo sincrónica.Although command execution is asynchronous, value fetching is still synchronous.

Dado que esta sobrecarga no admite un procedimiento de devolución de llamada, los desarrolladores deben sondear para determinar si el comando se ha IsCompleted completado, utilizando IAsyncResult la propiedad del BeginExecuteXmlReader devuelto por el método o esperar a que finalice. de uno o más comandos que usan AsyncWaitHandle la propiedad del devuelto. IAsyncResultBecause 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.

Si utiliza ExecuteReader o BeginExecuteReader para obtener acceso a los datos XML, SQL Server devuelve los resultados XML de más de 2.033 caracteres de longitud en varias filas de 2.033 caracteres cada uno.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. Para evitar este comportamiento, use ExecuteXmlReader o BeginExecuteXmlReader para leer las consultas for XML.To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries.

Consulte también:

BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object) BeginExecuteXmlReader(AsyncCallback, Object)

Inicia la ejecución asincrónica de la instrucción de Transact-SQL o del procedimiento almacenado que describe SqlCommand y devuelve los resultados como un objeto XmlReader a través de un procedimiento de devolución de llamada.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, using a callback procedure.

public:
 IAsyncResult ^ BeginExecuteXmlReader(AsyncCallback ^ callback, System::Object ^ stateObject);
public IAsyncResult BeginExecuteXmlReader (AsyncCallback callback, object stateObject);
member this.BeginExecuteXmlReader : AsyncCallback * obj -> IAsyncResult
Public Function BeginExecuteXmlReader (callback As AsyncCallback, stateObject As Object) As IAsyncResult

Parámetros

callback
AsyncCallback AsyncCallback AsyncCallback AsyncCallback

Delegado AsyncCallback que se invoca cuando finaliza la ejecución del comando.An AsyncCallback delegate that is invoked when the command's execution has completed. Pase null (Nothing en Microsoft Visual Basic) para indicar que no se requiere ninguna devolución de llamada.Pass null (Nothing in Microsoft Visual Basic) to indicate that no callback is required.

stateObject
Object Object Object Object

Objeto de estado definido por el usuario que se pasa al procedimiento de devolución de llamada.A user-defined state object that is passed to the callback procedure. Recupere este objeto desde el procedimiento de devolución de llamada mediante la propiedad AsyncState.Retrieve this object from within the callback procedure using the AsyncState property.

Devoluciones

IAsyncResult que se puede utilizar para sondear y/o esperar los resultados; este valor también es necesario cuando se llama a EndExecuteXmlReader(IAsyncResult), que devuelve los resultados del comando como XML.An IAsyncResult that can be used to poll, wait for results, or both; this value is also needed when the EndExecuteXmlReader(IAsyncResult) is called, which returns the results of the command as XML.

Excepciones

Un valor SqlDbType distinto a Binary o VarBinary se usó cuando Value se estableció en Stream.A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

O bien-or-

Un valor SqlDbType distinto a Char, NChar, NVarChar, VarChar o Xml se usó cuando Value se estableció en TextReader.A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

O bien-or-

Se usó un SqlDbType distinto de Xml cuando Value se estableció en XmlReader.A SqlDbType other than Xml was used when Value was set to XmlReader.

Cualquier error que se produjo al ejecutar el texto del comando.Any error that occurred while executing the command text.

o bien-or-

Se agotó el tiempo de espera durante una operación de streaming.A timeout occurred during a streaming operation. Para más información sobre el streaming, vea Compatibilidad con el streaming de SqlClient.For more information about streaming, see SqlClient Streaming Support.

El par nombre/valor "Asynchronous Processing=true" no se incluyó dentro de la cadena de conexión que define la conexión para este SqlCommand.The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

o bien-or-

La SqlConnection se cerró o se interrumpió durante una operación de streaming.The SqlConnection closed or dropped during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

Se produjo un error en un objeto Stream, XmlReader o TextReader durante una operación de streaming.An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

El objeto Stream, XmlReader o TextReader se cerró durante una operación de streaming.The Stream, XmlReader or TextReader object was closed during a streaming operation. Para obtener más información sobre el streaming, vea Compatibilidad de transmisión de datos de SqlClient.For more information about streaming, see SqlClient Streaming Support.

Ejemplos

La siguiente aplicación Windows muestra el uso del método BeginExecuteXmlReader, mediante la ejecución de una instrucción Transact-SQL que incluye un retraso de algunos segundos (al emular un comando cuya ejecución tiene una larga duración).The following Windows application demonstrates the use of the BeginExecuteXmlReader method, executing a Transact-SQL statement that includes a delay of a few seconds (emulating a long-running command). En este ejemplo se pasa el SqlCommand objeto de ejecución stateObject como parámetro; de este modo, resulta sencillo recuperar el SqlCommand objeto desde el procedimiento de devolución de llamada, de modo que el código EndExecuteXmlReader pueda llamar al método correspondiente a la llamada inicial a BeginExecuteXmlReader.This example passes the executing SqlCommand object as the stateObject parameter--doing so makes it simple to retrieve the SqlCommand object from within the callback procedure, so that the code can call the EndExecuteXmlReader method corresponding to the initial call to BeginExecuteXmlReader.

En este ejemplo se muestran muchas técnicas importantes.This example demonstrates many important techniques. Esto incluye llamar a un método que interactúa con el formulario desde un subproceso independiente.This includes calling a method that interacts with the form from a separate thread. Además, en este ejemplo se muestra cómo debe impedir que los usuarios ejecuten un comando varias veces simultáneamente y cómo debe asegurarse de que el formulario no se cierra antes de que se llame al procedimiento de devolución de llamada.In addition, this example demonstrates how you must block users from executing a command multiple times concurrently, and how you must make sure that the form does not close before the callback procedure is called.

Para realizar este ejemplo, cree una nueva aplicación Windows.To set up this example, create a new Windows application. Coloque un Button control, un ListBox control y un Label control en el formulario (aceptando el nombre predeterminado de cada control).Put a Button control, a ListBox control, and a Label control on the form (accepting the default name for each control). Agregue el código siguiente a la clase del formulario y modifique la cadena de conexión según sea necesario para su entorno.Add the following code to the form's class, modifying the connection string as needed for your environment.

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

namespace Microsoft.AdoDotNet.CodeSamples
{
    public partial class Form1 : Form
    {
        // Hook up the form's Load event handler and then add 
        // this code to the form's class:
        // You need these delegates in order to display text from a thread
        // other than the form's thread. See the HandleCallback
        // procedure for more information.
        private delegate void DisplayInfoDelegate(string Text);
        private delegate void DisplayReaderDelegate(XmlReader reader);

        private bool isExecuting;

        // This example maintains the connection object 
        // externally, so that it is available for closing.
        private SqlConnection connection;

        public Form1()
        {
            InitializeComponent();
        }

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

            // If you do not include the Asynchronous Processing=true name/value pair,
            // you wo not be able to execute the command asynchronously.
            return "Data Source=(local);Integrated Security=true;" +
            "Initial Catalog=AdventureWorks; Asynchronous Processing=true";
        }

        private void DisplayStatus(string Text)
        {
            this.label1.Text = Text;
        }

        private void ClearProductInfo()
        {
            // Clear the list box.
            this.listBox1.Items.Clear();
        }

        private 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")
                {
                    this.listBox1.Items.Add(String.Format("{0}: {1:C}",
                        reader["Name"], Convert.ToDecimal(reader["ListPrice"])));
                }
            }
            DisplayStatus("Ready");
        }

        private void Form1_FormClosing(object sender, 
            System.Windows.Forms.FormClosingEventArgs e)
        {
            if (isExecuting)
            {
                MessageBox.Show(this, "Cannot close the form until " +
                    "the pending asynchronous command has completed. Please wait...");
                e.Cancel = true;
            }
        }

        private void button1_Click(object sender, System.EventArgs e)
        {
            if (isExecuting)
            {
                MessageBox.Show(this, 
                    "Already executing. Please wait until the current query " +
                    "has completed.");
            }
            else
            {
                SqlCommand command = null;
                try
                {
                    ClearProductInfo();
                    DisplayStatus("Connecting...");
                    connection = new SqlConnection(GetConnectionString());

                    // To emulate a long-running query, wait for 
                    // a few seconds before working with the data.
                    string commandText =
                        "WAITFOR DELAY '00:00:03';" +
                        "SELECT Name, ListPrice FROM Production.Product " +
                        "WHERE ListPrice < 100 " +
                        "FOR XML AUTO, XMLDATA";

                    command = new SqlCommand(commandText, connection);
                    connection.Open();

                    DisplayStatus("Executing...");
                    isExecuting = true;
                    // Although it is not required that you pass the 
                    // SqlCommand object as the second parameter in the 
                    // BeginExecuteXmlReader call, doing so makes it easier
                    // to call EndExecuteXmlReader in the callback procedure.
                    AsyncCallback callback = new AsyncCallback(HandleCallback);
                    command.BeginExecuteXmlReader(callback, command);

                }
                catch (Exception ex)
                {
                    isExecuting = false;
                    DisplayStatus(string.Format("Ready (last error: {0})", ex.Message));
                    if (connection != null)
                    {
                        connection.Close();
                    }
                }
            }
        }

        private void HandleCallback(IAsyncResult result)
        {
            try
            {
                // Retrieve the original command object, passed
                // to this procedure in the AsyncState property
                // of the IAsyncResult parameter.
                SqlCommand command = (SqlCommand)result.AsyncState;
                XmlReader reader = command.EndExecuteXmlReader(result);

                // You may not interact with the form and its contents
                // from a different thread, and this callback procedure
                // is all but guaranteed to be running from a different thread
                // than the form. 

                // Instead, you must call the procedure from the form's thread.
                // One simple way to accomplish this is to call the Invoke
                // method of the form, which calls the delegate you supply
                // from the form's thread. 
                DisplayReaderDelegate del = new DisplayReaderDelegate(DisplayProductInfo);
                this.Invoke(del, reader);

            }
            catch (Exception ex)
            {
                // Because you are now running code in a separate thread, 
                // if you do not handle the exception here, none of your other
                // code catches the exception. Because none of 
                // your code is on the call stack in this thread, there is nothing
                // higher up the stack to catch the exception if you do not 
                // handle it here. You can either log the exception or 
                // invoke a delegate (as in the non-error case in this 
                // example) to display the error on the form. In no case
                // can you simply display the error without executing a delegate
                // as in the try block here. 

                // You can create the delegate instance as you 
                // invoke it, like this:
                this.Invoke(new DisplayInfoDelegate(DisplayStatus),
                String.Format("Ready(last error: {0}", ex.Message));
            }
            finally
            {
                isExecuting = false;
                if (connection != null)
                {
                    connection.Close();
                }
            }
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
            this.button1.Click += new System.EventHandler(this.button1_Click);
            this.FormClosing += new System.Windows.Forms.
                FormClosingEventHandler(this.Form1_FormClosing);
        }
    }
}
Imports System.Data.SqlClient
Imports System.Xml

Public Class Form1
    ' Add this code to the form's class:
    ' You need these delegates in order to display text from a thread
    ' other than the form's thread. See the HandleCallback
    ' procedure for more information.
    Private Delegate Sub DisplayInfoDelegate(ByVal Text As String)
    Private Delegate Sub DisplayReaderDelegate(ByVal reader As XmlReader)

    Private isExecuting As Boolean

    ' This example maintains the connection object 
    ' externally, so that it is available for closing.
    Private connection As SqlConnection

    Private Function GetConnectionString() As String
        ' 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"
    End Function

    Private Sub DisplayStatus(ByVal Text As String)
        Me.Label1.Text = Text
    End Sub

    Private Sub ClearProductInfo()
        ' Clear the list box.
        Me.ListBox1.Items.Clear()
    End Sub

    Private Sub DisplayProductInfo(ByVal reader As XmlReader)
        ' 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" Then
                Me.ListBox1.Items.Add(String.Format("{0}: {1:C}", _
                    reader("Name"), CSng(reader("ListPrice"))))
            End If
        End While
        DisplayStatus("Ready")
    End Sub

    Private Sub Form1_FormClosing(ByVal sender As Object, _
        ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        If isExecuting Then
            MessageBox.Show(Me, "Cannot close the form until " & _
                "the pending asynchronous command has completed. Please wait...")
            e.Cancel = True
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
        If isExecuting Then
            MessageBox.Show(Me, "Already executing. Please wait until the current query " & _
                "has completed.")
        Else
            Dim command As SqlCommand
            Try
                ClearProductInfo()
                DisplayStatus("Connecting...")
                connection = New SqlConnection(GetConnectionString())
                ' To emulate a long-running query, wait for 
                ' a few seconds before working with the data.
                Dim commandText As String = _
                    "WAITFOR DELAY '00:00:03';" & _
                    "SELECT Name, ListPrice " & _
                    "FROM Production.Product WHERE ListPrice < 100 " & _
                    "FOR XML AUTO, XMLDATA"

                command = New SqlCommand(commandText, connection)
                connection.Open()

                DisplayStatus("Executing...")
                isExecuting = True
                ' Although it is not required that you pass the 
                ' SqlCommand object as the second parameter in the 
                ' BeginExecuteXmlReader call, doing so makes it easier
                ' to call EndExecuteXmlReader in the callback procedure.
                Dim callback As New AsyncCallback(AddressOf HandleCallback)
                command.BeginExecuteXmlReader(callback, command)

            Catch ex As Exception
                isExecuting = False
                DisplayStatus(String.Format("Ready (last error: {0})", ex.Message))
                If connection IsNot Nothing Then
                    connection.Close()
                End If
            End Try
        End If
    End Sub

    Private Sub HandleCallback(ByVal result As IAsyncResult)
        Try
            ' Retrieve the original command object, passed
            ' to this procedure in the AsyncState property
            ' of the IAsyncResult parameter.
            Dim command As SqlCommand = CType(result.AsyncState, SqlCommand)
            Dim reader As XmlReader = command.EndExecuteXmlReader(result)

            ' You may not interact with the form and its contents
            ' from a different thread, and this callback procedure
            ' is all but guaranteed to be running from a different thread
            ' than the form. 

            ' Instead, you must call the procedure from the form's thread.
            ' One simple way to accomplish this is to call the Invoke
            ' method of the form, which calls the delegate you supply
            ' from the form's thread. 
            Dim del As New DisplayReaderDelegate(AddressOf DisplayProductInfo)
            Me.Invoke(del, reader)

        Catch ex As Exception
            ' Because you are now running code in a separate thread, 
            ' if you do not handle the exception here, none of your other
            ' code catches the exception. Because none of 
            ' your code is on the call stack in this thread, there is nothing
            ' higher up the stack to catch the exception if you do not 
            ' handle it here. You can either log the exception or 
            ' invoke a delegate (as in the non-error case in this 
            ' example) to display the error on the form. In no case
            ' can you simply display the error without executing a delegate
            ' as in the Try block here. 

            ' You can create the delegate instance as you 
            ' invoke it, like this:
            Me.Invoke(New DisplayInfoDelegate(AddressOf DisplayStatus), _
                String.Format("Ready(last error: {0}", ex.Message))
        Finally
            isExecuting = False
            If connection IsNot Nothing Then
                connection.Close()
            End If
        End Try
    End Sub
End Class

Comentarios

El BeginExecuteXmlReader método inicia el proceso de ejecución asincrónica de una instrucción de Transact-SQL o un procedimiento almacenado que devuelve filas como XML, de modo que se puedan ejecutar otras tareas simultáneamente mientras se ejecuta la instrucción.The BeginExecuteXmlReader method starts the process of asynchronously executing a Transact-SQL statement or stored procedure that returns rows as XML, so that other tasks can run concurrently while the statement is executing. Cuando la instrucción se haya completado, los desarrolladores deben EndExecuteXmlReader llamar al método para finalizar la operación y recuperar los datos XML solicitados.When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the requested XML data. El BeginExecuteXmlReader método se devuelve inmediatamente, pero hasta que el código ejecuta la EndExecuteXmlReader llamada al método correspondiente, no debe ejecutar ninguna otra llamada que inicie una ejecución sincrónica o asincrónica en el mismo SqlCommand objeto.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. Llamar a SqlCommand antes de que se complete la ejecución del comando hace que el objeto se bloquee hasta que finalice la ejecución. EndExecuteXmlReaderCalling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

La CommandText propiedad normalmente especifica una instrucción Transact-SQL con una cláusula for XML válida.The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. Sin embargo CommandText , también puede especificar una instrucción que devuelva datos que contengan XML válido.However, CommandText can also specify a statement that returns data that contains valid XML. Este método también se puede utilizar para recuperar un conjunto de resultados de una sola fila y una sola columna.This method can also be used to retrieve a single-row, single-column result set. En este caso, si se devuelve más de una fila, el EndExecuteXmlReader método asocia el XmlReader al valor de la primera fila y descarta el resto del conjunto de resultados.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.

Se puede BeginExecuteXmlReader dar formato A una consulta típica como en C# el ejemplo siguiente:A typical BeginExecuteXmlReader query can be formatted as in the following C# example:

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

Este método también se puede utilizar para recuperar un conjunto de resultados de una sola fila y una sola columna.This method can also be used to retrieve a single-row, single-column result set. En este caso, si se devuelve más de una fila, el EndExecuteXmlReader método asocia el XmlReader al valor de la primera fila y descarta el resto del conjunto de resultados.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.

La característica de conjunto de resultados activos múltiples (MARS) permite que varias acciones usen la misma conexión.The multiple active result set (MARS) feature lets multiple actions use the same connection.

El callback parámetro permite especificar un AsyncCallback delegado al que se llama cuando se ha completado la instrucción.The callback parameter lets you specify an AsyncCallback delegate that is called when the statement has completed. Puede llamar al EndExecuteXmlReader método desde este procedimiento delegado o desde cualquier otra ubicación dentro de la aplicación.You can call the EndExecuteXmlReader method from within this delegate procedure, or from any other location within your application. Además, puede pasar cualquier objeto en el stateObject parámetro y el procedimiento de devolución de llamada puede recuperar esta información mediante la AsyncState propiedad.In addition, you can pass any object in the stateObject parameter, and your callback procedure can retrieve this information using the AsyncState property.

Tenga en cuenta que el texto y los parámetros del comando se envían al servidor de forma sincrónica.Note that the command text and parameters are sent to the server synchronously. Si se envía un comando grande o muchos parámetros, este método puede bloquearse durante las operaciones de escritura.If a large command or many parameters is sent, this method may block during writes. Después de enviar el comando, el método vuelve inmediatamente sin esperar una respuesta del servidor; es decir, las lecturas son asincrónicas.After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous.

Todos los errores que se producen durante la ejecución de la operación se producen como excepciones en el procedimiento de devolución de llamada.All errors that occur during the execution of the operation are thrown as exceptions in the callback procedure. Debe controlar la excepción en el procedimiento de devolución de llamada, no en la aplicación principal.You must handle the exception in the callback procedure, not in the main application. Vea el ejemplo de este tema para obtener información adicional sobre el control de excepciones en el procedimiento de devolución de llamada.See the example in this topic for additional information on handling exceptions in the callback procedure.

Si usa ExecuteReader o BeginExecuteReader para tener acceso a los datos XML, SQL Server devolverá los resultados XML de más de 2.033 caracteres de longitud en varias filas de 2.033 caracteres cada uno.If you use ExecuteReader or BeginExecuteReader to access XML data, SQL Server will return any XML results greater than 2,033 characters in length in multiple rows of 2,033 characters each. Para evitar este comportamiento, use ExecuteXmlReader o BeginExecuteXmlReader para leer las consultas for XML.To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries.

Consulte también:

Se aplica a