SqlCommand.BeginExecuteXmlReader Método

Definição

Inicia a execução assíncrona da instrução Transact-SQL ou do procedimento armazenado descrito por este SqlCommand e retorna resultados como um 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()

Inicia a execução assíncrona da instrução Transact-SQL ou do procedimento armazenado descrito por este SqlCommand e retorna resultados como um 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)

Inicia a execução assíncrona da instrução Transact-SQL ou do procedimento armazenado descrito por este SqlCommand e retorna resultados como um objeto XmlReader usando um procedimento de retorno de chamada.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()

Inicia a execução assíncrona da instrução Transact-SQL ou do procedimento armazenado descrito por este SqlCommand e retorna resultados como um 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

Retornos

IAsyncResult

Um IAsyncResult que pode ser usado para sondar ou aguardar resultados ou ambos. Esse valor também é necessário ao invocar EndExecuteXmlReader, que retorna um valor único de XML.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.

Exceções

Um SqlDbType diferente de Binary ou varbinary foi usado quando Value foi definido como Stream .A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

- ou --or-

Um SqlDbType diferente de Char, nchar, nvarchar, varchar ou XML foi usado quando Value foi definido como TextReader .A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

- ou --or-

Um SqlDbType diferente de Xml foi usado quando Value foi definido como XmlReader.A SqlDbType other than Xml was used when Value was set to XmlReader.

Qualquer erro ocorrido ao executar o texto do comando.Any error that occurred while executing the command text.

- ou --or-

Ocorreu um tempo limite durante uma operação de streaming.A timeout occurred during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

O par nome-valor "Asynchronous Processing=true" não foi incluído na cadeia de conexão que define a conexão para este SqlCommand.The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

- ou --or-

O SqlConnection foi fechado ou removido durante uma operação de streaming.The SqlConnection closed or dropped during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

Ocorreu um erro em um objeto Stream, XmlReader ou TextReader durante uma operação de streaming.An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

O objeto Stream, XmlReader ou TextReader foi fechado durante uma operação de streaming.The Stream, XmlReader or TextReader object was closed during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

Exemplos

O aplicativo de console a seguir inicia o processo de recuperação de dados XML de forma assíncrona.The following console application starts the process of retrieving XML data asynchronously. Enquanto aguarda os resultados, esse aplicativo simples fica em um loop, investigando o IsCompleted valor da propriedade.While waiting for the results, this simple application sits in a loop, investigating the IsCompleted property value. Depois que o processo for concluído, o código recuperará o XML e exibirá seu conteúdo.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

Comentários

O BeginExecuteXmlReader método inicia o processo de execução assíncrona de uma instrução Transact-SQL que retorna linhas como XML, para que outras tarefas possam ser executadas simultaneamente enquanto a instrução estiver em execução.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. Quando a instrução for concluída, os desenvolvedores deverão chamar o EndExecuteXmlReader método para concluir a operação e recuperar o XML retornado pelo comando.When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the XML returned by the command. O BeginExecuteXmlReader método retorna imediatamente, mas até que o código execute a EndExecuteXmlReader chamada de método correspondente, ele não deve executar nenhuma outra chamada que inicie uma execução síncrona ou assíncrona no mesmo 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. Chamar o EndExecuteXmlReader antes da execução do comando é concluído faz com que o SqlCommand objeto seja bloqueado até que a execução seja concluída.Calling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

A CommandText propriedade normalmente especifica uma instrução Transact-SQL com uma cláusula for XML válida.The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. No entanto, CommandText também pode especificar uma instrução que retorne ntext dados que contenham XML válido.However, CommandText can also specify a statement that returns ntext data that contains valid XML.

Uma BeginExecuteXmlReader consulta típica pode ser formatada como no exemplo de C# a seguir: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);  

Esse método também pode ser usado para recuperar um conjunto de resultados de linha única e coluna única.This method can also be used to retrieve a single-row, single-column result set. Nesse caso, se mais de uma linha for retornada, o EndExecuteXmlReader método anexará o XmlReader ao valor na primeira linha e descartará o restante do 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.

O recurso MARS (vários conjuntos de resultados ativos) permite que várias ações usem a mesma conexão.The multiple active result set (MARS) feature lets multiple actions use the same connection.

Observe que o texto e os parâmetros do comando são enviados para o servidor de forma síncrona.Note that the command text and parameters are sent to the server synchronously. Se um comando grande ou muitos parâmetros forem enviados, esse método poderá bloquear durante as gravações.If a large command or many parameters are sent, this method may block during writes. Depois que o comando é enviado, o método retorna imediatamente sem esperar por uma resposta do servidor, ou seja, leituras são assíncronas.After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Embora a execução do comando seja assíncrona, a busca de valor ainda é síncrona.Although command execution is asynchronous, value fetching is still synchronous.

Como essa sobrecarga não dá suporte a um procedimento de retorno de chamada, os desenvolvedores precisam fazer uma pesquisa para determinar se o comando foi concluído, usando a IsCompleted Propriedade do IAsyncResult retornado pelo BeginExecuteXmlReader método ou aguardar a conclusão de um ou mais comandos usando a AsyncWaitHandle Propriedade do retornado IAsyncResult .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.

Se você usar ExecuteReader ou BeginExecuteReader para acessar dados xml, SQL Server retornará quaisquer resultados XML com mais de 2.033 caracteres de comprimento em várias linhas de 2.033 caracteres cada.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 esse comportamento, use ExecuteXmlReader ou BeginExecuteXmlReader para ler consultas for XML.To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries.

Esse método ignora a CommandTimeout propriedade.This method ignores the CommandTimeout property.

Aplica-se a

BeginExecuteXmlReader(AsyncCallback, Object)

Inicia a execução assíncrona da instrução Transact-SQL ou do procedimento armazenado descrito por este SqlCommand e retorna resultados como um objeto XmlReader usando um procedimento de retorno de chamada.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

Parâmetros

callback
AsyncCallback

Um delegado AsyncCallback invocado quando a execução do comando for concluída.An AsyncCallback delegate that is invoked when the command's execution has completed. Passe null (Nothing no Microsoft Visual Basic) para indicar que não é necessário um retorno de chamada.Pass null (Nothing in Microsoft Visual Basic) to indicate that no callback is required.

stateObject
Object

Um objeto de estado definido pelo usuário passado para o procedimento de retorno de chamada.A user-defined state object that is passed to the callback procedure. Recupere esse objeto de dentro do procedimento de retorno de chamada usando a propriedade AsyncState.Retrieve this object from within the callback procedure using the AsyncState property.

Retornos

IAsyncResult

Um IAsyncResult que pode ser usado para sondar, aguardar resultados ou ambos. Esse valor também é necessário ao chamar EndExecuteXmlReader(IAsyncResult), que retorna os resultados do 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.

Exceções

Um SqlDbType diferente de Binary ou varbinary foi usado quando Value foi definido como Stream .A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

- ou --or-

Um SqlDbType diferente de Char, nchar, nvarchar, varchar ou XML foi usado quando Value foi definido como TextReader .A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

- ou --or-

Um SqlDbType diferente de Xml foi usado quando Value foi definido como XmlReader.A SqlDbType other than Xml was used when Value was set to XmlReader.

Qualquer erro ocorrido ao executar o texto do comando.Any error that occurred while executing the command text.

- ou --or-

Ocorreu um tempo limite durante uma operação de streaming.A timeout occurred during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

O par nome-valor "Asynchronous Processing=true" não foi incluído na cadeia de conexão que define a conexão para este SqlCommand.The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

- ou --or-

O SqlConnection foi fechado ou removido durante uma operação de streaming.The SqlConnection closed or dropped during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

Ocorreu um erro em um objeto Stream, XmlReader ou TextReader durante uma operação de streaming.An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

O objeto Stream, XmlReader ou TextReader foi fechado durante uma operação de streaming.The Stream, XmlReader or TextReader object was closed during a streaming operation. Para saber mais sobre o streaming, consulte SqlClient Streaming Support (Suporte de streaming do SqlClient).For more information about streaming, see SqlClient Streaming Support.

Exemplos

O aplicativo do Windows a seguir demonstra o uso do método BeginExecuteXmlReader, executando uma instrução Transact-SQL que inclui um atraso de alguns segundos (emulando um comando de longa execução).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). Este exemplo passa o objeto em execução SqlCommand como o stateObject parâmetro--fazendo isso torna simples recuperar o SqlCommand objeto de dentro do procedimento de retorno de chamada, para que o código possa chamar o EndExecuteXmlReader método correspondente à chamada inicial para 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.

Este exemplo demonstra muitas técnicas importantes.This example demonstrates many important techniques. Isso inclui chamar um método que interage com o formulário de um thread separado.This includes calling a method that interacts with the form from a separate thread. Além disso, este exemplo demonstra como você deve impedir que os usuários executem um comando várias vezes simultaneamente e como você deve certificar-se de que o formulário não é fechado antes de o procedimento de retorno de chamada ser chamado.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 configurar este exemplo, crie um aplicativo do Windows.To set up this example, create a new Windows application. Coloque um Button controle, um ListBox controle e um Label controle no formulário (aceitando o nome padrão de cada controle).Put a Button control, a ListBox control, and a Label control on the form (accepting the default name for each control). Adicione o código a seguir à classe do formulário, modificando a cadeia de conexão conforme necessário para o seu ambiente.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

Comentários

O BeginExecuteXmlReader método inicia o processo de execução assíncrona de uma instrução Transact-SQL ou um procedimento armazenado que retorna linhas como XML, para que outras tarefas possam ser executadas simultaneamente enquanto a instrução está em execução.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. Quando a instrução for concluída, os desenvolvedores deverão chamar o EndExecuteXmlReader método para concluir a operação e recuperar os dados XML solicitados.When the statement has completed, developers must call the EndExecuteXmlReader method to finish the operation and retrieve the requested XML data. O BeginExecuteXmlReader método retorna imediatamente, mas até que o código execute a EndExecuteXmlReader chamada de método correspondente, ele não deve executar nenhuma outra chamada que inicie uma execução síncrona ou assíncrona no mesmo 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. Chamar o EndExecuteXmlReader antes da execução do comando é concluído faz com que o SqlCommand objeto seja bloqueado até que a execução seja concluída.Calling the EndExecuteXmlReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

A CommandText propriedade normalmente especifica uma instrução Transact-SQL com uma cláusula for XML válida.The CommandText property ordinarily specifies a Transact-SQL statement with a valid FOR XML clause. No entanto, CommandText também pode especificar uma instrução que retorne dados que contenham XML válido.However, CommandText can also specify a statement that returns data that contains valid XML. Esse método também pode ser usado para recuperar um conjunto de resultados de linha única e coluna única.This method can also be used to retrieve a single-row, single-column result set. Nesse caso, se mais de uma linha for retornada, o EndExecuteXmlReader método anexará o XmlReader ao valor na primeira linha e descartará o restante do 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.

Uma BeginExecuteXmlReader consulta típica pode ser formatada como no exemplo de C# a seguir: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);  

Esse método também pode ser usado para recuperar um conjunto de resultados de linha única e coluna única.This method can also be used to retrieve a single-row, single-column result set. Nesse caso, se mais de uma linha for retornada, o EndExecuteXmlReader método anexará o XmlReader ao valor na primeira linha e descartará o restante do 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.

O recurso MARS (vários conjuntos de resultados ativos) permite que várias ações usem a mesma conexão.The multiple active result set (MARS) feature lets multiple actions use the same connection.

O callback parâmetro permite especificar um AsyncCallback delegado que é chamado quando a instrução é concluída.The callback parameter lets you specify an AsyncCallback delegate that is called when the statement has completed. Você pode chamar o EndExecuteXmlReader método de dentro deste procedimento delegado ou de qualquer outro local dentro de seu aplicativo.You can call the EndExecuteXmlReader method from within this delegate procedure, or from any other location within your application. Além disso, você pode passar qualquer objeto no stateObject parâmetro e o procedimento de retorno de chamada pode recuperar essas informações usando a AsyncState propriedade.In addition, you can pass any object in the stateObject parameter, and your callback procedure can retrieve this information using the AsyncState property.

Observe que o texto e os parâmetros do comando são enviados para o servidor de forma síncrona.Note that the command text and parameters are sent to the server synchronously. Se um comando grande ou muitos parâmetros forem enviados, esse método poderá bloquear durante as gravações.If a large command or many parameters is sent, this method may block during writes. Depois que o comando é enviado, o método retorna imediatamente sem esperar por uma resposta do servidor, ou seja, leituras são assíncronas.After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous.

Todos os erros que ocorrem durante a execução da operação são lançados como exceções no procedimento de retorno de chamada.All errors that occur during the execution of the operation are thrown as exceptions in the callback procedure. Você deve tratar a exceção no procedimento de retorno de chamada, não no aplicativo principal.You must handle the exception in the callback procedure, not in the main application. Consulte o exemplo neste tópico para obter informações adicionais sobre como tratar exceções no procedimento de retorno de chamada.See the example in this topic for additional information on handling exceptions in the callback procedure.

Se você usar ExecuteReader ou BeginExecuteReader para acessar dados xml, SQL Server retornará quaisquer resultados XML com mais de 2.033 caracteres de comprimento em várias linhas de 2.033 caracteres cada.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 esse comportamento, use ExecuteXmlReader ou BeginExecuteXmlReader para ler consultas for XML.To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries.

Esse método ignora a CommandTimeout propriedade.This method ignores the CommandTimeout property.

Confira também

Aplica-se a