SqlCommand.BeginExecuteXmlReader 方法

参考

定义

命名空间:: System.Data.SqlClient

程序集:: System.Data.dll

程序集:: System.Data.SqlClient.dll

包:: System.Data.SqlClient v4.8.6

重要

一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。

启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行，并将结果作为 XmlReader 对象返回。

重载

BeginExecuteXmlReader()	启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行，并将结果作为 XmlReader 对象返回。
BeginExecuteXmlReader(AsyncCallback, Object)	通过使用回调过程，启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行，并将结果作为 XmlReader 对象返回。

BeginExecuteXmlReader()

启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行，并将结果作为 XmlReader 对象返回。

public:
 IAsyncResult ^ BeginExecuteXmlReader();

public IAsyncResult BeginExecuteXmlReader ();

member this.BeginExecuteXmlReader : unit -> IAsyncResult

Public Function BeginExecuteXmlReader () As IAsyncResult

IAsyncResult

可用于轮询和/或等待结果的 IAsyncResult；当调用会返回单个 XML 值的 EndExecuteXmlReader 时，也需要该值。

例外

InvalidCastException

将 Value 设置为 Stream 时，使用的是 SqlDbType 而不是 Binary 或 VarBinary。有关流式处理的详细信息，请参阅 SqlClient 流支持。

- 或 -

SqlDbType设置为时Value TextReader，使用了 Char、NChar、NVarChar、VarChar 或 Xml 以外的其他项。

- 或 -

将 Value 设置为 XmlReader 时，使用的是 SqlDbType 而不是 Xml。

SqlException

执行命令文本时出现的任何错误。

- 或 -

流式处理操作期间发生了超时。有关流式处理的详细信息，请参阅 SqlClient 流支持。

InvalidOperationException

名称/值对“Asynchronous Processing=true”未包含在为此 SqlCommand 定义连接的连接字符串中。

- 或 -

流式处理操作期间关闭或删除了 SqlConnection。有关流式处理的详细信息，请参阅 SqlClient 流支持。

IOException

流式处理操作期间在 Stream、XmlReader 或 TextReader 对象中出现了错误。有关流式处理的详细信息，请参阅 SqlClient 流支持。

ObjectDisposedException

流式处理操作期间关闭了 Stream、XmlReader 或 TextReader 对象。有关流式处理的详细信息，请参阅 SqlClient 流支持。

示例

以下控制台应用程序启动异步检索 XML 数据的过程。在等待结果时，此简单应用程序位于循环中，调查 IsCompleted 属性值。该过程完成后，代码将检索 XML 并显示其内容。

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

注解

方法 BeginExecuteXmlReader 启动异步执行 Transact-SQL 语句的过程，该语句将行作为 XML 返回，以便其他任务可以在语句执行时并发运行。语句完成后，开发人员必须调用 EndExecuteXmlReader 方法来完成操作并检索命令返回的 XML。方法 BeginExecuteXmlReader 会立即返回，但在代码执行相应的 EndExecuteXmlReader 方法调用之前，它不得执行针对同 SqlCommand 一对象启动同步或异步执行的任何其他调用。 EndExecuteXmlReader在命令执行完成之前调用会导致SqlCommand对象阻塞，直到执行完成。

属性 CommandText 通常指定具有有效 FOR XML 子句的 Transact-SQL 语句。但是， CommandText 也可以指定返回包含有效 XML 的数据的 ntext 语句。

典型 BeginExecuteXmlReader 查询的格式可以设置为以下 C# 示例中的格式：

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

此方法还可用于检索单行、单列结果集。在这种情况下，如果返回了多行，该方法 EndExecuteXmlReader 会将 XmlReader 附加到第一行的值，并放弃结果集的其余部分。

MARS) 功能 (多个活动结果集允许多个操作使用相同的连接。

请注意，命令文本和参数以同步方式发送到服务器。如果发送了大型命令或多个参数，此方法可能会在写入期间阻止。发送命令后，方法会立即返回，而无需等待服务器的答案，即读取是异步的。虽然命令执行是异步的，但值提取仍然是同步的。

由于此重载不支持回调过程，因此开发人员需要使用方法返回的的 IAsyncResult 属性轮询以确定命令是否已完成IsCompleted;或者使用AsyncWaitHandle返回IAsyncResult的的属性等待一个或多个命令完成。BeginExecuteXmlReader

如果使用 ExecuteReader 或 BeginExecuteReader 访问 XML 数据，SQL Server将返回长度超过 2,033 个字符的任何 XML 结果，每行 2,033 个字符。若要避免此行为，请使用 ExecuteXmlReader 或 BeginExecuteXmlReader 读取 FOR XML 查询。

此方法忽略 CommandTimeout 属性。

另请参阅

适用于

BeginExecuteXmlReader(AsyncCallback, Object)

通过使用回调过程，启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行，并将结果作为 XmlReader 对象返回。

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

参数

callback: AsyncCallback

命令执行完成时调用的 AsyncCallback 委托。传递 null（在 Microsoft Visual Basic 中为 Nothing）以指示不需要回调。

stateObject: Object

传递到回调过程的用户定义的状态对象。使用 AsyncState 属性从回调过程内检索此对象。

IAsyncResult

可用于轮询和/或等待结果的 IAsyncResult；当调用 EndExecuteXmlReader(IAsyncResult) 时，也需要该值，用于将命令的结果作为 XML 返回。

例外

InvalidCastException

将 Value 设置为 Stream 时，使用的是 SqlDbType 而不是 Binary 或 VarBinary。有关流式处理的详细信息，请参阅 SqlClient 流支持。

- 或 -

SqlDbType设置为时Value TextReader，使用了 Char、NChar、NVarChar、VarChar 或 Xml 以外的其他项。

- 或 -

将 Value 设置为 XmlReader 时，使用的是 SqlDbType 而不是 Xml。

SqlException

执行命令文本时出现的任何错误。

- 或 -

流式处理操作期间发生了超时。有关流式处理的详细信息，请参阅 SqlClient 流支持。

InvalidOperationException

名称/值对“Asynchronous Processing=true”未包含在为此 SqlCommand 定义连接的连接字符串中。

- 或 -

流式处理操作期间关闭或删除了 SqlConnection。有关流式处理的详细信息，请参阅 SqlClient 流支持。

IOException

流式处理操作期间在 Stream、XmlReader 或 TextReader 对象中出现了错误。有关流式处理的详细信息，请参阅 SqlClient 流支持。

ObjectDisposedException

流式处理操作期间关闭了 Stream、XmlReader 或 TextReader 对象。有关流式处理的详细信息，请参阅 SqlClient 流支持。

示例

以下 Windows 应用程序演示如何使用 BeginExecuteXmlReader 方法，以执行包含几秒钟延迟（模拟长时间运行的命令）的 Transact-SQL 语句。此示例将正在执行 SqlCommand 的对象作为 stateObject 参数进行传递-这样做使得从回调过程中检索 SqlCommand 对象变得简单，以便代码可以调用 EndExecuteXmlReader 对应于对的初始调用 BeginExecuteXmlReader的方法。

此示例演示了许多重要的技术。这包括从单独的线程调用与窗体交互的方法。此外，此示例演示了如何阻止用户多次同时执行命令，以及如何在调用回调过程之前确保窗体不会关闭。

若要设置此示例，请创建新的 Windows 应用程序。将 Button 控件、 ListBox 控件和 Label 控件放在窗体上， (接受每个控件的默认名称) 。将以下代码添加到窗体的类，根据环境需要修改连接字符串。

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

注解

方法 BeginExecuteXmlReader 启动异步执行 Transact-SQL 语句或将行作为 XML 返回的存储过程的过程，以便其他任务可以在语句执行时并发运行。语句完成后，开发人员必须调用 EndExecuteXmlReader 方法来完成操作并检索请求的 XML 数据。方法 BeginExecuteXmlReader 会立即返回，但在代码执行相应的 EndExecuteXmlReader 方法调用之前，它不得执行针对同 SqlCommand 一对象启动同步或异步执行的任何其他调用。 EndExecuteXmlReader在命令执行完成之前调用会导致SqlCommand对象阻塞，直到执行完成。

属性 CommandText 通常指定具有有效 FOR XML 子句的 Transact-SQL 语句。但是， CommandText 也可以指定返回包含有效 XML 的数据的语句。此方法还可用于检索单行、单列结果集。在这种情况下，如果返回了多行，该方法 EndExecuteXmlReader 会将 XmlReader 附加到第一行的值，并放弃结果集的其余部分。

典型 BeginExecuteXmlReader 查询的格式可以设置为以下 C# 示例中的格式：

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

此方法还可用于检索单行、单列结果集。在这种情况下，如果返回了多行，该方法 EndExecuteXmlReader 会将 XmlReader 附加到第一行的值，并放弃结果集的其余部分。

MARS) 功能 (多个活动结果集允许多个操作使用相同的连接。

参数 callback 允许指定在 AsyncCallback 语句完成时调用的委托。可以从此委托过程内部或应用程序内的任何其他位置调用 EndExecuteXmlReader 方法。此外，可以传递参数中的任何 stateObject 对象，并且回调过程可以使用属性检索此信息 AsyncState 。

请注意，命令文本和参数以同步方式发送到服务器。如果发送了大型命令或多个参数，此方法可能会在写入期间阻止。发送命令后，方法会立即返回，而无需等待服务器的答案，即读取是异步的。

在执行操作期间发生的所有错误都会在回调过程中作为异常引发。必须在回调过程中处理异常，而不是在main应用程序中。有关在回调过程中处理异常的其他信息，请参阅本主题中的示例。

此方法忽略 CommandTimeout 属性。

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SqlCommand.BeginExecuteXmlReader 方法

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BeginExecuteXmlReader()

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BeginExecuteXmlReader(AsyncCallback, Object)

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