SqlCommand.BeginExecuteXmlReader 方法

定义

命名空间:

System.Data.SqlClient

程序集:

System.Data.dll

启动由此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并将结果作为对象 XmlReader 返回。

重载

名称	说明
BeginExecuteXmlReader()	启动由此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并将结果作为对象 XmlReader 返回。
BeginExecuteXmlReader(AsyncCallback, Object)	启动由此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并使用回调过程将结果作为 XmlReader 对象返回。

BeginExecuteXmlReader()

启动由此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并将结果作为对象 XmlReader 返回。

public:
 IAsyncResult ^ BeginExecuteXmlReader();

public IAsyncResult BeginExecuteXmlReader();

member this.BeginExecuteXmlReader : unit -> IAsyncResult

Public Function BeginExecuteXmlReader () As IAsyncResult

返回

IAsyncResult

IAsyncResult可用于轮询或等待结果或两者的结果;调用EndExecuteXmlReader时也需要此值，这将返回单个 XML 值。

例外

InvalidCastException

如果设置为 Stream，则使用Value二进制或 VarBinary 以外的其他SqlDbType项。 有关流式处理的详细信息，请参阅 SqlClient 流式处理支持。

-或-

设置为 /a0> 时使用了除 Char、NChar、NVarChar、VarChar 或 Xml 以外的其他项。

-或-

设置为ValueXmlReader时使用了 Xml 以外的其他SqlDbType项。

SqlException

执行命令文本时发生的任何错误。

-或-

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

InvalidOperationException

名称/值对“异步处理=true”不包括在定义此 SqlCommand连接的连接字符串中。

-或-

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

IOException

流式处理操作期间发生错误StreamXmlReader或TextReader对象。 有关流式处理的详细信息，请参阅 SqlClient 流式处理支持。

ObjectDisposedException

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

示例

以下控制台应用程序启动异步检索 XML 数据的过程。 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

该方法 BeginExecuteXmlReader 启动异步执行将行作为 XML 返回的 Transact-SQL 语句的过程，以便其他任务在执行语句时可以并发运行。 语句完成后，开发人员必须调用 EndExecuteXmlReader 该方法来完成操作并检索命令返回的 XML。 该方法 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在命令的执行完成之前调用该对象会导致SqlCommand对象阻塞，直到执行完成。

该 CommandText 属性通常使用有效的 FOR XML 子句指定 Transact-SQL 语句。 但是， CommandText 还可以指定返回包含有效 XML 的数据的语句 ntext 。

典型 BeginExecuteXmlReader 查询的格式可以如下 C# 示例所示：

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，如果返回了多个行，该方法 EndExecuteXmlReader 将附加到 XmlReader 第一行上的值，并放弃结果集的其余部分。

多个活动结果集 （MARS） 功能允许多个操作使用相同的连接。

请注意，命令文本和参数将同步发送到服务器。 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，该方法会立即返回，而无需等待来自服务器的答案，即读取是异步的。 尽管命令执行是异步的，但提取值仍然同步。

由于此重载不支持回调过程，因此开发人员需要轮询以确定命令是否已完成，使用IsCompleted方法返回BeginExecuteXmlReader的属性IAsyncResult;或使用返回IAsyncResult的属性等待完成一个或多个命令AsyncWaitHandle。

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，每行包含 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

• 在 ADO.NET 中连接和检索数据
• 使用适用于 SQL Server 的 .NET Framework 数据提供程序
• ADO.NET 概述

BeginExecuteXmlReader(AsyncCallback, Object)

启动由此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并使用回调过程将结果作为 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 AsyncCallback 执行完成后调用的委托。 传入 null （Nothing Microsoft Visual Basic）以指示不需要回调。

stateObject

Object

传递给回调过程的用户定义状态对象。 使用 AsyncState 属性从回调过程中检索此对象。

返回

IAsyncResult

IAsyncResult可用于轮询、等待结果或同时轮询结果;调用时EndExecuteXmlReader(IAsyncResult)也需要此值，它将命令的结果返回为 XML。

例外

InvalidCastException

如果设置为 Stream，则使用Value二进制或 VarBinary 以外的其他SqlDbType项。 有关流式处理的详细信息，请参阅 SqlClient 流式处理支持。

-或-

设置为 /a0> 时使用了除 Char、NChar、NVarChar、VarChar 或 Xml 以外的其他项。

-或-

设置为ValueXmlReader时使用了 Xml 以外的其他SqlDbType项。

SqlException

执行命令文本时发生的任何错误。

-或-

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

InvalidOperationException

名称/值对“异步处理=true”不包括在定义此 SqlCommand连接的连接字符串中。

-或-

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

IOException

流式处理操作期间发生错误StreamXmlReader或TextReader对象。 有关流式处理的详细信息，请参阅 SqlClient 流式处理支持。

ObjectDisposedException

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

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。 此示例将执行 SqlCommand 的对象作为 stateObject 参数传递-这样做使从回调过程中检索 SqlCommand 对象变得简单，以便代码可以调用 EndExecuteXmlReader 与初始调用 BeginExecuteXmlReader对应的方法。

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表单交互的方法。 此外，此示例演示了如何阻止用户多次并发执行命令，以及如何确保在调用回调过程之前窗体未关闭。

若要设置此示例，请创建新的 Windows 应用程序。 Button将控件、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，根据需要修改环境的连接字符串。

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.Windows.Forms
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

该方法 BeginExecuteXmlReader 开始异步执行以 XML 形式返回行的 Transact-SQL 语句或存储过程的过程，以便其他任务在执行语句时可以并发运行。 语句完成后，开发人员必须调用 EndExecuteXmlReader 该方法来完成操作并检索请求的 XML 数据。 该方法 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在命令的执行完成之前调用该对象会导致SqlCommand对象阻塞，直到执行完成。

该 CommandText 属性通常使用有效的 FOR XML 子句指定 Transact-SQL 语句。 但是， CommandText 还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，如果返回了多个行，该方法 EndExecuteXmlReader 将附加到 XmlReader 第一行上的值，并放弃结果集的其余部分。

典型 BeginExecuteXmlReader 查询的格式可以如下 C# 示例所示：

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，如果返回了多个行，该方法 EndExecuteXmlReader 将附加到 XmlReader 第一行上的值，并放弃结果集的其余部分。

多个活动结果集 （MARS） 功能允许多个操作使用相同的连接。

通过该 callback 参数，可以指定在 AsyncCallback 语句完成时调用的委托。 可以从此委托过程或应用程序中的任何其他位置调用 EndExecuteXmlReader 该方法。 此外，还可以在参数中 stateObject 传递任何对象，回调过程可以使用该属性检索此信息 AsyncState 。

请注意，命令文本和参数将同步发送到服务器。 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，该方法会立即返回，而无需等待来自服务器的答案，即读取是异步的。

在执行操作期间发生的所有错误都会作为回调过程中的异常引发。 必须在回调过程中处理异常，而不是在主应用程序中处理。 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。

如果使用 ExecuteReader 或 BeginExecuteReader 访问 XML 数据，SQL Server 将返回长度大于 2,033 个字符的任何 XML 结果，每行包含 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

• EndExecuteXmlReader(IAsyncResult)
• ExecuteXmlReader()
• ADO.NET 概述