SqlCommand.BeginExecuteXmlReader 方法

參考

定義

命名空間:: System.Data.SqlClient

組件:: System.Data.dll

組件:: netstandard.dll, System.Data.SqlClient.dll

套件:: System.Data.SqlClient v4.8.6

重要

部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。

啟始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並傳回結果做為 XmlReader 物件。

多載

BeginExecuteXmlReader()	啟始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並傳回結果做為 XmlReader 物件。
BeginExecuteXmlReader(AsyncCallback, Object)	使用回呼程序，啟始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並傳回結果做為 XmlReader 物件。

BeginExecuteXmlReader()

啟始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並傳回結果做為 XmlReader 物件。

public:
 IAsyncResult ^ BeginExecuteXmlReader();

public IAsyncResult BeginExecuteXmlReader ();

member this.BeginExecuteXmlReader : unit -> IAsyncResult

Public Function BeginExecuteXmlReader () As IAsyncResult

傳回

IAsyncResult

IAsyncResult，可用於輪詢或等待結果 (或兩者)；叫用傳回單一 XML 值的 EndExecuteXmlReader 時，也需要這個值。

例外狀況

InvalidCastException

當 Value 設為 Stream 時，使用 Binary 或 VarBinary 以外的 SqlDbType。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

-或-

SqlDbType當設定TextReader為時Value，會使用 Char、NChar、NVarChar、VarChar 或 Xml 以外的。

-或-

當 Value 設定為 XmlReader 時，使用 Xml 以外的 SqlDbType。

SqlException

執行命令文字時發生的任何錯誤。

-或-

串流作業期間發生逾時。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

InvalidOperationException

定義這個 SqlCommand 之連接的連接字串中未包含名稱/值組 "Asynchronous Processing=true"。

-或-

在串流作業期間已關閉或卸除的 SqlConnection。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

IOException

Stream、XmlReader 或 TextReader 物件在串流作業期間發生錯誤。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

ObjectDisposedException

Stream、XmlReader 或 TextReader 物件在串流作業期間已關閉。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

範例

下列主控台應用程式會啟動異步擷取 XML 資料的程式。等候結果時，這個簡單的應用程式會位於迴圈中，並 IsCompleted 調查屬性值。程式完成後，程式代碼會擷取 XML 並顯示其內容。

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

備註

方法 BeginExecuteXmlReader 會啟動異步執行 Transact-SQL 語句以 XML 傳回數據列的程式，讓其他工作可以在語句執行時同時執行。當語句完成時，開發人員必須呼叫 EndExecuteXmlReader 方法來完成作業，並擷取命令傳回的 XML。方法 BeginExecuteXmlReader 會立即傳回，但在程式代碼執行對應的 EndExecuteXmlReader 方法呼叫之前，它不得執行任何其他針對相同 SqlCommand 對象啟動同步或異步執行的呼叫。 EndExecuteXmlReader在命令的執行完成之前呼叫，會導致SqlCommand對象封鎖直到執行完成為止。

屬性 CommandText 通常會指定具有有效 FOR XML 子句的 Transact-SQL 語句。不過， CommandText 也可以指定語句來傳回 ntext 包含有效 XML 的數據。

一般 BeginExecuteXmlReader 查詢的格式可以如下列 C# 範例所示：

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

這個方法也可以用來擷取單一數據列的單一數據行結果集。在此情況下，如果傳回多個數據列，方法會將 EndExecuteXmlReader 附加 XmlReader 至第一個數據列的值，並捨棄結果集的其餘部分。

MARS) 功能的多個作用中結果集 (可讓多個動作使用相同的連線。

請注意，命令文字和參數會同步傳送至伺服器。如果傳送大型命令或許多參數，這個方法可能會在寫入期間封鎖。傳送命令之後，方法會立即傳回，而不需要等候來自伺服器的答案，也就是讀取是異步的。雖然命令執行是異步的，但值擷取仍是同步的。

因為這個多載不支援回呼程式，所以開發人員需要輪詢以判斷命令是否已完成，使用 IsCompleted 方法所BeginExecuteXmlReader傳回的 IAsyncResult 屬性;或使用傳回 IAsyncResult的屬性等候完成一或多個命令AsyncWaitHandle。

如果您使用 ExecuteReader 或 BeginExecuteReader 來存取 XML 數據，SQL Server 會傳回長度大於 2,033 個字元的任何 XML 結果，每個數據列 2,033 個字元。若要避免這種行為，請使用 ExecuteXmlReader 或 BeginExecuteXmlReader 來讀取 FOR XML 查詢。

這個方法會 CommandTimeout 忽略屬性。

另請參閱

連接和擷取 ADO.NET 中的數據
使用 .NET Framework Data Provider for SQL Server
ADO.NET 概觀 \(部分機器翻譯\)

適用於

BeginExecuteXmlReader(AsyncCallback, Object)

使用回呼程序，啟始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並傳回結果做為 XmlReader 物件。

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

參數

callback: AsyncCallback

完成執行命令時叫用的 AsyncCallback 委派。傳遞 null (在 Microsoft Visual Basic 中為 Nothing) 以表示不需要回呼。

stateObject: Object

已傳遞至回呼程序的使用者定義狀態物件。使用 AsyncState 屬性，從回呼程序內擷取這個物件。

傳回

IAsyncResult

IAsyncResult，可用於輪詢或等待結果 (或兩者)；此外，呼叫會以 XML 傳回命令結果的 EndExecuteXmlReader(IAsyncResult) 時，也需要這個值。

例外狀況

InvalidCastException

當 Value 設為 Stream 時，使用 Binary 或 VarBinary 以外的 SqlDbType。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

-或-

SqlDbType當設定TextReader為時Value，會使用 Char、NChar、NVarChar、VarChar 或 Xml 以外的。

-或-

當 Value 設定為 XmlReader 時，使用 Xml 以外的 SqlDbType。

SqlException

執行命令文字時發生的任何錯誤。

-或-

串流作業期間發生逾時。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

InvalidOperationException

定義這個 SqlCommand 之連接的連接字串中未包含名稱/值組 "Asynchronous Processing=true"。

-或-

在串流作業期間已關閉或卸除的 SqlConnection。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

IOException

Stream、XmlReader 或 TextReader 物件在串流作業期間發生錯誤。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

ObjectDisposedException

Stream、XmlReader 或 TextReader 物件在串流作業期間已關閉。如需串流的詳細資訊，請參閱 SqlClient 串流支援。

範例

下列 Windows 應用程式示範如何使用 BeginExecuteXmlReader 方法，來執行包含數秒延遲的 Transact-SQL 陳述式 (模擬長時間執行的命令)。這個範例會將執行 SqlCommand 中的對象當做 stateObject 參數傳遞，如此即可輕鬆地從回呼程式內擷取 SqlCommand 物件，讓程式代碼可以呼叫 EndExecuteXmlReader 對應至初始呼叫 BeginExecuteXmlReader的方法。

此範例示範許多重要的技術。這包括呼叫從個別線程與窗體互動的方法。此外，此範例示範如何封鎖使用者多次同時執行命令，以及如何確保表單在呼叫回呼程式之前不會關閉。

若要設定此範例，請建立新的 Windows 應用程式。 Button將控件、ListBox控件和Label控件放在窗體上， (接受每個控件的預設名稱) 。將下列程式代碼新增至表單的類別，視您的環境需要修改連接字串。

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

備註

方法 BeginExecuteXmlReader 會啟動異步執行 Transact-SQL 語句或以 XML 傳回數據列的預存程式的程式，讓其他工作可以在語句執行時同時執行。當語句完成時，開發人員必須呼叫 EndExecuteXmlReader 方法來完成作業，並擷取要求的 XML 數據。方法 BeginExecuteXmlReader 會立即傳回，但在程式代碼執行對應的 EndExecuteXmlReader 方法呼叫之前，它不得執行任何其他針對相同 SqlCommand 對象啟動同步或異步執行的呼叫。 EndExecuteXmlReader在命令的執行完成之前呼叫，會導致SqlCommand對象封鎖直到執行完成為止。

屬性 CommandText 通常會指定具有有效 FOR XML 子句的 Transact-SQL 語句。不過， CommandText 也可以指定語句來傳回包含有效 XML 的數據。這個方法也可以用來擷取單一數據列的單一數據行結果集。在此情況下，如果傳回多個數據列，方法會將 EndExecuteXmlReader 附加 XmlReader 至第一個數據列的值，並捨棄結果集的其餘部分。

一般 BeginExecuteXmlReader 查詢的格式可以如下列 C# 範例所示：

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

這個方法也可以用來擷取單一數據列的單一數據行結果集。在此情況下，如果傳回多個數據列，方法會將 EndExecuteXmlReader 附加 XmlReader 至第一個數據列的值，並捨棄結果集的其餘部分。

MARS) 功能的多個作用中結果集 (可讓多個動作使用相同的連線。

參數 callback 可讓您指定 AsyncCallback 語句完成時所呼叫的委派。您可以從這個委派程式內，或從應用程式內的任何其他位置呼叫 EndExecuteXmlReader 方法。此外，您可以在參數中 stateObject 傳遞任何物件，而回呼程式可以使用屬性擷取這項資訊 AsyncState 。

請注意，命令文字和參數會同步傳送至伺服器。如果傳送大型命令或許多參數，這個方法可能會在寫入期間封鎖。傳送命令之後，方法會立即傳回，而不需要等候來自伺服器的答案，也就是讀取是異步的。

在作業執行期間發生的所有錯誤都會在回呼程式中擲回為例外狀況。您必須在回呼程式中處理例外狀況，而不是在主應用程式中。如需在回呼程式中處理例外狀況的其他資訊，請參閱本主題中的範例。

如果您使用 ExecuteReader 或 BeginExecuteReader 存取 XML 數據，SQL Server 會傳回長度超過 2,033 個字元的任何 XML 結果，每個數據列 2,033 個字元。若要避免這種行為，請使用 ExecuteXmlReader 或 BeginExecuteXmlReader 來讀取 FOR XML 查詢。

這個方法會 CommandTimeout 忽略屬性。

另請參閱

適用於

共用方式為

SqlCommand.BeginExecuteXmlReader 方法

定義

多載

BeginExecuteXmlReader()

傳回

例外狀況

範例

備註

另請參閱

適用於

BeginExecuteXmlReader(AsyncCallback, Object)

參數

傳回

例外狀況

範例

備註

另請參閱

適用於

其他資源