SqlCommand.BeginExecuteReader 方法
定義
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。
起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並從伺服器擷取一或多個結果集。
多載
| BeginExecuteReader() |
起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並從伺服器擷取一或多個結果集。 |
| BeginExecuteReader(CommandBehavior) |
藉由使用其中一個 CommandBehavior 值，起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行。 |
| BeginExecuteReader(AsyncCallback, Object) |
指定回呼程序和狀態資訊時，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，並且從伺服器擷取一或多個結果集。 |
| BeginExecuteReader(AsyncCallback, Object, CommandBehavior) |
指定回呼程序和狀態資訊時，使用其中一個 |
BeginExecuteReader()
起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並從伺服器擷取一或多個結果集。
public:
IAsyncResult ^ BeginExecuteReader();public IAsyncResult BeginExecuteReader ();member this.BeginExecuteReader : unit -> IAsyncResultPublic Function BeginExecuteReader () As IAsyncResult傳回
IAsyncResult，可用於輪詢或等待結果 (或兩者)；叫用 EndExecuteReader(IAsyncResult) 以傳回可用以擷取所傳回資料列的 SqlDataReader 執行個體時，也需要這個值。
例外狀況
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-
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。
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。
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數據讀取器的程式。 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 SqlDataReader 並顯示其內容。
using System.Data.SqlClient;
class Class1
{
static void Main()
{
// This is a simple example that demonstrates the usage of the
// BeginExecuteReader functionality
// The WAITFOR statement simply adds enough time to prove the
// asynchronous nature of the command.
string commandText =
"WAITFOR DELAY '00:00:03';" +
"SELECT LastName, FirstName FROM Person.Contact " +
"WHERE LastName LIKE 'M%'";
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))
{
try
{
SqlCommand command = new SqlCommand(commandText, connection);
connection.Open();
IAsyncResult result = command.BeginExecuteReader();
// Although it is not necessary, the following code
// 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)
{
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.
System.Threading.Thread.Sleep(100);
}
using (SqlDataReader reader = command.EndExecuteReader(result))
{
DisplayResults(reader);
}
}
catch (SqlException ex)
{
Console.WriteLine("Error ({0}): {1}", ex.Number, ex.Message);
}
catch (InvalidOperationException ex)
{
Console.WriteLine("Error: {0}", ex.Message);
}
catch (Exception ex)
{
// You might want to pass these errors
// back out to the caller.
Console.WriteLine("Error: {0}", ex.Message);
}
}
}
private static void DisplayResults(SqlDataReader reader)
{
// Display the data within the reader.
while (reader.Read())
{
// Display all the columns.
for (int i = 0; i < reader.FieldCount; i++)
Console.Write("{0} ", reader.GetValue(i));
Console.WriteLine();
}
}
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
Module Module1
Sub Main()
' This is a simple example that demonstrates the usage of the
' BeginExecuteReader functionality.
' 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 LastName, FirstName FROM Person.Contact " & _
"WHERE LastName LIKE 'M%'"
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.BeginExecuteReader()
' 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
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
' Once the IAsyncResult object signals that it is done
' waiting for results, you can retrieve the results.
Using reader As SqlDataReader = command.EndExecuteReader(result)
DisplayResults(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 DisplayResults(ByVal reader As SqlDataReader)
' Display the data within the reader.
While reader.Read()
' Display all the columns.
For i As Integer = 0 To reader.FieldCount - 1
Console.Write("{0} ", reader.GetValue(i))
Next
Console.WriteLine()
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 BeginExecuteReader 會啟動異步執行 Transact-SQL 語句或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 EndExecuteReader 方法來完成作業，並擷取 SqlDataReader 命令所傳回的 。 方法 BeginExecuteReader 會立即傳回，但在程式代碼執行對應的 EndExecuteReader 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。 EndExecuteReader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。
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。 這表示，如果需要更多數據，以及基礎網路的讀取作業區塊，呼叫 Read 可能會封鎖。
由於此多載不支援回呼程式，因此開發人員必須輪詢以判斷命令是否已完成，方法是使用 IsCompleted 方法所BeginExecuteReader傳回 的 IAsyncResult 屬性，或等候使用AsyncWaitHandle所傳回 IAsyncResult之 屬性完成一或多個命令。
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
BeginExecuteReader(CommandBehavior)
藉由使用其中一個 CommandBehavior 值，起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行。
public:
IAsyncResult ^ BeginExecuteReader(System::Data::CommandBehavior behavior);public IAsyncResult BeginExecuteReader (System.Data.CommandBehavior behavior);member this.BeginExecuteReader : System.Data.CommandBehavior -> IAsyncResultPublic Function BeginExecuteReader (behavior As CommandBehavior) As IAsyncResult參數
- behavior
- CommandBehavior
其中一個 CommandBehavior 值，表示陳述式執行和資料擷取的選項。
傳回
IAsyncResult，可用於輪詢或等待結果 (或兩者)；叫用 EndExecuteReader(IAsyncResult) (會傳回可用以擷取所傳回資料列的 SqlDataReader 執行個體) 時，也需要這個值。
例外狀況
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-
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。
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。
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數據讀取器的程式。 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 SqlDataReader 取 並顯示其內容。
這個範例也會傳遞 CommandBehavior.CloseConnection behavior 參數中的 和 CommandBehavior.SingleRow 值，導致關閉與傳 SqlDataReader 回的連接關閉，並針對單一數據列結果優化。
using System.Data.SqlClient;
class Class1
{
static void Main()
{
// This example is not terribly useful, 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 ProductID, Name FROM Production.Product WHERE ListPrice < 100";
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.
try
{
// The code does not need to handle closing the connection explicitly--
// the use of the CommandBehavior.CloseConnection option takes care
// of that for you.
SqlConnection connection = new SqlConnection(connectionString);
SqlCommand command = new SqlCommand(commandText, connection);
connection.Open();
IAsyncResult result = command.BeginExecuteReader(
CommandBehavior.CloseConnection);
// Although it is not necessary, the following code
// 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);
}
using (SqlDataReader reader = command.EndExecuteReader(result))
{
DisplayResults(reader);
}
}
catch (SqlException ex)
{
Console.WriteLine("Error ({0}): {1}", ex.Number, ex.Message);
}
catch (InvalidOperationException ex)
{
Console.WriteLine("Error: {0}", ex.Message);
}
catch (Exception ex)
{
// You might want to pass these errors
// back out to the caller.
Console.WriteLine("Error: {0}", ex.Message);
}
}
private static void DisplayResults(SqlDataReader reader)
{
// Display the data within the reader.
while (reader.Read())
{
// Display all the columns.
for (int i = 0; i < reader.FieldCount; i++)
{
Console.Write("{0}\t", reader.GetValue(i));
}
Console.WriteLine();
}
}
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
Module Module1
Sub Main()
' This example is not terribly useful, 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 ProductID, Name FROM Production.Product WHERE ListPrice < 100"
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.
Try
' The code does not need to handle closing the connection explicitly--
' the use of the CommandBehavior.CloseConnection option takes care
' of that for you.
Dim connection As New SqlConnection(connectionString)
Dim command As New SqlCommand(commandText, connection)
connection.Open()
Dim result As IAsyncResult = _
command.BeginExecuteReader(CommandBehavior.CloseConnection)
' Although it is not necessary, the following code
' 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
' The "using" statement closes the SqlDataReader when it is
' done executing.
Using reader As SqlDataReader = command.EndExecuteReader(result)
DisplayResults(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 Sub
Private Sub DisplayResults(ByVal reader As SqlDataReader)
' Display the data within the reader.
While reader.Read()
' Display all the columns.
For i As Integer = 0 To reader.FieldCount - 1
Console.Write("{0} ", reader.GetValue(i))
Next
Console.WriteLine()
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 BeginExecuteReader 會啟動異步執行 Transact-SQL 語句或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 EndExecuteReader 方法來完成作業，並擷取 SqlDataReader 命令所傳回的 。 方法 BeginExecuteReader 會立即傳回，但在程式代碼執行對應的 EndExecuteReader 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。 EndExecuteReader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。
參數 behavior 可讓您指定控制命令及其連線行為的選項。 這些值可以使用程式設計語言的 OR 運算子) ，將這些值結合在一起 (;一般而言，開發人員會使用 CommandBehavior.CloseConnection 值，確保運行時間關閉時 SqlDataReader 會關閉連接。
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。 這表示，如果需要更多數據，以及基礎網路的讀取作業區塊，呼叫 Read 可能會封鎖。
由於此多載不支援回呼程式，因此開發人員必須輪詢以判斷命令是否已完成，方法是使用 IsCompleted 方法所BeginExecuteNonQuery傳回 的 IAsyncResult 屬性，或等候使用AsyncWaitHandle所傳回 IAsyncResult之 屬性完成一或多個命令。
如果您使用 ExecuteReader 或 BeginExecuteReader 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
適用於
BeginExecuteReader(AsyncCallback, Object)
指定回呼程序和狀態資訊時，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，並且從伺服器擷取一或多個結果集。
public:
IAsyncResult ^ BeginExecuteReader(AsyncCallback ^ callback, System::Object ^ stateObject);public IAsyncResult BeginExecuteReader (AsyncCallback callback, object stateObject);member this.BeginExecuteReader : AsyncCallback * obj -> IAsyncResultPublic Function BeginExecuteReader (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，可用於輪詢或等待結果 (或兩者)；叫用 EndExecuteReader(IAsyncResult) 以傳回一個可用於擷取所傳回資料列的 SqlDataReader 執行個體時，也需要這個值。
例外狀況
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-
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。
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。
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 BeginExecuteReader 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)。 因為範例會以異步方式執行命令，所以窗體在等候結果時仍會保持回應。 這個範例會將執行 SqlCommand 中的對象當做 stateObject 參數傳遞;這麼做可讓您輕鬆地從回呼程式內擷取 SqlCommand 物件，讓程式碼可以呼叫 EndExecuteReader 對應至的初始呼叫 BeginExecuteReader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、DataGridView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;
namespace Microsoft.AdoDotNet.CodeSamples
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
// Hook up the form's Load event handler (you can double-click on
// the form's design surface in Visual Studio), and then add
// this code to the form's class:
// You need this delegate in order to fill the grid from
// a thread other than the form's thread. See the HandleCallback
// procedure for more information.
private delegate void FillGridDelegate(SqlDataReader reader);
// You need this delegate to update the status bar.
private delegate void DisplayStatusDelegate(string Text);
// This flag ensures that the user does not attempt
// to restart the command or close the form while the
// asynchronous command is executing.
private bool isExecuting = false;
// Because the overloaded version of BeginExecuteReader
// demonstrated here does not allow you to have the connection
// closed automatically, this example maintains the
// connection object externally, so that it is available for closing.
private SqlConnection connection = null;
private void DisplayStatus(string Text)
{
this.label1.Text = Text;
}
private void FillGrid(SqlDataReader reader)
{
try
{
DataTable table = new DataTable();
table.Load(reader);
this.dataGridView1.DataSource = table;
DisplayStatus("Ready");
}
catch (Exception ex)
{
// Because you are guaranteed this procedure
// is running from within the form's thread,
// it can directly interact with members of the form.
DisplayStatus(string.Format("Ready (last attempt failed: {0})",
ex.Message));
}
finally
{
// Do not forget to close the connection, as well.
if (reader != null)
{
reader.Close();
}
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;
SqlDataReader reader = command.EndExecuteReader(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. Therefore you cannot simply call code that
// fills the grid, like this:
// FillGrid(reader);
// 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.
FillGridDelegate del = new FillGridDelegate(FillGrid);
this.Invoke(del, reader);
// Do not close the reader here, because it is being used in
// a separate thread. Instead, have the procedure you have
// called close the reader once it is done with it.
}
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 there is none of
// your code 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 DisplayStatusDelegate(DisplayStatus),
"Error: " + ex.Message);
}
finally
{
isExecuting = false;
}
}
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 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
{
DisplayStatus("Connecting...");
connection = new SqlConnection(GetConnectionString());
// To emulate a long-running query, wait for
// a few seconds before retrieving the real data.
command = new SqlCommand("WAITFOR DELAY '0:0:5';" +
"SELECT ProductID, Name, ListPrice, Weight FROM Production.Product",
connection);
connection.Open();
DisplayStatus("Executing...");
isExecuting = true;
// Although it is not required that you pass the
// SqlCommand object as the second parameter in the
// BeginExecuteReader call, doing so makes it easier
// to call EndExecuteReader in the callback procedure.
AsyncCallback callback = new AsyncCallback(HandleCallback);
command.BeginExecuteReader(callback, command);
}
catch (Exception ex)
{
DisplayStatus("Error: " + ex.Message);
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 FormClosingEventHandler(Form1_FormClosing);
}
void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (isExecuting)
{
MessageBox.Show(this, "Cannot close the form until " +
"the pending asynchronous command has completed. Please wait...");
e.Cancel = true;
}
}
}
}
Imports System.Data.SqlClient
Public Class Form1
' Add this code to the form's class:
' You need this delegate in order to fill the grid from
' a thread other than the form's thread. See the HandleCallback
' procedure for more information.
Private Delegate Sub FillGridDelegate(ByVal reader As SqlDataReader)
' You need this delegate to update the status bar.
Private Delegate Sub DisplayStatusDelegate(ByVal Text As String)
' This flag ensures that the user does not attempt
' to restart the command or close the form while the
' asynchronous command is executing.
Private isExecuting As Boolean
' Because the overloaded version of BeginExecuteReader
' demonstrated here does not allow you to have the connection
' closed automatically, this example maintains the
' connection object externally, so that it is available for closing.
Private connection As SqlConnection
Private Sub DisplayStatus(ByVal Text As String)
Me.Label1.Text = Text
End Sub
Private Sub FillGrid(ByVal reader As SqlDataReader)
Try
Dim table As New DataTable
table.Load(reader)
Me.DataGridView1.DataSource = table
DisplayStatus("Ready")
Catch ex As Exception
' Because you are guaranteed this procedure
' is running from within the form's thread,
' it can directly interact with members of the form.
DisplayStatus(String.Format("Ready (last attempt failed: {0})", ex.Message))
Finally
' Do not forget to close the connection, as well.
If Not reader Is Nothing Then
reader.Close()
End If
If Not connection Is Nothing Then
connection.Close()
End If
End Try
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 SqlDataReader = command.EndExecuteReader(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. Therefore you cannot simply call code that
' fills the grid, like this:
' FillGrid(reader)
' 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 FillGridDelegate(AddressOf FillGrid)
Me.Invoke(del, reader)
' Do not close the reader here, because it is being used in
' a separate thread. Instead, have the procedure you have
' called close the reader once it is done with it.
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 there is none of
' your code 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 DisplayStatusDelegate(AddressOf DisplayStatus), _
"Error: " & ex.Message)
Finally
isExecuting = False
End Try
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 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"
End Function
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
DisplayStatus("Connecting...")
connection = New SqlConnection(GetConnectionString())
' To emulate a long-running query, wait for
' a few seconds before retrieving the real data.
command = New SqlCommand( _
"WAITFOR DELAY '0:0:5';" & _
"SELECT ProductID, Name, ListPrice, Weight FROM Production.Product", _
connection)
connection.Open()
DisplayStatus("Executing...")
isExecuting = True
' Although it is not required that you pass the
' SqlCommand object as the second parameter in the
' BeginExecuteReader call, doing so makes it easier
' to call EndExecuteReader in the callback procedure.
Dim callback As New AsyncCallback(AddressOf HandleCallback)
command.BeginExecuteReader(callback, command)
Catch ex As Exception
DisplayStatus("Error: " & ex.Message)
If connection IsNot Nothing Then
connection.Close()
End If
End Try
End If
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
End Class
備註
方法 BeginExecuteReader 會啟動異步執行 Transact-SQL 語句或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 EndExecuteReader 方法來完成作業，並擷取 SqlDataReader 命令所傳回的 。 方法 BeginExecuteReader 會立即傳回，但在程式代碼執行對應的 EndExecuteReader 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。 EndExecuteReader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。
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 EndExecuteReader 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。 雖然命令執行是異步的，但值擷取仍是同步的。 這表示，如果需要更多數據，以及基礎網路的讀取作業區塊，呼叫 Read 可能會封鎖。
由於回呼程式是從 Microsoft .NET 運行時間提供的背景線程內執行，因此請務必採取嚴格的方法來處理應用程式中的跨線程互動。 例如，您不得從回呼程式內與窗體的內容互動;如果必須更新表單，您必須切換回表單的線程，才能執行您的工作。 本主題中的範例示範此行為。
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 傳回長度大於 2,033 個字元的任何 XML 結果，每個數據列各 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
BeginExecuteReader(AsyncCallback, Object, CommandBehavior)
指定回呼程序和狀態資訊時，使用其中一個 CommandBehavior 值，起始這個 SqlCommand 所描述之 Transact-SQL 陳述式或預存程序的非同步執行，並從伺服器擷取一或多個結果集。
public:
IAsyncResult ^ BeginExecuteReader(AsyncCallback ^ callback, System::Object ^ stateObject, System::Data::CommandBehavior behavior);public IAsyncResult BeginExecuteReader (AsyncCallback callback, object stateObject, System.Data.CommandBehavior behavior);member this.BeginExecuteReader : AsyncCallback * obj * System.Data.CommandBehavior -> IAsyncResultPublic Function BeginExecuteReader (callback As AsyncCallback, stateObject As Object, behavior As CommandBehavior) 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。
- behavior
- CommandBehavior
其中一個 CommandBehavior 值，表示陳述式執行和資料擷取的選項。
傳回
IAsyncResult，可用於輪詢或等待結果 (或兩者)；叫用 EndExecuteReader(IAsyncResult) 以傳回一個可用於擷取所傳回資料列的 SqlDataReader 執行個體時，也需要這個值。
例外狀況
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-
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。
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。
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 BeginExecuteReader 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)。 因為範例會以異步方式執行命令，所以窗體在等候結果時仍會保持回應。 這個範例會將執行 SqlCommand 中的對象當做 stateObject 參數傳遞;這麼做可讓您輕鬆地從回呼程式內擷取 SqlCommand 物件，讓程式碼可以呼叫 EndExecuteReader 對應至的初始呼叫 BeginExecuteReader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、DataGridView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。
這個範例會 CommandBehavior.CloseConnection 傳遞 參數中的 behavior 值，導致傳 SqlDataReader 回的 在關閉時自動關閉其連接。
using System.Data.SqlClient;
namespace Microsoft.AdoDotNet.CodeSamples
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
// Hook up the form's Load event handler (you can double-click on
// the form's design surface in Visual Studio), and then add
// this code to the form's class:
// You need this delegate in order to fill the grid from
// a thread other than the form's thread. See the HandleCallback
// procedure for more information.
private delegate void FillGridDelegate(SqlDataReader reader);
// You need this delegate to update the status bar.
private delegate void DisplayStatusDelegate(string Text);
// This flag ensures that the user does not attempt
// to restart the command or close the form while the
// asynchronous command is executing.
private bool isExecuting;
private void DisplayStatus(string Text)
{
this.label1.Text = Text;
}
private void FillGrid(SqlDataReader reader)
{
try
{
DataTable table = new DataTable();
table.Load(reader);
this.dataGridView1.DataSource = table;
DisplayStatus("Ready");
}
catch (Exception ex)
{
// Because you are guaranteed this procedure
// is running from within the form's thread,
// it can directly interact with members of the form.
DisplayStatus(string.Format("Ready (last attempt failed: {0})",
ex.Message));
}
finally
{
// Closing the reader also closes the connection,
// because this reader was created using the
// CommandBehavior.CloseConnection value.
if (reader != null)
{
reader.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;
SqlDataReader reader = command.EndExecuteReader(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. Therefore you cannot simply call code that
// fills the grid, like this:
// FillGrid(reader);
// 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.
FillGridDelegate del = new FillGridDelegate(FillGrid);
this.Invoke(del, reader);
// Do not close the reader here, because it is being used in
// a separate thread. Instead, have the procedure you have
// called close the reader once it is done with it.
}
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 there is none of
// your code 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 DisplayStatusDelegate(DisplayStatus), "Error: " +
ex.Message);
}
finally
{
isExecuting = false;
}
}
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 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;
SqlConnection connection = null;
try
{
DisplayStatus("Connecting...");
connection = new SqlConnection(GetConnectionString());
// To emulate a long-running query, wait for
// a few seconds before retrieving the real data.
command = new SqlCommand("WAITFOR DELAY '0:0:5';" +
"SELECT ProductID, Name, ListPrice, Weight FROM Production.Product",
connection);
connection.Open();
DisplayStatus("Executing...");
isExecuting = true;
// Although it is not required that you pass the
// SqlCommand object as the second parameter in the
// BeginExecuteReader call, doing so makes it easier
// to call EndExecuteReader in the callback procedure.
AsyncCallback callback = new AsyncCallback(HandleCallback);
command.BeginExecuteReader(callback, command,
CommandBehavior.CloseConnection);
}
catch (Exception ex)
{
DisplayStatus("Error: " + ex.Message);
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 FormClosingEventHandler(Form1_FormClosing);
}
void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (isExecuting)
{
MessageBox.Show(this, "Cannot close the form until " +
"the pending asynchronous command has completed. Please wait...");
e.Cancel = true;
}
}
}
}
Imports System.Data.SqlClient
Public Class Form1
' Add this code to the form's class:
' You this delegate in order to fill the grid from
' a thread other than the form's thread. See the HandleCallback
' procedure for more information.
Private Delegate Sub FillGridDelegate(ByVal reader As SqlDataReader)
' You need this delegate to update the status bar.
Private Delegate Sub DisplayStatusDelegate(ByVal Text As String)
' This flag ensures that the user does not attempt
' to restart the command or close the form while the
' asynchronous command is executing.
Private isExecuting As Boolean
Private Sub DisplayStatus(ByVal Text As String)
Me.Label1.Text = Text
End Sub
Private Sub FillGrid(ByVal reader As SqlDataReader)
Try
Dim table As New DataTable
table.Load(reader)
Me.DataGridView1.DataSource = table
DisplayStatus("Ready")
Catch ex As Exception
' Because you are guaranteed this procedure
' is running from within the form's thread,
' it can directly interact with members of the form.
DisplayStatus(String.Format("Ready (last attempt failed: {0})", ex.Message))
Finally
' Closing the reader also closes the connection,
' because this reader was created using the
' CommandBehavior.CloseConnection value.
If reader IsNot Nothing Then
reader.Close()
End If
End Try
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 SqlDataReader = command.EndExecuteReader(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. Therefore you cannot simply call code that
' fills the grid, like this:
' FillGrid(reader)
' 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 FillGridDelegate(AddressOf FillGrid)
Me.Invoke(del, reader)
' Do not close the reader here, because it is being used in
' a separate thread. Instead, have the procedure you have
' called close the reader once it is done with it.
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 there is none of
' your code 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 DisplayStatusDelegate(AddressOf DisplayStatus), _
"Error: " & ex.Message)
Finally
isExecuting = False
End Try
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 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"
End Function
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 connection As SqlConnection
Dim command As SqlCommand
Try
DisplayStatus("Connecting...")
connection = New SqlConnection(GetConnectionString())
' To emulate a long-running query, wait for
' a few seconds before retrieving the real data.
command = New SqlCommand( _
"WAITFOR DELAY '0:0:5';" & _
"SELECT ProductID, Name, ListPrice, Weight FROM Production.Product", _
connection)
connection.Open()
DisplayStatus("Executing...")
isExecuting = True
' Although it is not required that you pass the
' SqlCommand object as the second parameter in the
' BeginExecuteReader call, doing so makes it easier
' to call EndExecuteReader in the callback procedure.
Dim callback As New AsyncCallback(AddressOf HandleCallback)
command.BeginExecuteReader(callback, command, _
CommandBehavior.CloseConnection)
Catch ex As Exception
DisplayStatus("Error: " & ex.Message)
If connection IsNot Nothing Then
connection.Close()
End If
End Try
End If
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
End Class
備註
方法 BeginExecuteReader 會啟動異步執行 Transact-SQL 語句或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 EndExecuteReader 方法來完成作業，並擷取 SqlDataReader 命令所傳回的 。 方法 BeginExecuteReader 會立即傳回，但在程式代碼執行對應的 EndExecuteReader 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。 EndExecuteReader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。
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 EndExecuteReader 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 。
參數 behavior 可讓您指定控制命令及其連線行為的選項。 這些值可以使用程式設計語言的 Or 運算子) ，將這些值結合在一起 (;一般而言，開發人員會使用 CloseConnection 值，確保運行時間關閉時 SqlDataReader 會關閉連接。 開發人員也可以在事先知道 Transact-SQL 語句或預存程式只傳回單一數據列時指定SingleRow值，藉此優化 SqlDataReader 的行為。
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。 這表示，如果需要更多數據，以及基礎網路的讀取作業區塊，呼叫 Read 可能會封鎖。
由於回呼程式是從 Microsoft .NET Common Language Runtime 所提供的背景線程內執行，因此您必須採取嚴格的方法來處理應用程式中的跨線程互動。 例如，您不得從回呼程式內與窗體的內容互動，您必須更新表單，您必須切換回表單的線程，才能執行您的工作。 本主題中的範例示範此行為。
執行作業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，而不是在主要應用程式中。 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 會以 2,033 個字元的多個數據列傳回長度大於 2,033 個字元的任何 XML 結果。 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。
