SqlCommand.BeginExecuteReader 方法
定义
重要
一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。 对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。
启动由此描述 SqlCommand的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。
重载
| 名称 | 说明 |
|---|---|
| BeginExecuteReader() |
启动由此描述 SqlCommand的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。 |
| BeginExecuteReader(CommandBehavior) |
使用其中一个CommandBehavior值启动 Transact-SQL 语句或存储过程SqlCommand的异步执行。 |
| BeginExecuteReader(AsyncCallback, Object) |
在给定回调过程和状态信息的情况下，启动对此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。 |
| BeginExecuteReader(AsyncCallback, Object, CommandBehavior) |
使用其中一个 |
BeginExecuteReader()
启动由此描述 SqlCommand的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。
public:
IAsyncResult ^ BeginExecuteReader();public IAsyncResult BeginExecuteReader();member this.BeginExecuteReader : unit -> IAsyncResultPublic Function BeginExecuteReader () As IAsyncResult返回
可用于轮询或等待结果或同时轮询结果的实例 IAsyncResult ;调用时 EndExecuteReader(IAsyncResult)也需要此值，该值返回 SqlDataReader 可用于检索返回的行的实例。
例外
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
-或-
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。
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。
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将启动异步检索数据读取器的过程。 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 SqlDataReader 并显示其内容。
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
注解
该方法 BeginExecuteReader 启动异步执行返回行的 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 EndExecuteReader 该方法来完成操作并检索 SqlDataReader 命令返回的操作。 该方法 BeginExecuteReader 会立即返回，但在代码执行相应的 EndExecuteReader 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。 EndExecuteReader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。
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。 这意味着，如果需要更多数据以及基础网络的读取操作块，则调用 Read 可能会阻止。
由于此重载不支持回调过程，因此开发人员必须轮询以确定命令是否已完成，使用IsCompleted方法返回BeginExecuteReader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
适用于
BeginExecuteReader(CommandBehavior)
使用其中一个CommandBehavior值启动 Transact-SQL 语句或存储过程SqlCommand的异步执行。
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参数
- behavior
- CommandBehavior
值之 CommandBehavior 一，指示语句执行和数据检索的选项。
返回
可用于轮询、等待结果或同时轮询结果的实例 IAsyncResult ;调用时 EndExecuteReader(IAsyncResult)也需要此值，该值返回 SqlDataReader 可用于检索返回的行的实例。
例外
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
-或-
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。
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。
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将启动异步检索数据读取器的过程。 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 SqlDataReader 检索并显示其内容。
此示例还传递 CommandBehavior.CloseConnection 行为参数中的和 CommandBehavior.SingleRow 值，导致关闭返回 SqlDataReader 的连接关闭，并针对单个行结果进行优化。
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
注解
该方法 BeginExecuteReader 启动异步执行返回行的 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 EndExecuteReader 该方法来完成操作并检索 SqlDataReader 命令返回的操作。 该方法 BeginExecuteReader 会立即返回，但在代码执行相应的 EndExecuteReader 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。 EndExecuteReader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。
通过此参数 behavior ，可以指定控制命令及其连接行为的选项。 这些值可以组合在一起（使用编程语言的 OR 运算符）;通常，开发人员使用 CommandBehavior.CloseConnection 该值来确保关闭时 SqlDataReader 运行时关闭连接。
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。 这意味着，如果需要更多数据以及基础网络的读取操作块，则调用 Read 可能会阻止。
由于此重载不支持回调过程，因此开发人员必须轮询以确定命令是否已完成，使用IsCompleted方法返回BeginExecuteNonQuery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
适用于
BeginExecuteReader(AsyncCallback, Object)
在给定回调过程和状态信息的情况下，启动对此描述 SqlCommand 的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。
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
可用于轮询、等待结果或同时轮询结果的实例 IAsyncResult ;调用时 EndExecuteReader(IAsyncResult)也需要此值，该值返回 SqlDataReader 可用于检索返回的行的实例。
例外
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
-或-
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。
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。
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 BeginExecuteReader 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。 由于示例以异步方式执行命令，因此窗体在等待结果时保持响应状态。 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 EndExecuteReader 与初始调用 BeginExecuteReader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、DataGridView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;
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
注解
该方法 BeginExecuteReader 启动异步执行返回行的 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 EndExecuteReader 该方法来完成操作并检索 SqlDataReader 命令返回的操作。 该方法 BeginExecuteReader 会立即返回，但在代码执行相应的 EndExecuteReader 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。 EndExecuteReader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。
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 EndExecuteReader 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。 尽管命令执行是异步的，但提取值仍然同步。 这意味着，如果需要更多数据以及基础网络的读取操作块，则调用 Read 可能会阻止。
由于回调过程从由 Microsoft .NET 运行时提供的后台线程内执行，因此必须采取严格的方法来处理应用程序中的跨线程交互。 例如，不得从回调过程中与表单的内容进行交互;如果必须更新窗体，必须切换回窗体的线程才能完成工作。 本主题中的示例演示了此行为。
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
适用于
BeginExecuteReader(AsyncCallback, Object, CommandBehavior)
使用其中一个CommandBehavior值并从服务器检索一个或多个结果集（给定回调过程和状态信息）启动由此SqlCommand描述的 Transact-SQL 语句或存储过程的异步执行。
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。
- behavior
- CommandBehavior
值之 CommandBehavior 一，指示语句执行和数据检索的选项。
返回
可用于轮询或等待结果或同时轮询结果的实例 IAsyncResult ;调用时 EndExecuteReader(IAsyncResult)也需要此值，该值返回 SqlDataReader 可用于检索返回的行的实例。
例外
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
-或-
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。
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。
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 BeginExecuteReader 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。 由于示例以异步方式执行命令，因此窗体在等待结果时保持响应状态。 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 EndExecuteReader 与初始调用 BeginExecuteReader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、DataGridView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。
此示例传递 CommandBehavior.CloseConnection 参数中的 behavior 值，导致返回 SqlDataReader 的值在关闭时自动关闭其连接。
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
注解
该方法 BeginExecuteReader 启动异步执行返回行的 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 EndExecuteReader 该方法来完成操作并检索 SqlDataReader 命令返回的操作。 该方法 BeginExecuteReader 会立即返回，但在代码执行相应的 EndExecuteReader 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。 EndExecuteReader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。
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 EndExecuteReader 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 。
通过此参数 behavior ，可以指定控制命令及其连接行为的选项。 这些值可以组合在一起（使用编程语言的 Or 运算符）;通常，开发人员使用 CloseConnection 该值来确保关闭时 SqlDataReader 运行时关闭连接。 开发人员还可以通过在事先知道 Transact-SQL 语句或存储过程仅返回单个行时指定SingleRow值来优化其行为SqlDataReader。
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。 这意味着，如果需要更多数据以及基础网络的读取操作块，则调用 Read 可能会阻止。
由于回调过程从 Microsoft .NET 公共语言运行时提供的后台线程内执行，因此，必须采取严格的方法来处理应用程序中的跨线程交互。 例如，你不得从回调过程中与窗体的内容进行交互--如果必须更新窗体，则必须切换回窗体的线程才能完成工作。 本主题中的示例演示了此行为。
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。
