使用回调的 Windows 应用程序
在大多数异步处理方案中，你希望启动数据库操作并继续运行其他进程，而无需等待数据库操作完成。 但是，许可场景需要在数据库操作结束后执行一些操作。 例如，在 Windows 应用程序中，你可能希望将长时间运行的操作委托给后台线程，同时允许用户界面线程保持响应。 但是，在数据库操作完成后，你需要使用结果来填充窗体。 这种类型的场景最好通过回调实现。
通过在 BeginExecuteNonQuery、BeginExecuteReader或 BeginExecuteXmlReader 方法中指定 AsyncCallback 委托来定义回调。 操作完成后，将调用该委托。 可以向委托传递对 SqlCommand 本身的引用，这样就可以轻松访问 SqlCommand 对象并调用适当的 End 方法，而无需使用全局变量。
示例
以下 Windows 应用程序演示如何使用 BeginExecuteNonQuery 方法，以执行包含几秒钟延迟（模拟长时间运行的命令）的 Transact-SQL 语句。
此示例演示了许多重要的方法，包括调用从单独线程与窗体进行交互的方法。 此外，此示例还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不会关闭。
若要设置此示例，请创建新的 Windows 应用程序。 将一个 Button 控件和两个 Label 控件置于窗体（接受每个控件的默认名称）。 将以下代码添加到窗体的类中，根据环境的需要修改连接字符串。
' Add these to the top of the class:
Imports System
Imports System.Data
Imports System.Data.SqlClient
' Add this code to the form's class:
' You'll need this delegate in order to display text from a
' thread other than the form's thread. See the HandleCallback
' procedure for more information.
' This same delegate matches both the DisplayStatus
' and DisplayResults methods.
Private Delegate Sub DisplayInfoDelegate(ByVal Text As String)
' This flag ensures that the user doesn't attempt
' to restart the command or close the form while the
' asynchronous command is executing.
Private isExecuting As Boolean
' This example maintains the connection object
' externally, so that it's 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 will not be able
' to execute asynchronously.
Return "..." & _
"Asynchronous Processing=true"
End Function
Private Sub DisplayStatus(ByVal Text As String)
Me.Label1.Text = Text
End Sub
Private Sub DisplayResults(ByVal Text As String)
Me.Label1.Text = Text
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, "Can't 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
DisplayResults("")
DisplayStatus("Connecting...")
connection = New SqlConnection(GetConnectionString())
' To emulate a long-running query, wait for
' a few seconds before working with the data.
' This command doesn't do much, but that's the point--
' it doesn't change your data, in the long run.
Dim commandText As String = _
"WAITFOR DELAY '0:0:05';" & _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint + 1 " & _
"WHERE ReorderPoint Is Not Null;" & _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint - 1 " & _
"WHERE ReorderPoint Is Not Null"
command = New SqlCommand(commandText, connection)
connection.Open()
DisplayStatus("Executing...")
isExecuting = True
' Although it's not required that you pass the
' SqlCommand object as the second parameter in the
' BeginExecuteNonQuery call, doing so makes it easier
' to call EndExecuteNonQuery in the callback procedure.
Dim callback As New _
AsyncCallback(AddressOf HandleCallback)
' Once the BeginExecuteNonQuery method is called,
' the code continues--and the user can interact with
' the form--while the server executes the query.
command.BeginExecuteNonQuery(callback, command)
Catch ex As Exception
isExecuting = False
DisplayStatus($"Ready (last error: {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 rowCount As Integer = _
command.EndExecuteNonQuery(result)
Dim rowText As String = " rows affected."
If rowCount = 1 Then
rowText = " row affected."
End If
rowText = rowCount & rowText
' 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 displays the results, like this:
' DisplayResults(rowText)
' 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 _
DisplayInfoDelegate(AddressOf DisplayResults)
Me.Invoke(del, rowText)
Catch ex As Exception
' Because you're now running code in a separate thread,
' if you don't handle the exception here, none of your
' other code will catch the exception. Because none of
' your code is on the call stack in this thread, there's
' nothing higher up the stack to catch the exception if
' you don't 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), _
$"Ready (last error: {ex.Message}")
Finally
isExecuting = False
If connection IsNot Nothing Then
connection.Close()
End If
End Try
End Sub
// Add these to the top of the class, if they're not already there:
using System;
using System.Data;
using System.Data.SqlClient;
// 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'll need this delegate in order to display text from a thread
// other than the form's thread. See the HandleCallback
// procedure for more information.
// This same delegate matches both the DisplayStatus
// and DisplayResults methods.
private delegate void DisplayInfoDelegate(string Text);
// This flag ensures that the user doesn't attempt
// to restart the command or close the form while the
// asynchronous command is executing.
private bool isExecuting;
// This example maintains the connection object
// externally, so that it's available for closing.
private SqlConnection connection;
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 will not be able
// to execute asynchronously.
return "..." + "Asynchronous Processing=true";
}
private void DisplayStatus(string Text)
{
this.label1.Text = Text;
}
private void DisplayResults(string Text)
{
this.label1.Text = Text;
DisplayStatus("Ready");
}
private void Form1_FormClosing(object sender, System.Windows.Forms.FormClosingEventArgs e)
{
if (isExecuting)
{
MessageBox.Show(this, "Can't 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
{
DisplayResults("");
DisplayStatus("Connecting...");
connection = new SqlConnection(GetConnectionString());
// To emulate a long-running query, wait for
// a few seconds before working with the data.
// This command doesn't do much, but that's the point--
// it doesn't change your data, in the long run.
string commandText =
"WAITFOR DELAY '0:0:05';" +
"UPDATE Production.Product " +
"SET ReorderPoint = ReorderPoint + 1 " +
"WHERE ReorderPoint Is Not Null;" +
"UPDATE Production.Product " +
"SET ReorderPoint = ReorderPoint - 1 " +
"WHERE ReorderPoint Is Not Null";
command = new SqlCommand(commandText, connection);
connection.Open();
DisplayStatus("Executing...");
isExecuting = true;
// Although it's not required that you pass the
// SqlCommand object as the second parameter in the
// BeginExecuteNonQuery call, doing so makes it easier
// to call EndExecuteNonQuery in the callback procedure.
AsyncCallback callback = new AsyncCallback(HandleCallback);
// Once the BeginExecuteNonQuery method is called,
// the code continues--and the user can interact with
// the form--while the server executes the query.
command.BeginExecuteNonQuery(callback, command);
}
catch (Exception ex)
{
isExecuting = false;
DisplayStatus($"Ready (last error: {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;
int rowCount = command.EndExecuteNonQuery(result);
string rowText = " rows affected.";
if (rowCount == 1)
{
rowText = " row affected.";
}
rowText = rowCount + rowText;
// 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
// displays the results, like this:
// DisplayResults(rowText)
// 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.
DisplayInfoDelegate del =
new DisplayInfoDelegate(DisplayResults);
this.Invoke(del, rowText);
}
catch (Exception ex)
{
// Because you're now running code in a separate thread,
// if you don't handle the exception here, none of your other
// code will catch the exception. Because none of your
// code is on the call stack in this thread, there's nothing
// higher up the stack to catch the exception if you don't
// 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),
$"Ready (last error: {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);
}