使用回呼的 Windows 應用程式
在大部分的非同步處理案例中，您想要啟動資料庫作業，並繼續執行其他處理序，而不需等候資料庫作業完成。 不過，許多案例都需要在資料庫作業結束後執行某些動作。 例如，在 Windows 應用程式中，您可以將長時間執行的作業委派給背景執行緒，同時讓使用者介面執行緒維持回應。 不過，當資料庫作業完成時，您想要使用結果來填入表單。 這種類型的案例最適合使用回呼來實作。
您可以藉由在 BeginExecuteNonQuery、BeginExecuteReader 或 BeginExecuteXmlReader 方法中指定 AsyncCallback 委派來定義回呼。 當作業完成時，即會呼叫委派。 您可以將 SqlCommand 本身的參考傳遞給委派，輕鬆存取 SqlCommand 物件，並呼叫適當的 End 方法，而不需使用全域變數。
範例
下列 Windows 應用程式示範如何使用 BeginExecuteNonQuery 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)。
此範例示範一些重要技術，包括從個別執行緒呼叫與表單互動的方法。 此外，此範例還會示範如何封鎖使用者多次執行命令，以及如何確保表單不會在呼叫回呼程序之前關閉。
若要設定此範例，請建立新的 Windows 應用程式。 在表單上放置一個 Button 控制項和兩個 Label 控制項 (接受每個控制項的預設名稱)。 將下列程式碼新增至表單的類別，並視您的環境需要修改連接字串。
' 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 "Data Source=(local);Integrated Security=SSPI;" & _
"Initial Catalog=AdventureWorks;" & _
"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 "Data Source=(local);Integrated Security=SSPI;" +
"Initial Catalog=AdventureWorks; 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);
}
另請參閱
- 非同步作業
- ADO.NET 概觀 \(部分機器翻譯\)