HOW TO：進行對 Windows Form 控制項的安全執行緒呼叫
更新：2007 年 11 月
如果您使用多執行緒處理來改進 Windows Form 應用程式的效能，您必須以安全執行緒的方式小心呼叫控制項。
範例
Windows Form 控制項的存取並非原本就採用安全執行緒的方式。如果您有兩個或多個執行緒管理控制項的狀態，就有可能強制控制項進入不一致的狀態。其他與執行緒有關的錯誤也有可能如此，包括競爭情形和死結。確定存取控制項是以安全執行緒的方式來進行，是很重要的。
.NET Framework 有助於偵測當您使用非安全執行緒的方式存取控制項的情況。當您在偵錯工具中執行應用程式，且有建立控制項之執行緒以外的執行緒嘗試呼叫該控制項時，偵錯工具會引發 InvalidOperationException 和訊息：「存取控制項 control name 時所使用的執行緒與建立控制項的執行緒不同」。
在偵錯期間這個例外狀況會經常發生，且在某些情況下會發生於執行階段。強烈建議您在發現問題時就加以修正。您可能會在針對使用 .NET Framework 2.0 版之前版本的 .NET Framework 所撰寫的應用程式進行偵錯時，發現這個例外狀況。
.gif "注意事項") 注意事項： |
|---|
您可以藉由將 CheckForIllegalCrossThreadCalls 屬性值設定為 false，停用這個例外狀況。這會造成您的控制項使用在 Visual Studio 2003 下執行的相同方式來執行。 |
下列程式碼範例示範如何以安全執行緒的方式呼叫 Windows Form 控制項，且不是以背景工作執行緒 (Worker Thread) 的安全執行緒方式呼叫。此範例示範使用非安全執行緒的方式設定 TextBox 控制項之 Text 屬性的方法，並示範兩種設定 Text 屬性的安全執行緒方式。
Imports System
Imports System.ComponentModel
Imports System.Threading
Imports System.Windows.Forms
Public Class Form1
Inherits Form
' This delegate enables asynchronous calls for setting
' the text property on a TextBox control.
Delegate Sub SetTextCallback([text] As String)
' This thread is used to demonstrate both thread-safe and
' unsafe ways to call a Windows Forms control.
Private demoThread As Thread = Nothing
' This BackgroundWorker is used to demonstrate the
' preferred way of performing asynchronous operations.
Private WithEvents backgroundWorker1 As BackgroundWorker
Private textBox1 As TextBox
Private WithEvents setTextUnsafeBtn As Button
Private WithEvents setTextSafeBtn As Button
Private WithEvents setTextBackgroundWorkerBtn As Button
Private components As System.ComponentModel.IContainer = Nothing
Public Sub New()
InitializeComponent()
End Sub
Protected Overrides Sub Dispose(disposing As Boolean)
If disposing AndAlso (components IsNot Nothing) Then
components.Dispose()
End If
MyBase.Dispose(disposing)
End Sub
' This event handler creates a thread that calls a
' Windows Forms control in an unsafe way.
Private Sub setTextUnsafeBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextUnsafeBtn.Click
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Me.ThreadProcUnsafe))
Me.demoThread.Start()
End Sub
' This method is executed on the worker thread and makes
' an unsafe call on the TextBox control.
Private Sub ThreadProcUnsafe()
Me.textBox1.Text = "This text was set unsafely."
End Sub
' This event handler creates a thread that calls a
' Windows Forms control in a thread-safe way.
Private Sub setTextSafeBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextSafeBtn.Click
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Me.ThreadProcSafe))
Me.demoThread.Start()
End Sub
' This method is executed on the worker thread and makes
' a thread-safe call on the TextBox control.
Private Sub ThreadProcSafe()
Me.SetText("This text was set safely.")
End Sub
' This method demonstrates a pattern for making thread-safe
' calls on a Windows Forms control.
'
' If the calling thread is different from the thread that
' created the TextBox control, this method creates a
' SetTextCallback and calls itself asynchronously using the
' Invoke method.
'
' If the calling thread is the same as the thread that created
' the TextBox control, the Text property is set directly.
Private Sub SetText(ByVal [text] As String)
' InvokeRequired required compares the thread ID of the
' calling thread to the thread ID of the creating thread.
' If these threads are different, it returns true.
If Me.textBox1.InvokeRequired Then
Dim d As New SetTextCallback(AddressOf SetText)
Me.Invoke(d, New Object() {[text]})
Else
Me.textBox1.Text = [text]
End If
End Sub
' This event handler starts the form's
' BackgroundWorker by calling RunWorkerAsync.
'
' The Text property of the TextBox control is set
' when the BackgroundWorker raises the RunWorkerCompleted
' event.
Private Sub setTextBackgroundWorkerBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextBackgroundWorkerBtn.Click
Me.backgroundWorker1.RunWorkerAsync()
End Sub
' This event handler sets the Text property of the TextBox
' control. It is called on the thread that created the
' TextBox control, so the call is thread-safe.
'
' BackgroundWorker is the preferred way to perform asynchronous
' operations.
Private Sub backgroundWorker1_RunWorkerCompleted( _
ByVal sender As Object, _
ByVal e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted
Me.textBox1.Text = _
"This text was set safely by BackgroundWorker."
End Sub
#Region "Windows Form Designer generated code"
Private Sub InitializeComponent()
Me.textBox1 = New System.Windows.Forms.TextBox()
Me.setTextUnsafeBtn = New System.Windows.Forms.Button()
Me.setTextSafeBtn = New System.Windows.Forms.Button()
Me.setTextBackgroundWorkerBtn = New System.Windows.Forms.Button()
Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
Me.SuspendLayout()
'
' textBox1
'
Me.textBox1.Location = New System.Drawing.Point(12, 12)
Me.textBox1.Name = "textBox1"
Me.textBox1.Size = New System.Drawing.Size(240, 20)
Me.textBox1.TabIndex = 0
'
' setTextUnsafeBtn
'
Me.setTextUnsafeBtn.Location = New System.Drawing.Point(15, 55)
Me.setTextUnsafeBtn.Name = "setTextUnsafeBtn"
Me.setTextUnsafeBtn.TabIndex = 1
Me.setTextUnsafeBtn.Text = "Unsafe Call"
'
' setTextSafeBtn
'
Me.setTextSafeBtn.Location = New System.Drawing.Point(96, 55)
Me.setTextSafeBtn.Name = "setTextSafeBtn"
Me.setTextSafeBtn.TabIndex = 2
Me.setTextSafeBtn.Text = "Safe Call"
'
' setTextBackgroundWorkerBtn
'
Me.setTextBackgroundWorkerBtn.Location = New System.Drawing.Point(177, 55)
Me.setTextBackgroundWorkerBtn.Name = "setTextBackgroundWorkerBtn"
Me.setTextBackgroundWorkerBtn.TabIndex = 3
Me.setTextBackgroundWorkerBtn.Text = "Safe BW Call"
'
' backgroundWorker1
'
'
' Form1
'
Me.ClientSize = New System.Drawing.Size(268, 96)
Me.Controls.Add(setTextBackgroundWorkerBtn)
Me.Controls.Add(setTextSafeBtn)
Me.Controls.Add(setTextUnsafeBtn)
Me.Controls.Add(textBox1)
Me.Name = "Form1"
Me.Text = "Form1"
Me.ResumeLayout(False)
Me.PerformLayout()
End Sub 'InitializeComponent
#End Region
<STAThread()> _
Shared Sub Main()
Application.EnableVisualStyles()
Application.Run(New Form1())
End Sub
End Class
using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;
namespace CrossThreadDemo
{
public class Form1 : Form
{
// This delegate enables asynchronous calls for setting
// the text property on a TextBox control.
delegate void SetTextCallback(string text);
// This thread is used to demonstrate both thread-safe and
// unsafe ways to call a Windows Forms control.
private Thread demoThread = null;
// This BackgroundWorker is used to demonstrate the
// preferred way of performing asynchronous operations.
private BackgroundWorker backgroundWorker1;
private TextBox textBox1;
private Button setTextUnsafeBtn;
private Button setTextSafeBtn;
private Button setTextBackgroundWorkerBtn;
private System.ComponentModel.IContainer components = null;
public Form1()
{
InitializeComponent();
}
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
components.Dispose();
}
base.Dispose(disposing);
}
// This event handler creates a thread that calls a
// Windows Forms control in an unsafe way.
private void setTextUnsafeBtn_Click(
object sender,
EventArgs e)
{
this.demoThread =
new Thread(new ThreadStart(this.ThreadProcUnsafe));
this.demoThread.Start();
}
// This method is executed on the worker thread and makes
// an unsafe call on the TextBox control.
private void ThreadProcUnsafe()
{
this.textBox1.Text = "This text was set unsafely.";
}
// This event handler creates a thread that calls a
// Windows Forms control in a thread-safe way.
private void setTextSafeBtn_Click(
object sender,
EventArgs e)
{
this.demoThread =
new Thread(new ThreadStart(this.ThreadProcSafe));
this.demoThread.Start();
}
// This method is executed on the worker thread and makes
// a thread-safe call on the TextBox control.
private void ThreadProcSafe()
{
this.SetText("This text was set safely.");
}
// This method demonstrates a pattern for making thread-safe
// calls on a Windows Forms control.
//
// If the calling thread is different from the thread that
// created the TextBox control, this method creates a
// SetTextCallback and calls itself asynchronously using the
// Invoke method.
//
// If the calling thread is the same as the thread that created
// the TextBox control, the Text property is set directly.
private void SetText(string text)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.textBox1.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetText);
this.Invoke(d, new object[] { text });
}
else
{
this.textBox1.Text = text;
}
}
// This event handler starts the form's
// BackgroundWorker by calling RunWorkerAsync.
//
// The Text property of the TextBox control is set
// when the BackgroundWorker raises the RunWorkerCompleted
// event.
private void setTextBackgroundWorkerBtn_Click(
object sender,
EventArgs e)
{
this.backgroundWorker1.RunWorkerAsync();
}
// This event handler sets the Text property of the TextBox
// control. It is called on the thread that created the
// TextBox control, so the call is thread-safe.
//
// BackgroundWorker is the preferred way to perform asynchronous
// operations.
private void backgroundWorker1_RunWorkerCompleted(
object sender,
RunWorkerCompletedEventArgs e)
{
this.textBox1.Text =
"This text was set safely by BackgroundWorker.";
}
#region Windows Form Designer generated code
private void InitializeComponent()
{
this.textBox1 = new System.Windows.Forms.TextBox();
this.setTextUnsafeBtn = new System.Windows.Forms.Button();
this.setTextSafeBtn = new System.Windows.Forms.Button();
this.setTextBackgroundWorkerBtn = new System.Windows.Forms.Button();
this.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
this.SuspendLayout();
//
// textBox1
//
this.textBox1.Location = new System.Drawing.Point(12, 12);
this.textBox1.Name = "textBox1";
this.textBox1.Size = new System.Drawing.Size(240, 20);
this.textBox1.TabIndex = 0;
//
// setTextUnsafeBtn
//
this.setTextUnsafeBtn.Location = new System.Drawing.Point(15, 55);
this.setTextUnsafeBtn.Name = "setTextUnsafeBtn";
this.setTextUnsafeBtn.TabIndex = 1;
this.setTextUnsafeBtn.Text = "Unsafe Call";
this.setTextUnsafeBtn.Click += new System.EventHandler(this.setTextUnsafeBtn_Click);
//
// setTextSafeBtn
//
this.setTextSafeBtn.Location = new System.Drawing.Point(96, 55);
this.setTextSafeBtn.Name = "setTextSafeBtn";
this.setTextSafeBtn.TabIndex = 2;
this.setTextSafeBtn.Text = "Safe Call";
this.setTextSafeBtn.Click += new System.EventHandler(this.setTextSafeBtn_Click);
//
// setTextBackgroundWorkerBtn
//
this.setTextBackgroundWorkerBtn.Location = new System.Drawing.Point(177, 55);
this.setTextBackgroundWorkerBtn.Name = "setTextBackgroundWorkerBtn";
this.setTextBackgroundWorkerBtn.TabIndex = 3;
this.setTextBackgroundWorkerBtn.Text = "Safe BW Call";
this.setTextBackgroundWorkerBtn.Click += new System.EventHandler(this.setTextBackgroundWorkerBtn_Click);
//
// backgroundWorker1
//
this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
//
// Form1
//
this.ClientSize = new System.Drawing.Size(268, 96);
this.Controls.Add(this.setTextBackgroundWorkerBtn);
this.Controls.Add(this.setTextSafeBtn);
this.Controls.Add(this.setTextUnsafeBtn);
this.Controls.Add(this.textBox1);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);
this.PerformLayout();
}
#endregion
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.Run(new Form1());
}
}
}
對 Windows Form 控制項進行非安全執行緒的呼叫
若要以非安全執行緒的方式對 Windows Form 控制項進行呼叫，就是直接從背景工作執行緒中呼叫。當您在偵錯應用程式時，偵錯工具會引發 InvalidOperationException，警告對控制項的呼叫並非採用安全執行緒的方式。
' This event handler creates a thread that calls a
' Windows Forms control in an unsafe way.
Private Sub setTextUnsafeBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextUnsafeBtn.Click
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Me.ThreadProcUnsafe))
Me.demoThread.Start()
End Sub
' This method is executed on the worker thread and makes
' an unsafe call on the TextBox control.
Private Sub ThreadProcUnsafe()
Me.textBox1.Text = "This text was set unsafely."
End Sub
// This event handler creates a thread that calls a
// Windows Forms control in an unsafe way.
private void setTextUnsafeBtn_Click(
object sender,
EventArgs e)
{
this.demoThread =
new Thread(new ThreadStart(this.ThreadProcUnsafe));
this.demoThread.Start();
}
// This method is executed on the worker thread and makes
// an unsafe call on the TextBox control.
private void ThreadProcUnsafe()
{
this.textBox1.Text = "This text was set unsafely.";
}
對 Windows Form 控制項進行安全執行緒呼叫
若要對 Windows Form 控制項進行安全執行緒呼叫
查詢控制項的 InvokeRequired 屬性。
如果 InvokeRequired 傳回 true，就使用對控制項進行實際呼叫的委派 (Delegate) 呼叫 Invoke。
如果 InvokeRequired 傳回 false，便直接呼叫控制項。
在下列程式碼範例中，會在稱為 SetText 的公用程式方法中實作此邏輯。名為 SetTextDelegate 的委派型別會封裝 SetText 方法。當 TextBox 控制項的 InvokeRequired 傳回 true 時，SetText 方法會建立 SetTextDelegate 的執行個體，並呼叫表單的 Invoke 方法。這會造成在建立 TextBox 控制項的執行緒上呼叫 SetText 方法，並且在這個執行緒內容中直接設定 Text 屬性。
' This event handler creates a thread that calls a
' Windows Forms control in a thread-safe way.
Private Sub setTextSafeBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextSafeBtn.Click
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Me.ThreadProcSafe))
Me.demoThread.Start()
End Sub
' This method is executed on the worker thread and makes
' a thread-safe call on the TextBox control.
Private Sub ThreadProcSafe()
Me.SetText("This text was set safely.")
End Sub
// This event handler creates a thread that calls a
// Windows Forms control in a thread-safe way.
private void setTextSafeBtn_Click(
object sender,
EventArgs e)
{
this.demoThread =
new Thread(new ThreadStart(this.ThreadProcSafe));
this.demoThread.Start();
}
// This method is executed on the worker thread and makes
// a thread-safe call on the TextBox control.
private void ThreadProcSafe()
{
this.SetText("This text was set safely.");
}
' This method demonstrates a pattern for making thread-safe
' calls on a Windows Forms control.
'
' If the calling thread is different from the thread that
' created the TextBox control, this method creates a
' SetTextCallback and calls itself asynchronously using the
' Invoke method.
'
' If the calling thread is the same as the thread that created
' the TextBox control, the Text property is set directly.
Private Sub SetText(ByVal [text] As String)
' InvokeRequired required compares the thread ID of the
' calling thread to the thread ID of the creating thread.
' If these threads are different, it returns true.
If Me.textBox1.InvokeRequired Then
Dim d As New SetTextCallback(AddressOf SetText)
Me.Invoke(d, New Object() {[text]})
Else
Me.textBox1.Text = [text]
End If
End Sub
// This method demonstrates a pattern for making thread-safe
// calls on a Windows Forms control.
//
// If the calling thread is different from the thread that
// created the TextBox control, this method creates a
// SetTextCallback and calls itself asynchronously using the
// Invoke method.
//
// If the calling thread is the same as the thread that created
// the TextBox control, the Text property is set directly.
private void SetText(string text)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.textBox1.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetText);
this.Invoke(d, new object[] { text });
}
else
{
this.textBox1.Text = text;
}
}
使用 BackgroundWorker 進行安全執行緒呼叫
在應用程式中實作多執行緒處理的慣用方式是使用 BackgroundWorker 元件。BackgroundWorker 元件會使用多執行緒處理的事件驅動 (Event-Driven) 模型。背景工作執行緒會執行您的 DoWork 事件處理常式，而建立控制項的執行緒則會執行 ProgressChanged 和 RunWorkerCompleted 事件處理常式。請小心，不要從 DoWork 事件處理常式呼叫任何控制項。
在下列程式碼範例中，沒有非同步執行的工作，因此沒有 DoWork 事件處理常式實作。TextBox 控制項的 Text 屬性是直接設定於 RunWorkerCompleted 事件處理常式。
' This event handler starts the form's
' BackgroundWorker by calling RunWorkerAsync.
'
' The Text property of the TextBox control is set
' when the BackgroundWorker raises the RunWorkerCompleted
' event.
Private Sub setTextBackgroundWorkerBtn_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) Handles setTextBackgroundWorkerBtn.Click
Me.backgroundWorker1.RunWorkerAsync()
End Sub
' This event handler sets the Text property of the TextBox
' control. It is called on the thread that created the
' TextBox control, so the call is thread-safe.
'
' BackgroundWorker is the preferred way to perform asynchronous
' operations.
Private Sub backgroundWorker1_RunWorkerCompleted( _
ByVal sender As Object, _
ByVal e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted
Me.textBox1.Text = _
"This text was set safely by BackgroundWorker."
End Sub
// This event handler starts the form's
// BackgroundWorker by calling RunWorkerAsync.
//
// The Text property of the TextBox control is set
// when the BackgroundWorker raises the RunWorkerCompleted
// event.
private void setTextBackgroundWorkerBtn_Click(
object sender,
EventArgs e)
{
this.backgroundWorker1.RunWorkerAsync();
}
// This event handler sets the Text property of the TextBox
// control. It is called on the thread that created the
// TextBox control, so the call is thread-safe.
//
// BackgroundWorker is the preferred way to perform asynchronous
// operations.
private void backgroundWorker1_RunWorkerCompleted(
object sender,
RunWorkerCompletedEventArgs e)
{
this.textBox1.Text =
"This text was set safely by BackgroundWorker.";
}
Windows Form 上的 ActiveX 控制項
如果您是在表單上使用 ActiveX 控制項，在偵錯工具下執行時可能會收到跨執行緒 (Cross-Thread) 的 InvalidOperationException。如果發生這種情況，就表示 ActiveX 控制項不支援多執行緒 (Multi-Thread)。如需在 Windows Form 中使用 ActiveX 控制項的詳細資訊，請參閱 Windows Form 和 Unmanaged 應用程式。
如果您使用的是 Visual Studio，則只要停用 Visual Studio 裝載處理序就可以防止發生這種例外狀況。
穩固程式設計
.gif "注意事項") 警告： |
|---|
在使用任何類型的多執行緒處理時，您的程式碼可能會接觸到極嚴重且複雜的錯誤。如需詳細資訊，請在實作使用多執行緒處理的任何方案之前參閱 Managed 執行緒處理的最佳實施方針。 |