使用多線程提高 Windows 表單應用程式的效能時，必須注意以安全執行緒方式調用控制項

 使用多線程提高 Windows 表單應用程式的效能時，必須注意以安全執行緒方式調用控制項。

樣本
訪問 Windows 表單控制項本質上不是安全執行緒的。如果有兩個或多個線程操作某一控制項的狀態，則可能會迫使該控制項進入一種不一致的狀態。還可能出現其他與線程相關的 bug，包括爭用情況和死結。確保以安全執行緒方式訪問控制項非常重要。

.NET Framework 有助於在以非安全執行緒方式訪問控制項時檢測到這一問題。在調試器中運行應用程式時，如果建立某控制項的線程之外的其他線程試圖調用該控制項，則調試器會引發一個 InvalidOperationException，並提示訊息：“從不是建立控制項 control name 的線程訪問它。”

此異常在調試期間和運行時的某些情況下可靠地發生。強烈建議您在顯示此錯誤資訊時修複此問題。在調試以 .NET Framework 2.0 版之前的 .NET Framework 編寫的應用程式時，可能會出現此異常。

注意
可以通過將 CheckForIllegalCrossThreadCalls 屬性的值設定為 false 來禁用此異常。這會使控制項以與在 Visual Studio 2003 下相同的方式運行。
 

下面的程式碼範例示範如何從輔助線程以安全執行緒方式和非安全執行緒方式調用 Windows 表單控制項。它示範一種以非安全執行緒方式設定 TextBox 控制項的 Text 屬性的方法，還示範兩種以安全執行緒方式設定 Text 屬性的方法。

Visual Basic  複製代碼
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 Not (components Is 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