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Control.Invoke, méthode (Delegate)

 

Exécute le délégué spécifié sur le thread qui détient le handle de fenêtre sous-jacent du contrôle.

Espace de noms:   System.Windows.Forms
Assembly:  System.Windows.Forms (dans System.Windows.Forms.dll)

public object Invoke(
	Delegate method
)

Paramètres

method

Délégué qui contient une méthode à appeler dans le contexte du thread du contrôle.

Valeur de retour

Type: System.Object

Valeur de retour du délégué appelé ou null si le délégué n'a pas de valeur de retour.

Delegates are similar to function pointers in C or C++ languages.Delegates encapsulate a reference to a method inside a delegate object.The delegate object can then be passed to code that calls the referenced method, and the method to be invoked can be unknown at compile time.Unlike function pointers in C or C++, delegates are object-oriented, type-safe, and more secure.

The Invoke method searches up the control's parent chain until it finds a control or form that has a window handle if the current control's underlying window handle does not exist yet.If no appropriate handle can be found, the Invoke method will throw an exception.Exceptions that are raised during the call will be propagated back to the caller.

System_CAPS_noteRemarque

In addition to the InvokeRequired property, there are four methods on a control that are thread safe: Invoke, BeginInvoke, EndInvoke, and CreateGraphics if the handle for the control has already been created.Calling CreateGraphics before the control's handle has been created on a background thread can cause illegal cross thread calls.For all other method calls, you should use one of the invoke methods to marshal the call to the control's thread.

The delegate can be an instance of EventHandler, in which case the sender parameter will contain this control, and the event parameter will contain EventArgs.Empty.The delegate can also be an instance of MethodInvoker, or any other delegate that takes a void parameter list.A call to an EventHandler or MethodInvoker delegate will be faster than a call to another type of delegate.

System_CAPS_noteRemarque

An exception might be thrown if the thread that should process the message is no longer active.

The following code example shows controls that contain a delegate.The delegate encapsulates a method that adds items to the list box, and this method is executed on the thread that owns the underlying handle of the form.When the user clicks on the button, Invoke runs the delegate.

/*
The following example demonstrates the 'Invoke(Delegate)' method of 'Control class.
A 'ListBox' and a 'Button' control are added to a form, containing a delegate
which encapsulates a method that adds items to the listbox.This function is executed
on the thread that owns the underlying handle of the form. When user clicks on button
the above delegate is executed using 'Invoke' method.


*/

using System;
using System.Drawing;
using System.Windows.Forms;
using System.Threading;

   public class MyFormControl : Form
   {
      public delegate void AddListItem();
      public AddListItem myDelegate;
      private Button myButton;
      private Thread myThread;
      private ListBox myListBox;
      public MyFormControl()
      {
         myButton = new Button();
         myListBox = new ListBox();
         myButton.Location = new Point(72, 160);
         myButton.Size = new Size(152, 32);
         myButton.TabIndex = 1;
         myButton.Text = "Add items in list box";
         myButton.Click += new EventHandler(Button_Click);
         myListBox.Location = new Point(48, 32);
         myListBox.Name = "myListBox";
         myListBox.Size = new Size(200, 95);
         myListBox.TabIndex = 2;
         ClientSize = new Size(292, 273);
         Controls.AddRange(new Control[] {myListBox,myButton});
         Text = " 'Control_Invoke' example";
         myDelegate = new AddListItem(AddListItemMethod);
      }
      static void Main()
      {
         MyFormControl myForm = new MyFormControl();
         myForm.ShowDialog();
      }
      public void AddListItemMethod()
      {
         String myItem;
         for(int i=1;i<6;i++)
         {
            myItem = "MyListItem" + i.ToString();
            myListBox.Items.Add(myItem);
            myListBox.Update();
            Thread.Sleep(300);
         }
      }
      private void Button_Click(object sender, EventArgs e)
      {
         myThread = new Thread(new ThreadStart(ThreadFunction));
         myThread.Start();
      }
      private void ThreadFunction()
      {
         MyThreadClass myThreadClassObject  = new MyThreadClass(this);
         myThreadClassObject.Run();
      }
   }

// The following code assumes a 'ListBox' and a 'Button' control are added to a form, 
// containing a delegate which encapsulates a method that adds items to the listbox.

   public class MyThreadClass
   {
      MyFormControl myFormControl1;
      public MyThreadClass(MyFormControl myForm)
      {
         myFormControl1 = myForm;
      }

      public void Run()
      {
         // Execute the specified delegate on the thread that owns
         // 'myFormControl1' control's underlying window handle.
         myFormControl1.Invoke(myFormControl1.myDelegate);
      }
   }

.NET Framework
Disponible depuis 1.1
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