Control.Invoke Method (Delegate)

Executes the specified delegate on the thread that owns the control's underlying window handle.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

public Object Invoke(
	Delegate method


Type: System.Delegate

A delegate that contains a method to be called in the control's thread context.

Return Value

Type: System.Object
The return value from the delegate being invoked, or null if the delegate has no return value.

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.



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.



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();
      public void AddListItemMethod()
         String myItem;
            myItem = "MyListItem" + i.ToString();
      private void Button_Click(object sender, EventArgs e)
         myThread = new Thread(new ThreadStart(ThreadFunction));
      private void ThreadFunction()
         MyThreadClass myThreadClassObject  = new MyThreadClass(this);

// 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.

.NET Framework


.NET Framework Client Profile

Supported in: 4, 3.5 SP1



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