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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
)
public Object Invoke (
	Delegate method
)
public function Invoke (
	method : Delegate
) : Object
Not applicable.

Parameters

method

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

Return Value

The return value from the delegate being invoked, or a null reference (Nothing in Visual Basic) 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.

NoteNote:

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.

NoteNote:

An exception might be thrown if the thread that should process the message is no longer active.

Windows Mobile 2003 for Pocket PC, Windows Mobile 2003 for Smartphone, Windows CE Platform Note: In .NET Compact Framework applications, the delegate must be an instance of EventHandler. For an example, see Delegates Sample.

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 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);
      }
   }

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0
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