Control.Invoke Method (Delegate, Object)
Executes the specified delegate, on the thread that owns the control's underlying window handle, with the specified list of arguments.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
- Type: System.Delegate
A delegate to a method that takes parameters of the same number and type that are contained in the args parameter.
- Type: System.Object
An array of objects to pass as arguments to the specified method. This parameter can be a null reference (Nothing in Visual Basic) if the method takes no arguments.
Return ValueType: System.Object
An Object that contains 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.
If the control's handle does not exist yet, this method searches up the control's parent chain until it finds a control or form that does have a window handle. If no appropriate handle can be found, this method throws an exception. Exceptions that are raised during the call will be propagated back to the caller.
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.
An exception might be thrown if the thread that should process the message is no longer active.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, using the specified arguments. When the user clicks on the button, Invoke runs the delegate.
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.