Dispatcher.Invoke Method (DispatcherPriority, Delegate) (System.Windows.Threading)

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Dispatcher.Invoke Method (DispatcherPriority, Delegate)
Executes the specified delegate synchronously at the specified priority on the thread on which the Dispatcher is associated with.

Namespace: System.Windows.Threading
Assembly: WindowsBase (in windowsbase.dll)

Syntax
Visual Basic (Declaration)
Public Function Invoke ( _
	priority As DispatcherPriority, _
	method As Delegate _
) As Object
Visual Basic (Usage)
Dim instance As Dispatcher
Dim priority As DispatcherPriority
Dim method As Delegate
Dim returnValue As Object

returnValue = instance.Invoke(priority, method)
C#
public Object Invoke (
	DispatcherPriority priority,
	Delegate method
)
C++
public:
Object^ Invoke (
	DispatcherPriority priority, 
	Delegate^ method
)
J#
public Object Invoke (
	DispatcherPriority priority, 
	Delegate method
)
JScript
public function Invoke (
	priority : DispatcherPriority, 
	method : Delegate
) : Object
XAML
You cannot use methods in XAML.

Parameters

priority

The priority, relative to the other pending operations in the Dispatcher event queue, the specified method is invoked.

method

A delegate to a method that takes no arguments, which is pushed onto the Dispatcher event queue.

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.
Exceptions
Exception type Condition

ArgumentException

priority is equal to Inactive.

InvalidEnumArgumentException

priority is not a valid priority.

ArgumentNullException

method is a null reference (Nothing in Visual Basic).

Remarks

In WPF, only the thread which created a DispatcherObject may access that object. For example, a background thread which is spun off from the main UI thread cannot update the contents of a Button which was created on the UI thread. In order for the background thread to access the Content property of the Button, the background thread must delegate the work to the Dispatcher associated with the UI thread. This is accomplished by using either Invoke or BeginInvoke. Invoke is synchronous and BeginInvoke is asynchronous. The operation is added to the event queue of the Dispatcher at the specified DispatcherPriority.

Invoke is a synchronous operation; therefore, control will not return to the calling object until after the callback returns.

Example

The following example places a delegate onto a Dispatcher at Normal using Invoke. For the full source of the sample, see Disable Command Source Via Dispatcher Timer Sample.

C#
// Places the delegate onto the UI Thread's Dispatcher
private void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    // Place delegate on the Dispatcher.
    this.Dispatcher.Invoke(DispatcherPriority.Normal,
        new TimerDispatcherDelegate(TimerWorkItem));
}
Platforms

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.

Version Information

.NET Framework

Supported in: 3.0
See Also