Dispatcher.BeginInvoke Method (DispatcherPriority, Delegate)

Executes the specified delegate asynchronously at the specified priority on the thread the Dispatcher is associated with.

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

'Declaration
<BrowsableAttribute(False)> _
Public Function BeginInvoke ( _
	priority As DispatcherPriority, _
	method As Delegate _
) As DispatcherOperation

Parameters

priority
Type: System.Windows.Threading.DispatcherPriority

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

method
Type: System.Delegate

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

Return Value

Type: System.Windows.Threading.DispatcherOperation
An object, which is returned immediately after BeginInvoke is called, that can be used to interact with the delegate as it is pending execution in the event queue.

ExceptionCondition
ArgumentNullException

method is Nothing.

InvalidEnumArgumentException

priority is not a valid DispatcherPriority.

If multiple BeginInvoke calls are made at the same DispatcherPriority, they will be executed in the order the calls were made.

BeginInvoke returns a DispatcherOperation object that can be used to interact with the delegate when the delegate is in the event queue.

The DispatcherOperation object returned by BeginInvoke can be used in several ways to interact with the specified delegate, such as:

  • Changing the DispatcherPriority of the delegate as it is pending execution in the event queue.

  • Removing the delegate from the event queue.

  • Waiting for the delegate to return.

  • Obtaining the value that the delegate returns after it is executed.

BeginInvoke is asynchronous; therefore, control returns immediately to the calling object after it is called.

In WPF, only the thread that created a DispatcherObject may access that object. For example, a background thread that is spun off from the main UI thread cannot update the contents of a Button that 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.

If BeginInvoke is called on a Dispatcher that has shut down, the status property of the returned DispatcherOperation is set to Aborted.

The following example shows how to place an operation onto a Dispatcher. For the full source code of this example, see Single-Threaded Application with Long-Running Calculation Sample.

First, a delegate is created that accepts no arguments.

Public Delegate Sub NextPrimeDelegate()

Next, BeginInvoke(DispatcherPriority, Delegate) is called. Because every DispatcherObject has a property that returns the Dispatcher it is associated with, the desired Dispatcher is obtained by querying the DispatcherObject, in this case a Button named startStopButton. The call to BeginInvoke(DispatcherPriority, Delegate) takes two parameters: the priority, which is set to DispatcherPriority.Normal, and the callback, which is passed in through an instance of the delegate NextPrimeDelegate.

startStopButton.Dispatcher.BeginInvoke(DispatcherPriority.Normal, New NextPrimeDelegate(AddressOf CheckNextNumber))

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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