Dispatcher.BeginInvoke Method (DispatcherPriority, Delegate, Object)


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

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

Public Function BeginInvoke (
	priority As DispatcherPriority,
	method As Delegate,
	arg As Object
) As DispatcherOperation


Type: System.Windows.Threading.DispatcherPriority

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

Type: System.Delegate

A delegate to a method that takes one argument, which is pushed onto the Dispatcher event queue.

Type: System.Object

The object to pass as an argument to the specified method.

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.

Exception Condition

method is null.


priority is not a valid DispatcherPriority.

Arg can be null if no arguments are needed.

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

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.

First, a delegate is created that accepts one argument, in this case a string.

Private Delegate Sub OneArgDelegate(ByVal arg As String)

Next, BeginInvoke(DispatcherPriority, Delegate, Object) 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 Grid named tomorrowsWeather. The call to BeginInvoke(DispatcherPriority, Delegate, Object) takes three parameters: the priority, which is set to DispatcherPriority.Normal; the callback, which is passed in through an instance of the delegate OneArgDelegate; and a string named weather, which is the argument for the callback.

' Schedule the update function in the UI thread.
tomorrowsWeather.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, New OneArgDelegate(AddressOf UpdateUserInterface), weather)

.NET Framework
Available since 3.0
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