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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)

'Declaration
Public Function Invoke ( _
	priority As DispatcherPriority, _
	method As Delegate _
) As Object
'Usage
Dim instance As Dispatcher 
Dim priority As DispatcherPriority 
Dim method As [Delegate]
Dim returnValue As Object 

returnValue = instance.Invoke(priority, _
	method)
You cannot use methods in XAML.

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

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

Return Value

Type: System.Object
The return value from the delegate being invoked or Nothing if the delegate has no return value.

ExceptionCondition
ArgumentException

priority is equal to Inactive.

InvalidEnumArgumentException

priority is not a valid priority.

ArgumentNullException

method is Nothing.

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.

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

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

// 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));
}

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

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.

.NET Framework

Supported in: 3.5, 3.0
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