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

Invoke Method (DispatcherPriority, Delegate)

.NET Framework Class Library

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)

public Object Invoke(
    DispatcherPriority priority,
    Delegate method
)

Visual C++

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: 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 nullNothingnullptra null reference (Nothing in Visual Basic) if the delegate has no return value.

Exceptions

Exception	Condition
ArgumentException	priority is equal to Inactive.
InvalidEnumArgumentException	priority is not a valid priority.
ArgumentNullException	method is nullNothingnullptra null reference (Nothing in Visual Basic).

Remarks

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.

Examples

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));
}

Platforms

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.

Version Information

.NET Framework

Supported in: 3.5, 3.0

Reference

Dispatcher Class

Dispatcher Members

Invoke Overload

System.Windows.Threading Namespace

Other Resources

Threading Model