Dispatcher.VerifyAccess Method ()


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Determines whether the calling thread has access to this Dispatcher.

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

Public Sub VerifyAccess

Exception Condition

The calling thread does not have access to this Dispatcher.

Only the thread the Dispatcher is created on may access the Dispatcher.

This method is public; therefore, any thread can check to see whether it has access to the Dispatcher.

The difference between CheckAccess and VerifyAccess is CheckAccess returns a Boolean if the calling thread does not have access to the Dispatcher and VerifyAccess throws an exception.

The following example uses VerifyAccess to determine whether a thread has access to the thread that a Button was created on. The method takes an object as an argument, which is cast to a Button. The VerifyAccess method on the Dispatcher of the Button is called to verify access to the thread.

If the calling thread has access to the Dispatcher, the Button is updated by just accessing the members of the Button.

If the calling thread does not have access, an InvalidOperationException is thrown. This example catches the exception and pushes a delegate, which accepts a Button as an argument, onto the Dispatcher of the Button. This Dispatcher will do the work of updating the Button.

' Uses the Dispatcher.VerifyAccess method to determine if 
' the calling thread has access to the thread the UI object is on.
Private Sub TryToUpdateButtonVerifyAccess(ByVal uiObject As Object)
	Dim theButton As Button = TryCast(uiObject, Button)

	If theButton IsNot Nothing Then
                  ' Check if this thread has access to this object.

                  ' The thread has access to the object, so update the UI.

                  ' Cannot access objects on the thread.
              Catch e As InvalidOperationException
                  ' Exception Error Message.
                  MessageBox.Show("Exception ToString: " & vbLf & vbLf & e.ToString(), "Execption Caught! Thrown During AccessVerify().")

                  MessageBox.Show("Pushing job onto UI Thread Dispatcher")

                  ' Placing job onto the Dispatcher of the UI Thread.
                  theButton.Dispatcher.BeginInvoke(DispatcherPriority.Normal, New UpdateUIDelegate(AddressOf UpdateButtonUI), theButton)
              End Try
	End If
      End Sub

.NET Framework
Available since 3.0
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