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DependencyObject.CheckAccess Method

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Determines whether the calling thread has access to this object.

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

Syntax

'Declaration
<EditorBrowsableAttribute(EditorBrowsableState.Never)> _
Public Function CheckAccess As Boolean

[EditorBrowsableAttribute(EditorBrowsableState.Never)]
public bool CheckAccess()

Return Value

Type: System.Boolean
true if the calling thread has access to this object; otherwise, false.

Remarks

CheckAccess is a utility method that determines whether a threading context that has obtained a DependencyObject is the same thread that the DependencyObject was created on.

  • If it is the same thread, CheckAccess returns true. You can perform actions on that DependencyObject. This might include setting properties, calling methods and so on.

  • If the current threading context is not the creating thread, CheckAccess returns false. Nearly all attempts to perform actions that attempt to change values on the DependencyObject will raise exceptions in this circumstance. However, you can retrieve the Dispatcher value from that DependencyObject, and use the Dispatcher API to invoke the action. For more information, see Dispatcher and Managed Threading Overview.

Any thread can check to see if it has access to a particular DependencyObject.

If CheckAccess returns false, this typically means that you are attempting to access a DependencyObject on the main UI thread, from a threading context that is not the main UI thread.

BackgroundWorker is a possible option for delegating threads to perform actions that are not using the main UI thread.

Examples

The following code example demonstrates how to use this method to check whether an action can be performed against a DependencyObject on the UI thread, and uses BeginInvoke in cases where CheckAccess returns false.

Private Delegate Sub AddTextDelegate(ByVal p As Panel, ByVal text As String)

Private Sub AddText(ByVal p As Panel, ByVal text As String)
    p.Children.Clear()
    Dim t As New TextBlock
    t.Text = text
    p.Children.Add(t)
End Sub

Private Sub TestBeginInvokeWithParameters(ByVal p As Panel)
    If p.Dispatcher.CheckAccess() _
        Then AddText(p, "Added directly.") _
        Else p.Dispatcher.BeginInvoke(New AddTextDelegate( _
            AddressOf AddText), p, "Added by Dispatcher.")
End Sub

private delegate void AddTextDelegate(Panel p, String text);

private void AddText(Panel p, String text)
{
    p.Children.Clear();
    p.Children.Add(new TextBlock { Text = text });
}

private void TestBeginInvokeWithParameters(Panel p)
{
    if (p.Dispatcher.CheckAccess()) AddText(p, "Added directly.");
    else p.Dispatcher.BeginInvoke(
        new AddTextDelegate(AddText), p, "Added by Dispatcher.");
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.