Silverlight 4 Clipboard Security Exception "access is not allowed" ?

Question

Hey,

I'm new in Silverlight and i am doing some tests. With my current test I try to display in real time the current Clipboard content. But there is a weird behaviors with this code :

namespace SilverlightTest
{
public partial class MainPage : UserControl
{
    private Timer _timer;

    public MainPage()
    {
        InitializeComponent();
        var dispatcher_timer = new DispatcherTimer {Interval = new TimeSpan(0, 0, 0, 5)};
        dispatcher_timer.Tick += new EventHandler(timer_Callback);
        dispatcher_timer.Start();
    }

    private void timer_Callback(object state, EventArgs eventArgs)
    {
        current_clip_board.Content = Clipboard.GetText();
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        current_clip_board.Content = Clipboard.GetText();
    }
}
}

The button Event and the timer Event are suppose to do exactly the same action. But it doesn't ! The Button works fine and set the clipboard text into the label but the timer throw an exception :

Clipboard access is not allowed

The question is : why ? :)

Thanks.

PS : I would bet on a thread problem :p

Answer 1

Have you tried this:

private void timer_Callback(object state, EventArgs eventArgs) 
{
    Dispatcher.Invoke(new System.Threading.ThreadStart(delegate()
    {
        current_clip_board.Content = Clipboard.GetText(); 
    }
} 

edit

After a quick search, it appears that Clipboard is only available in response to a user action see here and here.

In partial trust (the default mode for browser-hosted Silverlight-based applications), Silverlight also restricts clipboard access to its two key APIs GetText and SetText. These APIs can only be invoked from within a context that is determined by the Silverlight runtime to be in response to a user-initiated action. For example, clipboard access is valid from within a handler for a Click or KeyDown event. In contrast, clipboard access is not valid from a handler for Loaded or from a constructor, and access attempts throw exceptions.

Answer 2

Clipboard access, in a partial trust (in-browser) Silverlight application (the scenario you're likely referring to above), is restricted. The GetText property is accessible only in scenarios that the Silverlight runtime determines were initiated by the user. Your example is perfect -- by a button click for example. A dispatch timer however is not user initiated, so the property throws an exception (this is especially important within the context of a in-browser application, which could be a big security hole if you could create a Silverlight application that just ran silently in the browser, watching the user's clipboard updates without their knowledge).

See this clipboard documentation for more details.

Answer 3

If your only option is to use a timer, then don't do it at all. The clipboad is a shared resource, and you're going to raise "cannot open clipboard" errors in other programs as they try to access the clipboard. i.e. user copies something from WinWord, WinWord tries to open the clipboard, but can't, because you've got it locked while you're examining it.

Answer 4

Just trigger Clipboard.ContainsText() instead of Text. The method ContainsText is allowed!