Media video are not playing when recevied source?

Question

 void NavigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
            try
            {
                DataModel.YoutubeVideo value = e.NavigationParameter as DataModel.YoutubeVideo;
                if(!string.IsNullOrWhiteSpace(value.id))
                {
                    txttitle.Text = value.title;
                    Mediaplayer.Source = value.youtubelink;
                }
            }
            catch
            {
                throw new NotImplementedException();
            }
           
        }

<StackPanel>
                            <MediaElement Name="Mediaplayer" Width="650" Height="384" MediaFailed="Mediaplayer_MediaFailed" MediaOpened="Mediaplayer_MediaOpened" Margin="1,0,0,0" AutoPlay="True"></MediaElement>
                        </StackPanel>

Source :http://www.youtube.com/watch?v=_OBlgSz8sSM&feature=youtube_gdata

string uri youtubelink{get;set;}