SIlverlight使用WebClent上载图片出现的安全有关问题如何解决

 //开始上传
        private void OKButton_Click(object sender, RoutedEventArgs e)
        {
            if (string.IsNullOrEmpty(this.txbaddress.Text)) {
                this.txbMessage.Text = "请输入地址！";
            }
            else if (new Regex(@"http(s)?://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?").Match(this.txbaddress.Text).Success == false)
            {
                this.txbMessage.Text = "图片地址格式必须是(http://....jpg)！";
            }
            else {
                this.txbMessage.Text = "正在下载图片，请稍后...";
                this.txbaddress.IsEnabled = false;
                WebClient mywebclient = new WebClient();
                string url = this.txbaddress.Text.Trim();
                Uri endpoint = new Uri(url, UriKind.Absolute);
                mywebclient.OpenReadCompleted += new OpenReadCompletedEventHandler(mywebclient_OpenReadCompleted);
                mywebclient.DownloadProgressChanged += new DownloadProgressChangedEventHandler(mywebclient_DownloadProgressChanged);
                mywebclient.OpenReadAsync(endpoint);
            }
        }

        void mywebclient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
            if (e.Error != null)//这里一直报安全错误 谁遇到过
            {
                this.txbMessage.Text = "网络故障，下载失败！";
                this.txbaddress.IsEnabled = true;
                return;
            }
            if (e.Cancelled != true)
            {
                Stream clientStream = e.UserState as Stream;