Get WebView's content height - Windows Phone 8.1

问题

I try to get the height of the actual HTML content inside my WebView in order to set the height according to the content.

this is my script, but I get an empty string when the scripit is being invoked.

      private async Task LoadHTMLContent(ItemViewModel itemVm)
    {
        var htmlScript = "<script>function getDocHeight() { " +
                      "return document.getElementById('pageWrapper').offsetHeight; } </script>";

        var htmlConcat = string.Format("<html><head>{0}</head>" +
                                        "<body style=\"margin:0;padding:0;\" " +
                                        ">" +
                                        "<div id=\"pageWrapper\" style=\"width:100%;" +
                                        "\">{1}</div></body></html>", htmlScript, itemVm.Model.Content);

        webView.NavigationCompleted += webView_NavigationCompleted;
        webView.NavigateToString(htmlConcat);


    }

    async void webView_NavigationCompleted(WebView sender, WebViewNavigationCompletedEventArgs args)
    {
        string pageContentHeight = await webView.InvokeScriptAsync("getDocHeight", null);

    }