GetData from server Windows 8

I have next function:

private void getAllData()
    {
        HttpWebRequest request = HttpWebRequest.CreateHttp("http://webservice.com/wfwe");
        request.BeginGetResponse(new AsyncCallback(GetResponsetStreamCallback), request);
    }
        void GetResponsetStreamCallback(IAsyncResult callbackResult)
    {
        HttpWebRequest request = (HttpWebRequest)callbackResult.AsyncState;
        HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(callbackResult);
        using (StreamReader httpWebStreamReader = new StreamReader(response.GetResponseStream()))
        {
            string result = httpWebStreamReader.ReadToEnd();
            GetApplications(result);
        }
    }

And i fill in stack panels:

private void GetApplications(string result)
    {
        var ApplicationsList = JsonConvert.DeserializeObject<List<Applications>>(result);
        foreach (Applications A in ApplicationsList)
        {
            foreach (ApplicationRelation SCA in A.ApplicationRelations)
            {
                if (SCA.ApplicationSubcategory != null)
                {
                    #region Fill Customer Research Stack
                    if (SCA.ApplicationSubcategory.subcategoryName == "Customer Research")
                    {
                        if (TestStack.Children.Count == 0)
                        {
                            ApplicationTile AT = FillDataForApplicationTile(SCA);                                
                            AT.Margin = new Thickness(5, 0, 5, 0);
                            TestStack.Children.Add(AT);
                        }
                    }
                    #endregion
                }
            }
        }
    }

And code fails at:

if (TestStack.Children.Count == 0)

Error: The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

How can i rewrite my Request from void to string, so i could do something like this:

GetApplications(await getAllData())

EDIT 2 for dcastro:

EDIT 3:

Thanks it works, but i was looking for something like this:

//Modified your code:

GetApplications(getAllData2().Result);

private async Task<string> getAllData2()
    {
       string uri = "http://webservice.com/wfe";
       var client = new HttpClient();
       HttpResponseMessage response = await client.GetAsync(uri);
       var result = await response.Content.ReadAsStringAsync();
       return result.ToString();
    }

But somehow my construction doesn't enter GetApplication function...

Instead of using AsyncCallback (which I'm pretty sure is running GetResponsetStreamCallback in a non-UI thread), try fetching your data like this:

private async void getAllData()
   string uri = "http://webservice.com/wfwe";
   var client = new HttpClient();

   HttpResponseMessage response = await client.GetAsync(uri);

   string body = await response.Content.ReadAsStringAsync();

   GetApplications(body);
}

This will call your webservice asynchronously (at line await Client.sendMessageAsync(msg);), and return to the original UI thread when the response is received. This way, you can update UI elements, like your TestStack.

Edit fixed bug