A first chance exception of type 'System.InvalidCastException' occurred in PhoneApp4.dll

Question

Hi,

I was doing json parsing..But I was coming across an exception

 A first chance exception of type 'System.InvalidCastException' occurred in PhoneApp4.dll.

  public MainPage()

        {
            InitializeComponent();
            if (!System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable())
            {
                MessageBox.Show("No network connection available!");
                return;
            }
                     WebClient downloader = new WebClient();
            Uri uri = new Uri("http://gdata.youtube.com/feeds/api/playlists/PLhhKzfJ3WyA3INKPYXqR94iCIDNwBKdNr/?v=2&alt=json&start-index=1&max-results=7&feature=plcp");
            downloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Downloaded);
            downloader.DownloadStringAsync(uri);
        }

        void Downloaded(object sender, DownloadStringCompletedEventArgs e)
  {

          if (e.Result == null || e.Error != null)
            {
                MessageBox.Show("There was an error downloading the file!");
            }
            else
          {
            string s = Convert.ToString(e.Result);
                    //IEnumerable
           DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(IEnumerable<Item>));
              MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(s));
                 var list = (IEnumerable<Item>)serializer.ReadObject(ms);

   List<Item> datasource = new List<Item>();

                 

                    foreach (Item item in list)

                    {

                       datasource.Add(new Item{title =item.title});    

                                    listBox1.ItemsSource = datasource;
          }
         }
    }
}

public class Item

 {

public string title { get; set; }

    }

}

Exception was raising in this line   var list = (IEnumerable<Item>)serializer.ReadObject(ms);

Can anybody please help me with this..Many Thanks....

Answer 1

Your Item class does not appear to match the content of the JSON returned by that URL.

What do you get if you don't cast the result of ReadObject?

---

Eric Fleck, Windows Store and Windows Phone Developer Support. If you would like to provide feedback or suggestions for future improvements to the Windows Phone SDK please go to http://wpdev.uservoice.com/ where you can post your suggestions and/or cast your votes for existing suggestions.

Answer 2

Thank you.That means url is not having Item class property...

Answer 3

JSON returned by the service is not an array , thereby an exception is occurring at the specific line( considering IEnumerable being one), so you need to cast it as object (basically the json returned is a tree of objects which needs to be parsed)

I tried parsing it with a much simpler json parser SimpleJson , here's a sample snippet :

void Downloaded(object sender, DownloadStringCompletedEventArgs e)
        {

            if (e.Result == null || e.Error != null)
            {
                MessageBox.Show("There was an error!");
            }
            else
            {
               
                IDictionary<string, object> _GenericObj = null;
                IDictionary<string, object> _GenericObj1 = null;
                IDictionary<string, object> _GenericObj2 = null;

                _GenericObj = (IDictionary<string, object>)(SimpleJson.DeserializeObject(e.Result)); // all objects parsed
                _GenericObj1 = (IDictionary<string, object>)_GenericObj["feed"]; //root object 'feed' parsed
                _GenericObj2 =  (IDictionary<string, object>)_GenericObj1["id"]; //'id' object parsed from 'feed' object

// likewise you can parse the whole object tree(like done for 'id' , considering all objects as branches of 'feed' object)

---

http://developer.nokia.com/community/wiki/Using_Crypto%2B%2B_library_with_Windows_Phone_8

Answer 4

Hii.

I was getting error in Simplejson..

The name 'Simplejson' does not exist in the current context.

I executed (Install-Package SimpleJson) in Package Manager Console.I was also getting below message

'SimpleJson 0.38.0' already installed.
PhoneApp4 already has a reference to 'SimpleJson 0.38.0'.

Even though i was comming across the same error.Please help me with this.

ManyThanks...

Answer 5

Is SimpleJson.cs class present in project, as soon as SimpleJson is installed from Nuget, the class gets included in the project itself.

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http://developer.nokia.com/community/wiki/Using_Crypto%2B%2B_library_with_Windows_Phone_8

Answer 6

Ya, it was present eventhough same error was raising...

 The name 'Simplejson' does not exist in the current context..

Answer 7

That is bit strange,  can you try removing the SimpleJson and then add it again. Go to tools->Nuget Package Manager -> Manage Nuget Package for solution -> Installed packages - remove 'SimpleJson' and now again from package manager console  add it.

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http://developer.nokia.com/community/wiki/Using_Crypto%2B%2B_library_with_Windows_Phone_8

Answer 8

Ya thankyou I removed and added it again it was working fine.

How can i bind them to listbox.

I tried like this 

 _GenericObj1 = (IDictionary<string, object>)_GenericObj["feed"]; //root object 'feed' parsed
 _GenericObj2 = (IDictionary<string, object>)_GenericObj1["title"];
                listBox1.ItemsSource = _GenericObj2;

An exception was rising.

Thankyou..

Answer 9

Well you can take a observablecollection or simply a List to keep storing the id's or title to it and data bind that List in xaml page then.

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http://developer.nokia.com/community/wiki/Using_Crypto%2B%2B_library_with_Windows_Phone_8