JSON Object Casting Error with Facebook C# SDK for ASP.NET 3.5

Question

I am using the Facebook C# SDK using ASP.NET 3.5 and I am getting a weird error message for one of the calls from a sample code base provided here: http://www.thepcwizard.in/2013/07/sample-app-using-facebook-c-sharp-sdk.html. This example is an excellent walk-through of the basics using the Facebook C# SDK for FB login code but I have this one issue that I can't seem to figure out.

The error message is below:

Server Error in '/' Application.

Unable to cast object of type 'Facebook.JsonArray' to type 'System.Collections.Generic.IDictionary`2[System.String,System.Object]'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidCastException: Unable to cast object of type 'Facebook.JsonArray' to type 'System.Collections.Generic.IDictionary`2[System.String,System.Object]'.

Source Error: 

Line 271:        var athletes = (IDictionary<string, object>)me["favorite_athletes"];

Line 272:

Line 273:        foreach (var athlete in (JsonArray)athletes)

I have compiled the FB C# SDK version 3.5 as instructed and mostly everything is working fine. I had to replace the dynamic types to var types and cast them to IDictionary collections because ASP.NET 3.5 does not support dynamic types. All good. Everything works great including the friends list generation and display and all of my personal data being called but the favorite athletes call is causing the error (code is below). I think the issue lies in that fact that there is a "data" variable called for the friends lists and this "data" variable is not called for the athletes list. This is the only difference in how the JSON object is returned from what I can see. The code sample seems to recognize this by removing the "data" variable when the data is retrieved but still no luck on the error.

The favorite athletes JSON data as it comes back from Facebook is below:

{
  "favorite_athletes": [
    {
      "id": "14185406833", 
      "name": "Serena Williams"
    }, 
    {
      "id": "61801828075", 
      "name": "Venus Williams"
    }, 
    {
      "id": "386180624733248", 
      "name": "Robert Griffin III"
    }, 
    {
      "id": "164825930120", 
      "name": "Tiger Woods"
    }, 
    {
      "id": "363865183112", 
      "name": "Floyd Mayweather"
    }, 
    {
      "id": "142603419139653", 
      "name": "Michael Vick"
    }, 
    {
      "id": "64637653943", 
      "name": "LeBron James"
    }
  ], 
  "id": "8723497347"
}

The friends list data as it is returned from Facebook is below (fake user info for the purposes of this question because I do have far more than 4 friends :) ):

{
  "id": "8723497347", 
  "friends": {
    "data": [
      {
        "name": "Friend 1", 
        "id": "1234567"
      }, 
      {
        "name": "Friend 2", 
        "id": "567965"
      }, 
      {
        "name": "Friend 3", 
        "id": "9847634"
      }, 
      {
        "name": "Friend 4", 
        "id": "100005000106091"
      }
    ], 
    "paging": {
      "next": "https://graph.facebook.com/8723497347/friends?limit=5000&offset=5000&__after_id=100005000106091"
    }
  }
}

The code block that is throwing the error is below:

private void GetUserData(string accessToken)
{
    var fb = new FacebookClient(accessToken);

    var me = (IDictionary<string, object>)fb.Get("me?fields=friends,name,email,favorite_athletes");

    string id = (string)me["id"]; // Store in database
    string email = (string)me["email"]; // Store in database
    string FBName = (string)me["name"]; // Store in database            

    NameText.Visible = true;
    NameText.Text = FBName;

    ViewState["FBName"] = FBName; // Storing User's Name in ViewState

    var friends = (IDictionary<string, object>)me["friends"];

    foreach (var friend in (JsonArray)friends["data"])
    {
        ListItem item = new ListItem((string)(((JsonObject)friend)["name"]), (string)(((JsonObject)friend)["id"]));
        FriendList.Items.Add(item);
    }

    var athletes = (IDictionary<string, object>)me["favorite_athletes"];

    foreach (var athlete in (JsonArray)athletes)
    {
        SportsPersonList.Items.Add((string)(((JsonObject)athlete)["name"]));
    }

    Login.Text = "Log Out";
}

Any advice on this would be appreciated! Thanks in advance for your time.

Answer 1

When getting friends list, we receive a JSONObject and then later we extract the JSONArray from the object. But when extracting athletes, 'favorite_athletes' is itself returned as JSONArray and that's why there is a cast exception when converted into a IDictionary object. So,

Replace this code

var athletes = (IDictionary<string, object>)me["favorite_athletes"];

foreach (var athlete in (JsonArray)athletes)
{
    SportsPersonList.Items.Add((string)(((JsonObject)athlete)["name"]));
}

With this

foreach (var athlete in (JsonArray)me["favorite_athletes"])
{
    SportsPersonList.Items.Add((string)(((JsonObject)athlete)["name"]));
}

Hope this helps!