Question

I’m getting the following error when retrieving a document from MongoDB:

“Unable to determine actual type of
object to deserialize. NominalType is
System.Object and BsonType is Array.”

The object that I’m serializing has a Dictionary<string, object> property. I can put a simple string in the Dictionary and pull it out without getting an error, but if there is a List<string> then I get the deserialization error.

I’m using the official c# driver (v 1.1). I can query the document just fine using Mongo shell, so I’m pretty it’s an issue with MongoDB.Bson.

Any suggestions/workarounds?

Code sample as requested:

Example object being saved to MongoDB:

public class WebUser
{
  public int _id;
  private DateTime startTime;
  private DateTime stopTime;
  private string browser;
  private string sessionID;
  private string ip;
  public List<PageView> PageViews;
  public Dictionary<string, Object> Session;

  public Save(){/*Data access code here*/}
  public static Single(int id){/*Data access code here*/}
}

Data Access Code:

public T Single<T>(int id) where T : class, new()
{
  var server = MongoServer.Create(ConnectionString);
  var db = server.GetDatabase(DBName);
  var collection = db.GetCollection<T>(typeof(T).Name);   
  var value = collection.FindOneById(id);
  server.Disconnect();
  return value;
 }

This works just fine:

var wu = WebUser.single(1);
wu.Session.Add("string key", "value");
wu.Session.Add("int key", 1);
wu.Save();
wu = WebUser.single(1);

This is where I get an error:

var wu = WebUser.single(1);
wu.Session.Add("list of values", new List<string>() { "yada", "yada 2", "yada 3" });
wu.Save();
//deserialize error on the retrieve below
wu = WebUser.single(1);

Answer 1

I think the following simple example reproduces the issue. Use class:

public class C {
    public ObjectId Id;
    public object Obj;
}

and the following test code:

collection.RemoveAll();
var c = new C { Obj = new int[] { 1, 2, 3 } };
collection.Insert(c);

var r = collection.FindOneAs<C>(); // fails

The problem is that the document was serialized as:

> db.test.find()
{ "_id" : ObjectId("4e15b931e447ad6a54eb0114"), "Obj" : [ 1, 2, 3 ] }
>

and the value for “Obj” has no type information, so the deserializer doesn’t know what class to instantiate for “Obj”.

I’ve create a JIRA ticket for this:

https://jira.mongodb.org/browse/CSHARP-263