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Basicly i want to retrive data from following collection to my Class:

[BsonIgnoreExtraElements]
public class Topic
    [BsonElement("id")]
    public int Id { get; set; }
    [BsonElement("title")]
    public string Name { get; set; }
The problem is this code doesn't work -> executes with error message: 
 Cannot deserialize a 'Int32' from BsonType 'ObjectId',
but this one does:
[BsonIgnoreExtraElements]
public class Topic
    [BsonIgnore]
    public int Id { get; set; }
    [BsonElement("title")]
    public string Name { get; set; }
    [BsonElement("id")]
    public int IdTest { get; set; }
Seems like deserialization desperatly tries to match class property with name "Id" with the ObjectId in database which is not correct because i explicitly declare that i want to match it with BsonElement("id") and not ("_id").
I appreciate any ideas how to make it works as I need to.
Your "_id" stored in your mongo document is of type BsonType.ObjectId so when deserializing, the Serializer try to find a property with the name "Id" and with type of ObjectId. In your case it found matched name but not the right type which is int (int32 type in your class): my suggestion is to change 
public int Id { get; set; }
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }
    public ObjectId Id { get; set; }
Or to make your class inheriting from class Entity if you use MongoRepository :
     [BsonIgnoreExtraElements]
     public class Topic : Entity
       [BsonElement("title")]
       public string Name { get; set; }
and this:
var list = await col.Find(new BsonDocument()).ToListAsync().ConfigureAwait(false);
        foreach(var doc in list)
            if(doc.Name != null)
               topics.Add(new Topic{
                Id = doc.Identity,
                Name = doc.Name
        
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