C# MongoDB & Projection. Fails with serialization Exception

Question

Let's say I have a collection like :

 {
 id: "1"
 name: "collection 1"
 properties: "Some properties."
 }

With a class representation as

[BsonIgnoreExtraElements]
public class InfoPOCO {
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    [BsonElement("name")]
    public string Name { get; set; }

    [BsonElement("properties")]
    public string Properties { get; set; }
 } 

Now if am going to create a Projection like

Builders<InfoPOCO>.Projection.Include(_ => new{_.Name});

And call it with other params ( That is working fine without projection )

return GetDataBase().GetCollection<InfoPOCO>(collectionName).Find(Expr).
Project<InfoPOCO>(projectionDefinition).Skip(Offset).Limit(Limit).Sort(sort).ToList<InfoPOCO>()

Then I get the following error:

System.InvalidOperationException : Unable to determine the serialization information for
_ => new <>f__AnonymousType2`1


 Result StackTrace: 
at MongoDB.Driver.ExpressionFieldDefinition`1.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.SingleFieldProjectionDefinition`1.Render(IBsonSerializer`1 sourceSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.KnownResultTypeProjectionDefinitionAdapter`2.Render(IBsonSerializer`1 sourceSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.MongoCollectionImpl`1.CreateFindOperation[TProjection](FilterDefinition`1 filter, FindOptions`2 options)
   at MongoDB.Driver.MongoCollectionImpl`1.FindSync[TProjection](IClientSessionHandle session, FilterDefinition`1 filter, FindOptions`2 options, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.<>c__DisplayClass35_0`1.<FindSync>b__0(IClientSessionHandle session)
   at MongoDB.Driver.MongoCollectionImpl`1.UsingImplicitSession[TResult](Func`2 func, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.FindSync[TProjection](FilterDefinition`1 filter, FindOptions`2 options, CancellationToken cancellationToken)
   at MongoDB.Driver.FindFluent`2.ToCursor(CancellationToken cancellationToken)
   at MongoDB.Driver.IAsyncCursorSourceExtensions.ToList[TDocument](IAsyncCursorSource`1 source, CancellationToken cancellationToken)

What could be the reason for it, when am not using projection, it is fetching the entire collection fine. I don't know if I need to map the classes? (Thought Auto Map would take it) . I tried making the constructor in the class too. But still the same error. Any help would be highly appreciated. Thank you !

Answer 1

I finally understood! Yeah, I know, kinda slow, duh.

The issue is that when I was trying to map it like :

Builders<InfoPOCO>.Projection.Include(_ => new{_.Name, _.Properties});

It's a strongly typed expression and the result is mapped to an anonymous type. If am using forEach at the same time as am fetching it, then I can get those values. So the solution I am using right now is simple. I changed my projection to:

Builders<InfoPOCO>.Projection.Include(_ => _.Name).
                              Include(_ => _.Properties);

This works! Am not saying its the perfect solution or explanation but I see a lot of people struggling with it, so just mentioning a workaround. Hope it helps someone like me from going through hours of googling!