using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Driver.Linq;
using System.Linq;

internal class C
{
    [BsonRepresentation(BsonType.String)] public E E1 { get; set; }
    [BsonRepresentation(BsonType.String)] public E E2 { get; set; }
}

internal enum E { Zero, A, B }

public class TestClass
{
    public static void Main()
    {
        var pipeline = new EmptyPipelineDefinition<C>()
            .Project(d => new { E = d.E1 != E.Zero ? d.E1 : d.E2});

        var linqProvider = LinqProvider.V3;
        var pipelineRendered = pipeline.Render(BsonSerializer.LookupSerializer<C>(), BsonSerializer.SerializerRegistry, linqProvider).Documents.Select(d => d.ToString()).ToArray();
    }
}