for example :We want save string value of Enum to database,we set EnumRepresentationConvention Convention. then we can use it in match. but in aggregate group,it dosent work: public enum Gender { Male,Female} public class Person { public int Age { get; set; } public Gender Gender { get; set; } } [HttpGet("test")] public object Test() { var query=db._database.GetCollection("persons").Aggregate().Group(x => x.Age,g=>new { g.Key, MaleCount=g.Sum(x=>x.Gender==Gender.Male?1:0), FemaleCount = g.Sum(x => x.Gender == Gender.Female ? 1 : 0), }); var queryString = query.ToString(); return queryString; } //queryString will be: aggregate([{ "$group" : { "_id" : "$age", "MaleCount" : { "$sum" : { "$cond" : [{ "$eq" : ["$gender", 0] }, 1, 0] } }, "FemaleCount" : { "$sum" : { "$cond" : [{ "$eq" : ["$gender", 1] }, 1, 0] } } } }]) note:result "$eq" : ["$gender", 0],wish:$eq" : ["$gender", "Male"] if in Find(x=>x.Gender=Gender.Male) or Aggregate().Match(x=>x.Gender=Gender.Male),it result {"gender":"Male"} as we expect //ConventionPack follow var pack = new ConventionPack { new CamelCaseElementNameConvention(), new IgnoreExtraElementsConvention(true), new NamedIdMemberConvention("Id","ID"), new EnumRepresentationConvention(BsonType.String),// new StringObjectIdIdGeneratorConventionThatWorks() }; ConventionRegistry.Register("myconvention", pack, x => true); BsonSerializer.RegisterSerializer(typeof(DateTime), new DateTimeSerializer(DateTimeKind.Local));