c# - Grouping mongodb filtered results by reference id throws -

i have simplified model:

public class parent {     public objectid id { get; set; } }  public class child {     public objectid id { get; set; }      public objectid parentid { get; set; }      [bsonrepresentation(bsontype.string)]     public gender gender { get; set; } } 

i have queried parents filter on fields i've intentionally omitted. have parent id collection. want children gender.male parents had obtained first query , group them can have mapping between parent , list<child>.

var parentids = filteredparents.select(p => p.id).tolist(); var childrenfilter = builders<child>.filter.in(c => c.parentid, parentids) &                      builders<child>.filter.eq(c => c.gender, gender.male);  var aggregation = children     .aggregate()     .match(childrenfilter)     .group(c => c.parentid, g => new keyvaluepair<string, list<child>>(g.key, new list<child>(g)));  var groupcursor = await aggregation.tocursorasync(); 

this throws

system.notsupportedexception: not find member match constructor parameter collection on type list`1. 

a few small side questions

1. i assume i've written childrenfilter in such way executed on server , client won't receive gender.female children? if that's not true - proper way it? , if used overload linq.expression - filter executed on server or on client?

2. are children grouped on server or on client? how should write group in order executed on server should more efficient?

update 17.06

as per @craigwilson's suggestion tried g.tolist():

var aggregation = children     .aggregate()     .match(childrenfilter)     .group(c => c.parentid, g => new keyvaluepair<string, list<child>>(g.key, g.tolist())); 

which led different exception:

unhandled exception: system.aggregateexception: 1 or more errors occurred. --- system.invalidcastexception: unable cast object of type 'mongodb.driver.linq.expressions.serializationexpression' type 'system.linq.expressions.methodcallexpression'.