using System.Collections.Generic;
using MongoDB.Driver;
using MongoDB.Driver.Linq;

var settings = new MongoClientSettings { LinqProvider = LinqProvider.V3 };
var client = new MongoClient(settings);
var db = client.GetDatabase("test");
var coll = db.GetCollection<Foo>("foo");

var query = coll.AsQueryable().Select(x => new { MatchingBars = x.Bars.Where(y => y.Text == "foo") });
Console.WriteLine(query);

class Foo
{
    public Bar[] Bars { get; set; } // <-- IEnumerable<Bar> works, but other collection types do not
}

class Bar
{
    public string Text { get; set; }
}

Value type of serializer is Bar[] and does not match member type System.Collections.Generic.IEnumerable`1[[Bar, csharp4047, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]. (Parameter 'serializer')