> db.foo.find().pretty()
{
        "_id" : 1,
        "bars" : [
                {
                        "x" : 1,
                        "y" : 2
                },
                {
                        "x" : 2,
                        "y" : 3
                },
                {
                        "x" : 1,
                        "y" : 4
                }
        ]
}

        private class Foo
        {
            public int Id;

            public List<Bars> Bars;
        }

        private class UnwoundFoo
        {
            public int Id;
            [BsonElement("Bars")]
            public Bar Bar;
        }

        private class Bar
        {
            [BsonElement("x")]
            public int X;
            [BsonElement("y")]
            public int Y;
        }

    var list = await col.Aggregate()
                .Unwind<Foo, UnwoundFoo>(x => x.Bars)
                .Match(x => x.Bar.X == 1)
                .Project(x => new { X = x.Bar.X, Y = x.Bar.Y })
                .ToListAsync();