using MongoDB.Bson.Serialization.Serializers;
using MongoDB.Bson.Serialization;
using MongoDB.Bson;
using MongoDB.Driver;

BsonSerializer.RegisterSerializer(new GuidSerializer(GuidRepresentation.Standard));

var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("testdb");
var collection = database.GetCollection<BsonDocument>("testcollection");

var field = "guidField";

// Using Guid as the list type
var values = new List<Guid>()
{
    Guid.Parse("6cd6f392-8271-49bb-8564-e584ddf48890"),
    Guid.Parse("c7b1ebaf-4ac1-4fe0-b066-1282e072585a")
};

// Using dynamic as the list type
var dynamicValues = new List<dynamic>()
{
    "1234",
    Guid.Parse("6cd6f392-8271-49bb-8564-e584ddf48890"),
    Guid.Parse("c7b1ebaf-4ac1-4fe0-b066-1282e072585a")
};

var filter1 = Builders<BsonDocument>.Filter.In(field, values);  
var filter2 = Builders<BsonDocument>.Filter.In(field, dynamicValues);

var result1 = collection.Find(filter1).ToList();  // OK
var result2 = collection.Find(filter2).ToList();  // FAIL ('GuidSerializer cannot serialize a Guid when GuidRepresentation is Unspecified.')