using MongoDB.Driver;
using System;
using System.Threading.Tasks;

namespace MongoDBUpdateExample
{
    class Program
    {
        static async Task Main(string[] args)
        {
            // Initialize MongoDB client and get the collection
            var client = new MongoClient("connection_string");  // Replace with your MongoDB connection string
            var database = client.GetDatabase("db");  // Replace with your database name
            var collection = database.GetCollection<DbTargetRecord>("collection");  // Replace with your collection name

            FilterDefinition<DbTargetRecord> filter = Builders<DbTargetRecord>.Filter.Where(
                x => x._id == "1"
                && x.DistinctAnchors.Count <= 3
            );

            UpdateDefinition<DbTargetRecord> updateDefinition = Builders<DbTargetRecord>.Update.Set(z => z.LastUpdateTime, DateTime.UtcNow);

            UpdateResult result = await collection.UpdateOneAsync(filter, updateDefinition, new UpdateOptions() { IsUpsert = true });
        }
    }

    public class DbTargetRecord
    {
        public string _id { get; set; }
        public List<string> DistinctAnchors { get; set; }
        public DateTime LastUpdateTime { get; set; }
    }

    public class TargetAnchor
    {
        public string Target { get; set; }
        public string Anchor { get; set; }
    }
}