public class TransactionTest
{
    private const string DatabaseName = "PressureTest";
    private const string CollectionName = "Test";
    public const string ConnectionString = "";
    public MongoClient GetMongoClient(int timeout = 5)
    {
        var clientSettings = MongoClientSettings.FromConnectionString(ConnectionString);
        clientSettings.ConnectTimeout = TimeSpan.FromSeconds(5);
        clientSettings.ServerSelectionTimeout = TimeSpan.FromSeconds(timeout);
        clientSettings.AllowInsecureTls = true;
        var mongoClient = new MongoClient(clientSettings);
        return mongoClient;
    }

    public async Task TestTransactionAsync()
    {
        var client = GetMongoClient();
        var tasks = new List<Task>();
        for (int i = 0; i < 5; ++i)
        {
            //var client = GetMongoClient(i + 5);
            tasks.Add(DoAsync(client));
        }
        await Task.WhenAll(tasks);
    }

    private async Task DoAsync(IMongoClient mongoClient)
    {
        Console.WriteLine("Client hashcode: " + mongoClient.GetHashCode());
        var collection = mongoClient.GetDatabase(DatabaseName).GetCollection<BsonDocument>(CollectionName);

        while (true)
        {
            var uuid1 = Guid.NewGuid().ToString("N").Substring(24);
            var uuid2 = Guid.NewGuid().ToString("N").Substring(24);
            try
            {
                using (var session = await mongoClient.StartSessionAsync())
                {
                    session.StartTransaction();
                    await collection.InsertOneAsync(session, new BsonDocument("Uuid", uuid1));
                    await collection.InsertOneAsync(session, new BsonDocument("Uuid", uuid2));

                    await session.CommitTransactionAsync();
                }
                Console.WriteLine($"[{uuid1}] [{uuid2}]");
            }
            catch (Exception e)
            {
                Console.WriteLine("$$$ " + e.Message);
            }
        }
    }
}

public async Task TestTransactionAsync()
{
    var tasks = new List<Task>();
    for (int i = 0; i < 5; ++i)
    {
        var client = GetMongoClient(i + 5);
        tasks.Add(DoAsync(client));
    }
    await Task.WhenAll(tasks);
}