Why is using IAsyncEnumerable slower than returning async/await Task<T>?

Question

I'm currently testing out C# 8's async streams, and it seems that when I try to run the application using the old pattern of of using async/await and returning Task> it seems to be faster. (I measured it using a stopwatch and tried running it multiple times, and the result was that the old pattern I mentioned seems somewhat faster than using IAsyncEnumerable).

Here's a simple Console App that I wrote (I'm also thinking perhaps I'm loading the data from database the wrong way)

class Program
    {
        static async Task Main(string[] args)
        {

            // Using the old pattern 
            //Stopwatch stopwatch = Stopwatch.StartNew();
            //foreach (var person in await LoadDataAsync())
            //{
            //    Console.WriteLine($"Id: {person.Id}, Name: {person.Name}");
            //}
            //stopwatch.Stop();
            //Console.WriteLine(stopwatch.ElapsedMilliseconds);


            Stopwatch stopwatch = Stopwatch.StartNew();
            await foreach (var person in LoadDataAsyncStream())
            {
                Console.WriteLine($"Id: {person.Id}, Name: {person.Name}");
            }
            stopwatch.Stop();
            Console.WriteLine(stopwatch.ElapsedMilliseconds);


            Console.ReadKey();
        }


        static async Task<IEnumerable<Person>> LoadDataAsync()
        {
            string connectionString = "Server=localhost; Database=AsyncStreams; Trusted_Connection = True;";
            var people = new List<Person>();
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                //SqlDataReader
                await connection.OpenAsync();

                string sql = "Select * From Person";
                SqlCommand command = new SqlCommand(sql, connection);

                using (SqlDataReader dataReader = await command.ExecuteReaderAsync())
                {
                    while (await dataReader.ReadAsync())
                    {
                        Person person = new Person();
                        person.Id = Convert.ToInt32(dataReader[nameof(Person.Id)]);
                        person.Name = Convert.ToString(dataReader[nameof(Person.Name)]);
                        person.Address = Convert.ToString(dataReader[nameof(Person.Address)]);
                        person.Occupation = Convert.ToString(dataReader[nameof(Person.Occupation)]);
                        person.Birthday = Convert.ToDateTime(dataReader[nameof(Person.Birthday)]);
                        person.FavoriteColor = Convert.ToString(dataReader[nameof(Person.FavoriteColor)]);
                        person.Quote = Convert.ToString(dataReader[nameof(Person.Quote)]);
                        person.Message = Convert.ToString(dataReader[nameof(Person.Message)]);

                        people.Add(person);
                    }
                }

                await connection.CloseAsync();
            }

            return people;
        }

        static async IAsyncEnumerable<Person> LoadDataAsyncStream()
        {
            string connectionString = "Server=localhost; Database=AsyncStreams; Trusted_Connection = True;";
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                //SqlDataReader
                await connection.OpenAsync();

                string sql = "Select * From Person";
                SqlCommand command = new SqlCommand(sql, connection);

                using (SqlDataReader dataReader = await command.ExecuteReaderAsync())
                {
                    while (await dataReader.ReadAsync())
                    {
                        Person person = new Person();
                        person.Id = Convert.ToInt32(dataReader[nameof(Person.Id)]);
                        person.Name = Convert.ToString(dataReader[nameof(Person.Name)]);
                        person.Address = Convert.ToString(dataReader[nameof(Person.Address)]);
                        person.Occupation = Convert.ToString(dataReader[nameof(Person.Occupation)]);
                        person.Birthday = Convert.ToDateTime(dataReader[nameof(Person.Birthday)]);
                        person.FavoriteColor = Convert.ToString(dataReader[nameof(Person.FavoriteColor)]);
                        person.Quote = Convert.ToString(dataReader[nameof(Person.Quote)]);
                        person.Message = Convert.ToString(dataReader[nameof(Person.Message)]);

                        yield return person;
                    }
                }

                await connection.CloseAsync();
            }
        }

I would like to know whether IAsyncEnumerable is not best suited for this kind of scenario or there was something wrong with how I queried the data while using IAsyncEnumerable? I might be wrong but I actually expect using IAsyncEnumerable would be faster. (by the way...the difference are usually in hundreds of milliseconds)

I tried the application with a sample data of 10,000 rows.

Here's also the code for populating the data just in case...

static async Task InsertDataAsync()
        {
            string connectionString = "Server=localhost; Database=AsyncStreams; Trusted_Connection = True;";
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                string sql = $"Insert Into Person (Name, Address, Birthday, Occupation, FavoriteColor, Quote, Message) Values";


                for (int i = 0; i < 1000; i++)
                {
                    sql += $"('{"Randel Ramirez " + i}', '{"Address " + i}', '{new DateTime(1989, 4, 26)}', '{"Software Engineer " + i}', '{"Red " + i}', '{"Quote " + i}', '{"Message " + i}'),";
                }

                using (SqlCommand command = new SqlCommand(sql.Remove(sql.Length - 1), connection))
                {
                    command.CommandType = CommandType.Text;

                    await connection.OpenAsync();
                    await command.ExecuteNonQueryAsync();
                    await connection.CloseAsync();
                }

            }
        }

Answer 1

IAsyncEnumerable<T> is not inherently faster or slower than Task<T>. It depends on the implementation.

IAsyncEnumerable<T> is about asynchronously retrieving data providing individual values as soon as possible.

IAsyncEnumerable<T> allows batch producing values which will make some of the invocations of MoveNextAsync synchronous, as in the next example:

async Task Main()
{
    var hasValue = false;
    var asyncEnumerator = GetValuesAsync().GetAsyncEnumerator();
    do
    {
        var task = asyncEnumerator.MoveNextAsync();
        Console.WriteLine($"Completed synchronously: {task.IsCompleted}");
        hasValue = await task;
        if (hasValue)
        {
            Console.WriteLine($"Value={asyncEnumerator.Current}");
        }
    }
    while (hasValue);
    await asyncEnumerator.DisposeAsync();
}

async IAsyncEnumerable<int> GetValuesAsync()
{
    foreach (var batch in GetValuesBatch())
    {
        await Task.Delay(1000);
        foreach (var value in batch)
        {
            yield return value;
        }
    }
}
IEnumerable<IEnumerable<int>> GetValuesBatch()
{
    yield return Enumerable.Range(0, 3);
    yield return Enumerable.Range(3, 3);
    yield return Enumerable.Range(6, 3);
}

Output:

Completed synchronously: False
Value=0
Completed synchronously: True
Value=1
Completed synchronously: True
Value=2
Completed synchronously: False
Value=3
Completed synchronously: True
Value=4
Completed synchronously: True
Value=5
Completed synchronously: False
Value=6
Completed synchronously: True
Value=7
Completed synchronously: True
Value=8
Completed synchronously: True