SelectAsync With Max Degree Of Parallelism

Question

I have been looking for the best approach to creating an extension method that would give me the ability to select in a linq query using async/await with a max degree of parallelism. I'm aware that this can be achieved with the use of different libraries but wanted to create one without. Using postings I found online I made a few changes to convert a ForEachAsync to the following:

public static async Task<IEnumerable<TOut>> SelectAsync<TIn, TOut>(this IEnumerable<TIn> source, 
        Func<TIn, Task<TOut>> body, 
        int maxDegreeOfParallelism)
{
    var bag = new ConcurrentBag<TOut>();

    var selectAsync = Partitioner.Create(source)
        .GetPartitions(maxDegreeOfParallelism)
        .Select(async partition => 
        {
            using (partition)
            {
                while (partition.MoveNext())
                {
                    var item = await body(partition.Current);       
                    bag.Add(item);
                }
            }
        });

    await Task
        .WhenAll(selectAsync);

    return bag;
}

Here is the unit test:

[TestMethod]
public async Task SelectAsync_EnsureMaxConcurrency_SuccessfulOnChecksAsync()
{
    // Arrange
    const int itemsMax = 10;
    const int dop = 5;
    const int delayInMs = 1000;

    var itemsToProcess = Enumerable
        .Range(1, itemsMax);

    // Act
    var started = DateTime.Now;

    var result = await itemsToProcess
        .SelectAsync(async item =>
        {
            await Task.Delay(1000);
            return item;
        }, dop);

    var elapsed = (DateTime.Now - started)
        .TotalMilliseconds;

    const double expected = (double)itemsMax / dop * delayInMs;

    // Assert
    Debug.WriteLine($"Elapsed:{elapsed} Expected:{expected}");

    Assert.AreEqual(itemsMax, result.Count());
    Assert.IsTrue(elapsed > expected);
}