GetAsyncEnumerableIfAnyAsync.cs

var seq1 = GenerateNumbersAsync("A", 2, 1);
var seq2 = GenerateNumbersAsync("B", 0, 1);

await PrintSeqAsync(seq1);
await PrintSeqAsync(seq2);

async Task PrintSeqAsync(
    IAsyncEnumerable<int> seq,
    [System.Runtime.CompilerServices.CallerArgumentExpression(nameof(seq))] string label = "")
{
    Console.WriteLine($"========== {label} ==========");
    await PrintIfAny1Async(seq);
    Console.WriteLine("----- vs -----");
    await PrintIfAny2Async(seq);
    Console.WriteLine();
}

async IAsyncEnumerable<int> GenerateNumbersAsync(string label, int count, int delay)
{
    Console.WriteLine($"[{label}] START");
    for (var i = 0; i < count; i++)
    {
        await Task.Delay(delay);
        Console.WriteLine($"[{label}] YIELD");
        yield return i;
    }
    Console.WriteLine($"[{label}] END");
}

async Task PrintIfAny1Async<T>(IAsyncEnumerable<T> seq)
{
    // var (hasAny, items) = (await seq.CountAsync() != 0, seq); // even worse
    var (hasAny, items) = (await seq.AnyAsync(), seq);
    if (hasAny)
    {
        await foreach (var item in items)
        {
            Console.WriteLine(item);
        }
    }
    else
    {
        Console.WriteLine("No items");
    }
}

async Task PrintIfAny2Async<T>(IAsyncEnumerable<T> seq)
{
    var (hasAny, items) = await seq.GetAsyncEnumerableIfAnyAsync();
    if (hasAny)
    {
        await foreach (var item in items)
        {
            Console.WriteLine(item);
        }
    }
    else
    {
        Console.WriteLine("No items");
    }
}

public static class AsyncEnumerableExtensions
{
    public static async Task<(bool hasAny, IAsyncEnumerable<T> enumerable)> GetAsyncEnumerableIfAnyAsync<T>(
        this IAsyncEnumerable<T> source)
    {
        var enumerator = source.GetAsyncEnumerator();

        if (!await enumerator.MoveNextAsync())
        {
            return (false, AsyncEnumerable.Empty<T>());
        }

        return (true, Yield());

        async IAsyncEnumerable<T> Yield()
        {
            yield return enumerator.Current;
            while (await enumerator.MoveNextAsync())
            {
                yield return enumerator.Current;
            }
        }
    }
}