output.txt

started 1
Finished 1
started 2
Finished 2
started 3
started 4
Finished 3
Finished 4
started 5
Finished 5
started 6
Finished 6
started 7
Finished 7
started 8
Finished 8
started 9
Finished 9
started 10
Finished 10

wtf.cs

using System;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Linq;

var tasks = 
    Enumerable.Range(1,10)
    .ToAsyncEnum()
    .SelectAsync(async x => {
        Console.WriteLine($"started {x}");
        await Task.Delay(1000);
        return x;
    });

await Parallel.ForEachAsync(
    tasks,
    new ParallelOptions { MaxDegreeOfParallelism = 5 },
    (x, _) =>
    {
      Console.WriteLine($"Finished {x}");
      return ValueTask.CompletedTask;
    });

static class Ext {
  public static async IAsyncEnumerable<T> ToAsyncEnum<T>(this IEnumerable<T> e) {
    foreach (var item in e)
    {
      yield return item;
    }
  }
  
  public static async IAsyncEnumerable<U> SelectAsync<T, U>(this IAsyncEnumerable<T> source, Func<T, Task<U>> f)
  {
    await foreach (var item in source)
    {
      yield return await f(item);
    }
  }
}