using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

public static class MyExtensions

{

    public static async Task<IEnumerable<T>> Where<T>(this IEnumerable<T> source, Func<T, Task<bool>> func)

    {

        var tasks = new List<Task<bool>>();

        foreach (var element in source)

        {

            tasks.Add(func(element));

        }

​

        var results = await Task.WhenAll<bool>(tasks.ToArray());

        var trueIndex = results.Select((x, index) => new

        {

        x, index

        }).Where(x => x.x).Select(x => x.index).ToList();

        var filterSource = source.Where((x, index) => trueIndex.Contains(index));

        return filterSource;

    }

}

​

public class Program

{

    static async Task Main(string[] args)

    {

        var users = new List<User>{new User{Id = 1, Age = 18, Name = "Michael"}, new User{Id = 2, Age = 18, Name = "Mike"}, new User{Id = 3, Age = 5, Name = "Sunny"}, new User{Id = 4, Age = 70, Name = "John"}};

        var result = await users.Where(async x => await TestAsync(x));

        result.ToList().ForEach(x => Console.WriteLine(x.Id));

    }

​

    public static async Task<bool> TestAsync(User user)

    {

        await Task.Delay(100);

        return user.Age < 21;

    }

​

    public class User

    {

        public int Id

        {

            get;

            set;

        }

​

        public int Age

        {

            get;

            set;

        }

​

        public string Name

        {

            get;

            set;

        }

    }

}