using System;

using System.Collections.Generic;

using System.Threading.Tasks;

using System.Linq;

using System.Threading;

​

public class Program

{

    public async static Task Main()

    {

​

        List<string> names = ["Alice", "Bob", "Charlie"];

​

        var upperCaseNamesList = await names.SelectAsync(async name =>

            {

                // simulate an asynchronous operation

                await Task.Delay(100); 

                return name.ToUpper();

            });

​

​

        Console.WriteLine(string.Join(",", upperCaseNamesList));

    }

}

​

public static class AsyncEnumerableExtensions

{

    /// <summary>

    /// Projects each element of an enumerable sequence into a List by applying an asynchronous selector function to each member of the source sequence and awaiting the result.

    /// </summary>

    /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>

    /// <typeparam name="TResult">The type of the elements in the result sequence, obtained by running the selector function for each element in the source sequence and awaiting the result.</typeparam>

    /// <param name="source">A sequence of elements to invoke a transform function on.</param>

    /// <param name="selector">An asynchronous transform function to apply to each source element.</param>

    /// <param name="cancellationToken">The optional cancellation token to be used for cancelling the sequence at any time.</param>

    /// <returns>A list whose elements are the result of invoking the transform function on each element of the source sequence and awaiting the result.</returns>

    /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>

    public static async Task<IEnumerable<TResult>> SelectAsync<TSource, TResult>(

        this IEnumerable<TSource> source,

        Func<TSource, Task<TResult>> selector,

        CancellationToken cancellationToken = default

    )

    {

        return await source

            .ToAsyncEnumerable()

            .SelectAwait(async x => await selector(x))

            .ToListAsync(cancellationToken);

    }

}