Extension to run tasks concurrently and return an array of results when all have completed successfully

Extensions.cs

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static partial class Extensions
{
    public static Task WhenAllAsync<T>(this IEnumerable<T> source,
                                       Func<T, CancellationToken, Task> taskSelector,
                                       int maxConcurrency,
                                       CancellationToken cancellationToken) =>
        source.WhenAllAsync(async (e, cancellationToken) =>
                            {
                                await taskSelector(e, cancellationToken);
                                return 0;
                            },
                            maxConcurrency, cancellationToken);

    public static async Task<TResult[]>
        WhenAllAsync<T, TResult>(this IEnumerable<T> source,
                                 Func<T, CancellationToken, Task<TResult>> taskSelector,
                                 int maxConcurrency,
                                 CancellationToken cancellationToken)
    {
        if (source is null) throw new ArgumentNullException(nameof(source));
        if (taskSelector is null) throw new ArgumentNullException(nameof(taskSelector));
        if (maxConcurrency < 0) throw new ArgumentOutOfRangeException(nameof(maxConcurrency));

        if (maxConcurrency == 0)
        {
            return await Task.WhenAll(from item in source
                                      select taskSelector(item, cancellationToken));
        }

        using var semaphore = new SemaphoreSlim(maxConcurrency);

        return await Task.WhenAll(from item in source
                                  select RunAsync(item));

        async Task<TResult> RunAsync(T item)
        {
            await semaphore.WaitAsync(cancellationToken);

            try
            {
                return await taskSelector(item, cancellationToken);
            }
            finally
            {
                semaphore.Release();
            }
        }
    }
}