AsyncEnumerableExtensions.cs

 public static class AsyncEnumerableExtensions
 {
     public static Task<List<T>> ToListAsync<T>(this IAsyncEnumerable<T> source)
     {
         if (source == null)
         {
             throw new ArgumentNullException(nameof(source));
         }

         return ExecuteAsync();

         async Task<List<T>> ExecuteAsync()
         {
             var list = new List<T>();

             await foreach (var element in source)
             {
                 list.Add(element);
             }

             return list;
         }
     }
 }