composing queryables

using System;
using System.Collections.Specialized;
using System.Linq;
using System.Web;

namespace demo
{
    class Program
    {
        static void Main(string[] args)
        {
            // create a PagingOptions object from URL
            var parser = new PagingQueryOptionsParser();
            var options = parser.Parse(new Uri("https://localhost/foo?$top=5&$skip=5"));

            // retrieve data as a queryable
            var items = Enumerable.Range(0, 100).AsQueryable();

            // apply paging query options
            var result = options.Apply(items);

            // print result
            System.Console.WriteLine("{0}", string.Join(", ", result));
        }
    }


    public class PagingQueryOptions
    {
        public PagingQueryOptions(int? take, int? skip)
        {
            Take = take;
            Skip = skip;
        }

        public int? Take { get; }
        public int? Skip { get; }

        public IQueryable<T> Apply<T>(IQueryable<T> items)
        {
            // n.b. order of skip and take is important
            if (Skip.HasValue)
            {
                items = items.Skip(Skip.Value);
            }
            if (Take.HasValue)
            {
                items = items.Take(Take.Value);
            }
            return items;
        }
    }

    public class PagingQueryOptionsParser
    {
        public PagingQueryOptions Parse(Uri url)
        {
            var query = HttpUtility.ParseQueryString(url.Query);
            var take = GetSingleIntQueryOption("$top", query);
            var skip = GetSingleIntQueryOption("$skip", query);
            return new PagingQueryOptions(take, skip);
        }

        // get the value of the query option with name <name> and return it as int?
        // return null if the name is not present,
        // throw ArgumentException if value is not a number or the name is given more than once
        // return the value as an Int otherwise
        private static int? GetSingleIntQueryOption(string name, NameValueCollection query)
        {
            var values = query.GetValues(name);
            if (values == null)
            {
                return null;
            }
            if (values.Length > 1)
            {
                throw new ArgumentException($"Invalid query parameter, query option {name} must not appear more than once");
            }

            return Int32.TryParse(values.Single(), out var val) ? val :
                throw new ArgumentException($"Query Option {name}'s value is not a number");
        }
    }
}