using System;

using Microsoft.AspNetCore.Http.Extensions;

using System.Collections.Generic;

using Microsoft.AspNetCore.WebUtilities;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        string[] x= {

            "https://xxx.com/xxdda/what?state=xd3",

            "https://xxx.com/xxdda/what?state",

            "https://xxx.com/xxdda/what?",

            "https://xxx.com/xxdda/what",

            "https://xxx.com/xxdda/",

        };

        foreach(var url in x)

        {

            var xx = SupplyStateParameter(url,"xd");

            Console.WriteLine(xx);

        }

    }

            static private string SupplyStateParameter(string rawurl, string state)

        {

            var uri = new Uri(rawurl);

            var baseUri =  uri.GetComponents(UriComponents.Scheme | UriComponents.Host | UriComponents.Port | UriComponents.Path, UriFormat.UriEscaped);

            var query = QueryHelpers.ParseQuery(uri.Query);

            if (!query.ContainsKey(nameof(state)))

            {

                query[nameof(state)] = string.IsNullOrWhiteSpace(state) ? Guid.NewGuid().ToString() : state;

            }

​

            var items = query.SelectMany(x => x.Value, (col, value) => new KeyValuePair<string, string>(col.Key, value)).ToList();

            var qb = new QueryBuilder(items);

            return baseUri + qb.ToQueryString();

        }

}