using System;

using System.Web;

using System.Collections.Specialized;

​

public static class UriExtensions

{

    public static Uri WithQueryStringParameters(this Uri uri, NameValueCollection nameValueCollection)

    {

        var uriBuilder = new UriBuilder(uri);

​

        var query = HttpUtility.ParseQueryString(uriBuilder.Query);

        query.Add(nameValueCollection);

        uriBuilder.Query = query.ToString();

​

        return uriBuilder.Uri;

    }

}

public class Program

{

    public static void Main()

    {

        var ErrorOccurred = true;

        // https%3A%2F%2Fgantu-local.stitchmoney.com%2Freturn%3Ftype%3Dinstantpay

        // https%3a%2f%2fgantu-local.stitchmoney.com%2freturn%2f

        var ReturnUrl = "/connect/payment-request/redirect/f31fd48f-82b9-4e8c-8252-cca00395a92a?&redirect_uri=https%3A%2F%2Fgantu-local.stitchmoney.com%2Freturn%3Ftype%3Dinstantpay";

        var ClientQueryParameters = new NameValueCollection { { "status", ErrorOccurred ? "failed" : "closed" } };

        string[] ReturnUrlSections = ReturnUrl.Split('?');

        NameValueCollection QueryString = HttpUtility.ParseQueryString(ReturnUrlSections[1]);

        string ClientRedirectUri = QueryString["redirect_uri"] ?? string.Empty;

        Uri UpdatedClientRedirect = new Uri(ClientRedirectUri).WithQueryStringParameters(ClientQueryParameters);

        QueryString["redirect_uri"] = UpdatedClientRedirect.ToString();

        var returnUrlOldStyle = ReturnUrl + $"&status={(ErrorOccurred ? "failed" : "closed")}";

        var returnUrlNewStyle = "";

        Console.WriteLine($"ClientQueryParams -> {ClientQueryParameters}");

        Console.WriteLine($"ClientRedirectUri -> {ClientRedirectUri}");

        Console.WriteLine($"Old concatenation version -> {returnUrlOldStyle}");

        Console.WriteLine($"New dynamic version -> {returnUrlNewStyle}");

    }

}