using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

public class Program

{

    protected class QueryParameters

    {

      public string paramName { get; set; }

      public IEnumerable<object> paramValues { get; set; }

    }

​

    protected static string ObjectToQueryParametersString(IReadOnlyCollection<QueryParameters> parameters)

    {

      return parameters.Aggregate(new StringBuilder("?"), (sb, parameter) => sb.Append(parameter.paramValues.Aggregate(new StringBuilder(), (sb2, paramValue) => sb2.Append($"{parameter.paramName}={paramValue}&"))),

      (result) =>

      {

        result.Length-= "&".Length;

        return result.ToString();

      });

    }

​

    public static void Main()

    {

        var baseRoute = "http://localhost/api/users";

        var userIdParams = new QueryParameters {paramName = "userid", paramValues = new List<object> { 1, 2, 3} };

        var someOtherParams = new QueryParameters {paramName = "foo", paramValues = new List<string> {"x", "y", "z"} };

        var paramString = ObjectToQueryParametersString(new List<QueryParameters> { userIdParams, someOtherParams });

        Console.WriteLine(baseRoute + paramString);

    }

}