Xamarin OAuth2 Custom Class to get & use refresh_token

OAuth2Custom.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Xamarin.Auth;

namespace MyNetStandardLibrary.OAuth2
{
    // inspired by https://lostechies.com/jimmybogard/2014/11/13/mobile-authentication-with-xamarin-auth-and-refresh-tokens/
    public class CustomOAuth2Authenticator : OAuth2Authenticator
    {
        public CustomOAuth2Authenticator(string clientId, string scope, Uri authorizeUrl, Uri redirectUrl, GetUsernameAsyncFunc getUsernameAsync = null, bool isUsingNativeUI = false) : base(clientId, scope, authorizeUrl, redirectUrl, getUsernameAsync, isUsingNativeUI)
        {
        }

        public CustomOAuth2Authenticator(string clientId, string clientSecret, string scope, Uri authorizeUrl, Uri redirectUrl, Uri accessTokenUrl, GetUsernameAsyncFunc getUsernameAsync = null, bool isUsingNativeUI = false) : base(clientId, clientSecret, scope, authorizeUrl, redirectUrl, accessTokenUrl, getUsernameAsync, isUsingNativeUI)
        {
        }

        async public override Task<Uri> GetInitialUrlAsync(Dictionary<string, string> custom_query_parameters = null)
        {
            var initilUrl = await base.GetInitialUrlAsync(custom_query_parameters);
            return new Uri($"{initilUrl}&access_type=offline");
        }

        async public Task<Dictionary<string,string>> RequestValidAccessTokenAsync(string refreshToken)
        {
            var queryValues = new Dictionary<string, string>
            {
                {"refresh_token", refreshToken},
                {"client_id", this.ClientId},
                {"grant_type", "refresh_token"}
            };

            if (!string.IsNullOrEmpty(this.ClientSecret))
                queryValues["client_secret"] = this.ClientSecret;

            var accountProperties = await this.RequestAccessTokenAsync(queryValues);

            this.OnRetrievedAccountProperties(accountProperties);
            return new Dictionary<string,string>(accountProperties);


            //return this.RequestAccessTokenAsync(queryValues).ContinueWith(result =>
            //{
            //    var accountProperties = result.Result;

            //    this.OnRetrievedAccountProperties(accountProperties);

            //    return int.Parse(accountProperties["expires_in"]);
            //});
        }
    }
}