Prototype of Auzre AD OAuth2 client for RestSharp.Portable

AzureAdSystemClient.cs

using Newtonsoft.Json;
using RestSharp.Portable;
using RestSharp.Portable.Authenticators.OAuth2;
using RestSharp.Portable.Authenticators.OAuth2.Configuration;
using RestSharp.Portable.Authenticators.OAuth2.Infrastructure;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestOAuthAD
{
    class AzureAdSystemClient : OAuth2Client
    {
        private string _tenantId;
        private Endpoint _accessCodeServiceEndpoint;
        private Endpoint _accessTokenServiceEndpoint;

        public AzureAdSystemClient(IRequestFactory factory, IClientConfiguration configuration, string tenantId = "common")
            : base(factory, configuration)
        {
            _tenantId = tenantId;
        }

        public string TenantId { get { return _tenantId; } }
        public string PrincipalName { get; private set; }
        public string ObjectId { get; private set; }

        protected override Endpoint AccessCodeServiceEndpoint
        {
            get
            {
                if (_accessCodeServiceEndpoint == null)
                    _accessCodeServiceEndpoint = new Endpoint
                    {
                        BaseUri = "https://login.windows.net/",
                        Resource = string.Format("{0}/oauth2/authorize", _tenantId),
                    };
                return _accessCodeServiceEndpoint;
            }
        }

        protected override Endpoint AccessTokenServiceEndpoint
        {
            get
            {
                if (_accessTokenServiceEndpoint == null)
                    _accessTokenServiceEndpoint = new Endpoint
                    {
                        BaseUri = "https://login.windows.net/",
                        Resource = string.Format("{0}/oauth2/token", _tenantId),
                    };
                return _accessTokenServiceEndpoint;
            }
        }

        public override string Name
        {
            get
            {
                return "AzureAD";
            }
        }

        private static string FixBase64(string base64)
        {
            base64 = base64.Replace('-', '+').Replace('_', '/');
            switch (base64.Length % 4)
            {
                case 2:
                    base64 += "==";
                    break;
                case 3:
                    base64 += "=";
                    break;
            }
            return base64;
        }

        protected override void BeforeGetAccessToken(BeforeAfterRequestArgs args)
        {
            base.BeforeGetAccessToken(args);

            var res = args.Parameters["resource"].SingleOrDefault();
            if (!string.IsNullOrEmpty(res))
                args.Request.AddParameter("resource", res);
        }

        protected override void AfterGetAccessToken(BeforeAfterRequestArgs args)
        {
            dynamic response = JsonConvert.DeserializeObject(Encoding.UTF8.GetString(args.Response.RawBytes));
            var tokenId = (string)response.id_token;
            var parts = tokenId.Split('.');
            var jwtRaw = FixBase64(parts[1]);
            dynamic jwt = JsonConvert.DeserializeObject(Encoding.UTF8.GetString(Convert.FromBase64String(jwtRaw)));
            string newTenantId = jwt.tid;
            if (!string.IsNullOrEmpty(newTenantId))
                _tenantId = newTenantId;

            ObjectId = jwt.oid;
            PrincipalName = jwt.upn;

            base.AfterGetAccessToken(args);
        }

        protected override RestSharp.Portable.Authenticators.OAuth2.Models.UserInfo ParseUserInfo(string content)
        {
            throw new NotImplementedException();
        }

        protected override Endpoint UserInfoServiceEndpoint
        {
            get
            {
                throw new NotImplementedException();
            }
        }
    }
}

Add a resource parameter with value "https://graph.windows.net/" to the GetToken function to get access to the user information using the graph API.
You can use the graph API with the following lines:

var svcRoot = string.Format("https://graph.windows.net/{0}/", _authClient.TenantId);
var client = new Microsoft.Azure.ActiveDirectory.GraphClient.ActiveDirectoryClient(new Uri(svcRoot), async () => await _authClient.GetCurrentToken());

You need at least RestSharp.Portable v1.8.4 for this to work.