chaosbot/ChaosBot/WebServer/App/DiscordController.cs

using System.Collections.Generic;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Newtonsoft.Json;
using NLog;

namespace ChaosBot.WebServer.App
{
    [ApiController]
    [Route("/discord")]
    public class DiscordController : Controller
    {
        private static readonly HttpClient client = new HttpClient();
        private static readonly ILogger Logger = Program.Logger;
        
        [HttpGet]
        public async Task<IActionResult> Index(string code = null)
        {
            string redirectUri = $"{Request.Scheme}://{Request.Host}/discord";
            string clientId = Program.AppSettingsHandler.GetValue<string>("Discord:ClientId");
            string clientSecret = Program.AppSettingsHandler.GetValue<string>("Discord:ClientSecret");
            
            if (code == null)
                return Redirect($"https://discord.com/api/oauth2/authorize?client_id={clientId}&redirect_uri={redirectUri}&response_type=code&scope=identify%20guilds");

            Dictionary<string, string> values = new Dictionary<string, string>
            {
                { "client_id", clientId },
                { "client_secret", clientSecret },
                { "grant_type", "authorization_code" },
                { "code", code },
                { "redirect_uri", redirectUri },
                { "scope", "identify guild" }
            };

            FormUrlEncodedContent content = new FormUrlEncodedContent(values);
            HttpResponseMessage response = await client.PostAsync("https://discord.com/api/oauth2/token", content);
            string responseString = await response.Content.ReadAsStringAsync();
            DiscordOauthResponse responseObject = JsonConvert.DeserializeObject<DiscordOauthResponse>(responseString);

            return LocalRedirect($"/#/?access_token={responseObject.access_token}");
        }

        [HttpGet("user")]
        public IActionResult GetUser(string access_token)
        {
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", access_token);

            HttpResponseMessage response;
            using (HttpRequestMessage requestMessage =
                new HttpRequestMessage(HttpMethod.Get, "https://discord.com/api/v7/users/@me"))
            {
                requestMessage.Headers.Authorization =
                    new AuthenticationHeaderValue("Bearer", access_token);
                response = client.SendAsync(requestMessage).GetAwaiter().GetResult();
            }
            string responseString = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            DiscordUserResponse userResponse = JsonConvert.DeserializeObject<DiscordUserResponse>(responseString);

            return Json(new
            {
                id = userResponse.id,
                username = $"{userResponse.username}#{userResponse.discriminator}",
                avatar = userResponse.avatar
            });
        }

        [HttpGet("guilds")]
        public IActionResult GetGuilds(string access_token)
        {
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", access_token);

            HttpResponseMessage response;
            using (HttpRequestMessage requestMessage =
                new HttpRequestMessage(HttpMethod.Get, "https://discord.com/api/v7/users/@me/guilds"))
            {
                requestMessage.Headers.Authorization =
                    new AuthenticationHeaderValue("Bearer", access_token);
                response = client.SendAsync(requestMessage).GetAwaiter().GetResult();
            }
            string responseString = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            List<DiscordGuildResponse> userResponse = JsonConvert.DeserializeObject<List<DiscordGuildResponse>>(responseString);

            return Json(userResponse);
        }
    }

    internal class DiscordOauthResponse
    {
        public string error = null;
        public string error_description = null;
        
        public string access_token = null;
        public int expires_in = 0;
        public string refresh_token = null;
        public string scope = null;
        public string token_type = null;
    }

    public class DiscordUserResponse
    {
        public string id;
        public string username;
        public string discriminator;
        public string avatar = null;
        public bool bot = false;
        public bool system = false;
        public bool mfa_enabled = false;
        public string locale = null;
        public bool verified = false;
        public string email = null;
        public int flags = 0;
        public int premium_type = 0;
        public int public_flags = 0;
    }

    public class DiscordGuildResponse
    {
        public string id;
        public string name;
        public string icon;
        public bool owner;
        public int permissions;
        public int permissions_new;
    }
}