Add sample bots to the solution (#972)

* Add sample bots to the solution

* Fix missing attributes, show use of preconditions

samples/02_commands_framework/Services/CommandHandlingService.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Reflection;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using Discord;
+using Discord.Commands;
+using Discord.WebSocket;
+
+namespace _02_commands_framework.Services
+{
+    public class CommandHandlingService
+    {
+        private readonly CommandService _commands;
+        private readonly DiscordSocketClient _discord;
+        private readonly IServiceProvider _services;
+
+        public CommandHandlingService(IServiceProvider services)
+        {
+            _commands = services.GetRequiredService<CommandService>();
+            _discord = services.GetRequiredService<DiscordSocketClient>();
+            _services = services;
+
+            _discord.MessageReceived += MessageReceivedAsync;
+        }
+
+        public async Task InitializeAsync()
+        {
+            await _commands.AddModulesAsync(Assembly.GetEntryAssembly(), _services);
+        }
+
+        public async Task MessageReceivedAsync(SocketMessage rawMessage)
+        {
+            // Ignore system messages, or messages from other bots
+            if (!(rawMessage is SocketUserMessage message)) return;
+            if (message.Source != MessageSource.User) return;
+
+            // This value holds the offset where the prefix ends
+            var argPos = 0;
+            if (!message.HasMentionPrefix(_discord.CurrentUser, ref argPos)) return;
+
+            var context = new SocketCommandContext(_discord, message);
+            var result = await _commands.ExecuteAsync(context, argPos, _services);
+
+            if (result.Error.HasValue &&
+                result.Error.Value != CommandError.UnknownCommand) // it's bad practice to send 'unknown command' errors
+                await context.Channel.SendMessageAsync(result.ToString());
+        }
+    }
+}

samples/02_commands_framework/Services/PictureService.cs

@@ -0,0 +1,20 @@
+using System.IO;
+using System.Net.Http;
+using System.Threading.Tasks;
+
+namespace _02_commands_framework.Services
+{
+    public class PictureService
+    {
+        private readonly HttpClient _http;
+
+        public PictureService(HttpClient http)
+            => _http = http;
+
+        public async Task<Stream> GetCatPictureAsync()
+        {
+            var resp = await _http.GetAsync("https://cataas.com/cat");
+            return await resp.Content.ReadAsStreamAsync();
+        }
+    }
+}