とりあえず動いたのでヨシ！

CedBotSharp/Bot/CedBot.cs

@@ -6,34 +6,36 @@ using System.Threading;
 using System.Threading.Tasks;
 using Discord;
 using Discord.WebSocket;
+using Discord.Commands;
+using Microsoft.Extensions.DependencyInjection;
+using CedBotSharp.Bot.Services;
+using System.Net.Http;
 
 namespace CedBotSharp.Bot
 {
     class CedBot
     {
-        private readonly DiscordSocketClient _client;
-
-        public CedBot()
+        public CedBot(string token)
         {
-            _client = new DiscordSocketClient();
-
-            _client.Log += LogAsync;
-            _client.Ready += ReadyAsync;
-            _client.MessageReceived += MessageReceivedAsync;
+            _ = MainAsync(token);
         }
 
-        public async Task MainAsync(String token)
+        public async Task MainAsync(string token)
         {
-            await _client.LoginAsync(TokenType.Bot, token);
-            await _client.StartAsync();
+            using (var services = ConfigureServices())
+            {
+                var client = services.GetRequiredService<DiscordSocketClient>();
 
-            await Task.Delay(Timeout.Infinite);
-        }
+                client.Log += LogAsync;
+                services.GetRequiredService<CommandService>().Log += LogAsync;
 
-        private Task ReadyAsync()
-        {
-            Console.WriteLine($"{_client.CurrentUser} is connected!");
-            return Task.CompletedTask;
+                await client.LoginAsync(TokenType.Bot, token);
+                await client.StartAsync();
+
+                await services.GetRequiredService<CommandHandlingService>().InitializeAsync();
+
+                await Task.Delay(Timeout.Infinite);
+            }
         }
 
         private Task LogAsync(LogMessage log)
@@ -42,16 +44,15 @@ namespace CedBotSharp.Bot
             return Task.CompletedTask;
         }
 
-        private async Task MessageReceivedAsync(SocketMessage message)
+        private ServiceProvider ConfigureServices()
         {
-            if (message.Author.Id == _client.CurrentUser.Id) return;
-            if (message.Content == "cs.ping") await message.Channel.SendMessageAsync("pong!");
-        }
-
-        public async void Start(String token)
-        {
-            if (String.IsNullOrWhiteSpace(token)) throw new FormatException("The token has not been set.");
-            await MainAsync(token);
+            return new ServiceCollection()
+                .AddSingleton<DiscordSocketClient>()
+                .AddSingleton<CommandService>()
+                .AddSingleton<CommandHandlingService>()
+                .AddSingleton<HttpClient>()
+                .AddSingleton<PictureService>()
+                .BuildServiceProvider();
         }
     }
 }

CedBotSharp/Bot/Modules/PublicModule.cs

@@ -0,0 +1,62 @@
+using CedBotSharp.Bot.Services;
+using Discord;
+using Discord.Commands;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace CedBotSharp.Bot.Modules
+{
+    public class PublicModule : ModuleBase<SocketCommandContext>
+    {
+        // Dependency Injection will fill this value in for us
+        public PictureService PictureService { get; set; }
+
+        [Command("ping")]
+        [Alias("pong", "hello")]
+        public Task PingAsync()
+            => ReplyAsync("pong!");
+
+        [Command("cat")]
+        public async Task CatAsync()
+        {
+            var stream = await PictureService.GetCatPictureAsync();
+            stream.Seek(0, SeekOrigin.Begin);
+            await Context.Channel.SendFileAsync(stream, "cat.png");
+        }
+
+        [Command("userinfo")]
+        public async Task UserInfoAsync(IUser user = null)
+        {
+            user = user ?? Context.User;
+
+            await ReplyAsync(user.ToString());
+        }
+
+        [Command("ban")]
+        [RequireContext(ContextType.Guild)]
+        [RequireUserPermission(GuildPermission.BanMembers)]
+        [RequireBotPermission(GuildPermission.BanMembers)]
+        public async Task BanUserAsync(IGuildUser user, [Remainder] string reason = null)
+        {
+            await user.Guild.AddBanAsync(user, reason: reason);
+            await ReplyAsync("ok!");
+        }
+
+        [Command("echo")]
+        public Task EchoAsync([Remainder] string text)
+            => ReplyAsync('\u200B' + text);
+
+        [Command("list")]
+        public Task ListAsync(params string[] objects)
+            => ReplyAsync("You listed: " + string.Join("; ", objects));
+
+        [Command("guild_only")]
+        [RequireContext(ContextType.Guild, ErrorMessage = "Sorry, this command must be ran from within a server, not a DM!")]
+        public Task GuildOnlyCommand()
+            => ReplyAsync("Nothing to see here!");
+    }
+}

CedBotSharp/Bot/Services/CommandHandlingService.cs

@@ -0,0 +1,55 @@
+using Discord;
+using Discord.Commands;
+using Discord.WebSocket;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace CedBotSharp.Bot.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;
+
+            _commands.CommandExecuted += CommandExecutedAsync;
+            _discord.MessageReceived += MessageReceivedAsync;
+        }
+
+        public async Task InitializeAsync()
+        {
+            await _commands.AddModulesAsync(Assembly.GetEntryAssembly(), _services);
+        }
+
+        public async Task MessageReceivedAsync(SocketMessage rawMessage)
+        {
+            if (!(rawMessage is SocketUserMessage message)) return;
+            if (message.Source != MessageSource.User) return;
+
+            var argPos = 0;
+            if (!message.HasMentionPrefix(_discord.CurrentUser, ref argPos)) return;
+
+            var context = new SocketCommandContext(_discord, message);
+            await _commands.ExecuteAsync(context, argPos, _services);
+        }
+
+        public async Task CommandExecutedAsync(Optional<CommandInfo> command, ICommandContext context, IResult result)
+        {
+            if (!command.IsSpecified) return;
+            if (result.IsSuccess) return;
+
+            await context.Channel.SendMessageAsync($"error: {result}");
+        }
+    }
+}

CedBotSharp/Bot/Services/PictureService.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Net.Http;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace CedBotSharp.Bot.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();
+        }
+    }
+}

CedBotSharp/CedBotSharp.csproj

@@ -5,7 +5,7 @@
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
     <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
     <ProjectGuid>{3277E259-3695-4CF0-8D0A-F8EA6A7C9B68}</ProjectGuid>
-    <OutputType>WinExe</OutputType>
+    <OutputType>Exe</OutputType>
     <RootNamespace>CedBotSharp</RootNamespace>
     <AssemblyName>CedBotSharp</AssemblyName>
     <TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
@@ -36,6 +36,9 @@
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
   </PropertyGroup>
+  <PropertyGroup>
+    <StartupObject />
+  </PropertyGroup>
   <ItemGroup>
     <Reference Include="Discord.Net.Commands, Version=2.4.0.0, Culture=neutral, processorArchitecture=MSIL">
       <HintPath>..\packages\Discord.Net.Commands.2.4.0\lib\net461\Discord.Net.Commands.dll</HintPath>
@@ -61,6 +64,12 @@
     <Reference Include="Microsoft.Bcl.AsyncInterfaces, Version=5.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
       <HintPath>..\packages\Microsoft.Bcl.AsyncInterfaces.5.0.0\lib\net461\Microsoft.Bcl.AsyncInterfaces.dll</HintPath>
     </Reference>
+    <Reference Include="Microsoft.Extensions.DependencyInjection, Version=5.0.0.2, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
+      <HintPath>..\packages\Microsoft.Extensions.DependencyInjection.5.0.2\lib\net461\Microsoft.Extensions.DependencyInjection.dll</HintPath>
+    </Reference>
+    <Reference Include="Microsoft.Extensions.DependencyInjection.Abstractions, Version=5.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60, processorArchitecture=MSIL">
+      <HintPath>..\packages\Microsoft.Extensions.DependencyInjection.Abstractions.5.0.0\lib\net461\Microsoft.Extensions.DependencyInjection.Abstractions.dll</HintPath>
+    </Reference>
     <Reference Include="Microsoft.Win32.Primitives, Version=4.0.2.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
       <HintPath>..\packages\Microsoft.Win32.Primitives.4.3.0\lib\net46\Microsoft.Win32.Primitives.dll</HintPath>
       <Private>True</Private>
@@ -250,6 +259,9 @@
       <SubType>Code</SubType>
     </Compile>
     <Compile Include="Bot\CedBot.cs" />
+    <Compile Include="Bot\Modules\PublicModule.cs" />
+    <Compile Include="Bot\Services\CommandHandlingService.cs" />
+    <Compile Include="Bot\Services\PictureService.cs" />
     <Compile Include="MainWindow.xaml.cs">
       <DependentUpon>MainWindow.xaml</DependentUpon>
       <SubType>Code</SubType>

CedBotSharp/MainWindow.xaml.cs

@@ -21,26 +21,12 @@ namespace CedBotSharp
     /// </summary>
     public partial class MainWindow : Window
     {
-        private readonly Bot.CedBot client;
-
         public MainWindow()
         {
             DotNetEnv.Env.Load(".env");
+            new Bot.CedBot(DotNetEnv.Env.GetString("token"));
 
             InitializeComponent();
-            
-            try
-            {
-                client = new Bot.CedBot();
-                client.Start(DotNetEnv.Env.GetString("token"));
-            }
-            catch (FormatException error)
-            {
-                MessageBoxResult result = MessageBox.Show(error.Message,
-                    error.StackTrace,
-                    MessageBoxButton.OK,
-                    MessageBoxImage.Error);
-            }
         }
 
         private void ExitItem_Click(object sender, RoutedEventArgs e)

CedBotSharp/packages.config

@@ -9,6 +9,8 @@
   <package id="DiscordRichPresence" version="1.0.175" targetFramework="net472" />
   <package id="DotNetEnv" version="2.1.1" targetFramework="net472" />
   <package id="Microsoft.Bcl.AsyncInterfaces" version="5.0.0" targetFramework="net472" />
+  <package id="Microsoft.Extensions.DependencyInjection" version="5.0.2" targetFramework="net472" />
+  <package id="Microsoft.Extensions.DependencyInjection.Abstractions" version="5.0.0" targetFramework="net472" />
   <package id="Microsoft.NETCore.Platforms" version="5.0.2" targetFramework="net472" />
   <package id="Microsoft.Win32.Primitives" version="4.3.0" targetFramework="net472" />
   <package id="NETStandard.Library" version="2.0.3" targetFramework="net472" />