Discord.Net/samples/TextCommandFramework/Modules/PublicModule.cs

using Discord;
using Discord.Commands;
using System.IO;
using System.Threading.Tasks;
using TextCommandFramework.Services;

namespace TextCommandFramework.Modules
{
    // Modules must be public and inherit from an IModuleBase
    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()
        {
            // Get a stream containing an image of a cat
            var stream = await PictureService.GetCatPictureAsync();
            // Streams must be seeked to their beginning before being uploaded!
            stream.Seek(0, SeekOrigin.Begin);
            await Context.Channel.SendFileAsync(stream, "cat.png");
        }

        // Get info on a user, or the user who invoked the command if one is not specified
        [Command("userinfo")]
        public async Task UserInfoAsync(IUser user = null)
        {
            user ??= Context.User;

            await ReplyAsync(user.ToString());
        }

        // Ban a user
        [Command("ban")]
        [RequireContext(ContextType.Guild)]
        // make sure the user invoking the command can ban
        [RequireUserPermission(GuildPermission.BanMembers)]
        // make sure the bot itself can ban
        [RequireBotPermission(GuildPermission.BanMembers)]
        public async Task BanUserAsync(IGuildUser user, [Remainder] string reason = null)
        {
            await user.Guild.AddBanAsync(user, reason: reason);
            await ReplyAsync("ok!");
        }

        // [Remainder] takes the rest of the command's arguments as one argument, rather than splitting every space
        [Command("echo")]
        public Task EchoAsync([Remainder] string text)
            // Insert a ZWSP before the text to prevent triggering other bots!
            => ReplyAsync('\u200B' + text);

        // 'params' will parse space-separated elements into a list
        [Command("list")]
        public Task ListAsync(params string[] objects)
            => ReplyAsync("You listed: " + string.Join("; ", objects));

        // Setting a custom ErrorMessage property will help clarify the precondition error
        [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!");
    }
}