Discord.Net/docs/guides/samples/command_handler.cs

using System.Threading.Tasks;
using System.Reflection;
using Discord;
using Discord.Commands;

public class Program
{
    private CommandService commands;
    private DiscordSocketClient client;

    static void Main(string[] args) => new Program().Start().GetAwaiter().GetResult();

    public async Task Start()
    {
        client = new DiscordSocketClient();
        commands = new CommandService();

        string token = "bot token here";

        await InstallCommands();

        await client.LoginAsync(TokenType.Bot, token);
        await client.ConnectAsync();

        await Task.Delay(-1);
    }

    public async Task InstallCommands()
    {
        // Hook the MessageReceived Event into our Command Handler
        client.MessageReceived += HandleCommand;
		// Discover all of the commands in this assembly and load them.
        await commands.LoadAssembly(Assembly.GetEntryAssembly());
    }
    public async Task HandleCommand(IMessage msg)
	{
		// Internal integer, marks where the command begins
		int argPos = 0;
		// Get the current user (used for Mention parsing)
		var currentUser = await client.GetCurrentUserAsync();
		// Determine if the message is a command, based on if it starts with '!' or a mention prefix
		if (msg.HasCharPrefix('!', ref argPos) || msg.HasMentionPrefix(currentUser, ref argPos)) 
		{
			// Execute the command. (result does not indicate a return value, 
			// rather an object stating if the command executed succesfully)
			var result = await _commands.Execute(msg, argPos);
			if (!result.IsSuccess)
				await msg.Channel.SendMessageAsync(result.ErrorReason);
		}
	}

}