DemoPlugin.cs

class Program
{
	static void Main(string[] args)
	{
		Console.WriteLine("Load plugin cetak nota..\n");

		var loader = new PluginLoader();
		loader.LoadPlugins();

		foreach (var plugin in PluginLoader.Plugins)
		{
			Console.WriteLine("Nama plugin: {0}", plugin.Name);
			Console.WriteLine("Contoh nota:\n");
			plugin.Cetak();

			Console.WriteLine("\n----------------------------------------------------------------\n");
		}

		Console.ReadKey();
	}
}

class PluginLoader
{
	public static List<IPluginCetakNota> Plugins { get; set; }

	public void LoadPlugins()
	{
		Plugins = new List<IPluginCetakNota>();

		var folderPlugins = Path.Combine(Directory.GetCurrentDirectory(), "Plugins");

		//Load the DLLs from the Plugins directory
		if (Directory.Exists(folderPlugins))
		{
			var files = Directory.GetFiles(folderPlugins);
			foreach (string file in files)
			{
				if (file.EndsWith(".dll"))
				{
					Assembly.LoadFile(Path.GetFullPath(file));
				}
			}
		}

		var interfaceType = typeof(IPluginCetakNota);

		//Fetch all types that implement the interface IPlugin and are a class
		var types = AppDomain.CurrentDomain.GetAssemblies()
			.SelectMany(a => a.GetTypes())
			.Where(p => interfaceType.IsAssignableFrom(p) && p.IsClass)
			.ToArray();

		foreach (var type in types)
		{
			//Create a new instance of all found types
			Plugins.Add((IPluginCetakNota)Activator.CreateInstance(type));
		}
	}
}