class Program
    {
        static void Main(string[] args)
        {
            var pls = PluginLoader.LoadPlugins(Directory.GetCurrentDirectory());

            foreach(var pl in pls)
            {
                pl.DoSmth("plugin call");
                pl.DoSmth("plugin call");
                pl.DoSmth("plugin call");
            }

            Console.ReadLine();
            
        }
    }

    public interface IPlugin
    {
        string Id { get; }
        void DoSmth(string txt);
    }

    public static class PluginLoader
    {
        public static IReadOnlyCollection<IPlugin> LoadPlugins(string path)
        {
            var result = new List<IPlugin>();
            var potencialPlugins = Directory.GetFiles(path, "*.dll");

            foreach (string item in potencialPlugins)
            {
                if (Path.GetFileName(item) == "PluginCommon.dll")
                    continue;

                Assembly ass = Assembly.LoadFrom(item);
                List<Type> targetTypes = ass.GetExportedTypes().Where(t => typeof(IPlugin).IsAssignableFrom(t)).ToList();

                if (targetTypes.Count == 0)
                    continue;

                foreach (Type pluginType in targetTypes)
                    result.Add((IPlugin) Activator.CreateInstance(pluginType));
            }

            return result.AsReadOnly();
        }
    }

    public class PluginClass : IPlugin
    {
        public string Id { get => "Plugin1"; }

        public void DoSmth(string txt)
        {
            Console.WriteLine($"{Id} - {txt}");
        }
    }

    public class Plugin2 : IPlugin
    {
        public string Id => "Plugin2";

        public void DoSmth(string txt)
        {
            Console.WriteLine($"{Id} - {txt}");
        }
    }

Plugin1 - plugin call
Plugin1 - plugin call
Plugin1 - plugin call
Plugin2 - plugin call
Plugin2 - plugin call
Plugin2 - plugin call