using System;

    using System.Linq;

    public class HelloWorldAttribute : System.Attribute  

    {  

        private string _someString;

        public HelloWorldAttribute(string text)  

        {  

            _someString = text;

        }  

        public void MyAttributeMethod()

        {

            Console.WriteLine("HelloWorldAttribute says '{0}'.", _someString);

        }

    }  

    public class MyClass

    {

        [HelloWorld("This is a test!!")]

        public void MyClassMethod(string text)

        {

            Console.WriteLine("MyClassMethod says '{0}'", text);

        }

    }

    public class Program

    {

        public static void Main()

        {

            //By itself, the attribute does nothing

            var c = new MyClass();

            c.MyClassMethod("Main call");

            //You need to write code to detect the attribute and make it do something

            typeof(MyClass)

                .GetMethod("MyClassMethod")

                .GetCustomAttributes(true)

                .OfType<HelloWorldAttribute>()

                .First()

                .MyAttributeMethod();

            c.MyClassMethod("Main call");

        }

    }