class Program
{
    static void Main(string[] args)
    {
      while(true){
        Somefunc();
      }
    }

    static void Somefunc()
    {
       Console.WriteLine("Hello World");
    }
}

class Program
 {
     static void Main(string[] args)
     {
       while(true){
          if(App1.SomeFuncIsCalled)
             InjectCode();
         }
     }

    static void InjectCode()
    {
       App1.Console.WriteLine("Hello World Injected");
    }
 }

Hello World
Hello World Injected

class Program
{
    [ImportMany("AddinContractName", typeof(IRunMe))]
    public IEnumerable<IRunMe> ThingsToRun { get; set; }

    void SomeFunc()
    {
        foreach(IRunMe thing in ThingsToRun)
        {
            thing.Run();
        }
        /* do whatever else ... */
    }
}