C# 4.0 project targeting earlier framework-Collection of common programming errors

In the specific case of optional parameters, compatibility will work as default values to use are stored in the caller’s assembly and not in the called assembly so compatibility with other assemblies is ensured. If it compiles, it will run.

Optional parameters are just a syntaxic sugar. The following code compiles and run for a target framework 2.0 :

internal class Program
{
    public static class DummyClass
    {
        public static string Bar(int b = 10, int a = 12)
        {
            return a.ToString();
        }
    }

    private static void Main(string[] args)
    {
        Console.WriteLine("{0}", DummyClass.Bar(a: 8));

        Console.ReadKey();
    }
}

Read a full explanation by Mr Botelho