public class Program
{
    public static void Main()
    {
        Outer.CreateInstance(5);
    }
}

public static class Outer
{
    public static void CreateInstance(int a)
    {
        new Nested(a);
    }

    private class Nested
    {
        int a;
        private Nested(int a)
        {
            this.a = a;
        }
    }
}