How to pass an instance of of a class from within that class

I am wondering about something which can be achieved in c++ but I want to do it in C#.

How can I pass a "pointer" to a class, from within the class that contains the code, allowing me to write code in the form of the Foo.Bar()?

namespace Example
{
    public class Foo
    {
        MainClass MainClass;

        public void Bar()
        {
            var somethingElse = MainClass.A.SomethingElse;
            var somethingDifferent = MainClass.B.SomethingDifferent;
        }
    }

    public class MainClass
    {
        public string Something;

        public SubClassA A = new SubClassA(this); // <<-- not valid in c#
        public SubClassB B = new SubClassB(this); // <<-- not valid in c#
    }

    public class SubClassA
    {
        private MainClass _MainClass;

        public SubClassA(MainClass mainClass)
        {
            _MainClass = mainClass;
        }

        public string SomethingElse {
            get {
                return _MainClass.Something + " Else";
            }
        }
    }

    public class SubClassB
    {
        private MainClass _MainClass;

        public SubClassB(MainClass mainClass)
        {
            _MainClass = mainClass;
        }

        public string SomethingDifferent
        {
            get
            {
                return _MainClass.Something + " Different";
            }
        }
    }
}

I should state that I know I can restructure my code in a million ways to achieve a workable result, but I have a specific question I am hoping to answer! :)

Thanks