C# class extensibility pattern

gistfile1.cs

public interface ISomeExtension
    {
        object Private { get; set; }
    }

    public static class SomeExtension
    {
        class State
        {
            public string data;
        }

        public static void DoThis(this ISomeExtension that, string message)
        {
            that.Private = new State { data = message };
        }

        public static string DoThat(this ISomeExtension that, string message)
        {
            var state = that.Private as State;
            if ( state != null )
            {
                return state.data + message;
            }
            else
            {
                return message;
            }
        }
    }

    class MyClass : ISomeExtension
    {
        object ISomeExtension.Private { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyClass mc = new MyClass();
            mc.DoThis("Data");
            Console.WriteLine(mc.DoThat("AndData"));
        }
    }