using System;

public class Program

{

    public static void Main()

    {

        BaseClass data = new BaseClass();

        ExtendedClass test1 = new ExtendedClass(data);

        ExtendedClass test2 = new ExtendedClass(data);

​

        test1.SomeData = "new data";

​

        Console.WriteLine("Data = '{0}', wanted it to be: '{1}'", test2.SomeData,test1.SomeData);

    }

    class ExtendedClass

    {

        private BaseClass _data;

        public ExtendedClass(BaseClass data)

        {

            _data = data;

            SomeSpecificClientData = 20;

        }

​

        public int SomeSpecificClientData { get; private set; }

        public string SomeData

        {

            get => _data.SomeData;

            set => _data.SomeData = value;

        }

    }

    class BaseClass

    {

        public BaseClass()

        {

            SomeData = "data";

        }

​

        public string SomeData { get; set; }

    }

}