Using base class and base interface in C#

Question

I am reshaping an entire system that does not use base classes and base interfaces.

My idea to do so is to extract all the common methods to a base classes and base interfaces.
So basically, we would have:

• A base class SomeClassBase implementing an interface ISomeClassBase

• A derived class SomeClassDerived implementing ISomeClassDerived (this interface deriving from ISomeClassBase)

Now the problem, how can I instantiate "_mySession" in the derived class (which has a different cast than in the base class), while preserving all the methods from the base class:

public class SomeClassBase : ISomeClassBase 
{
 public IMySessionBase _mySession = MySession.Instance();

 public SomeClassBase ()
 {
   _mySession.connect();  // Needed??
 }

 public void doSomething()
 {
  _mySession.doSomething();
 }
}

public class SomeClassDerived : SomeClassBase, ISomeClassDerived  
{
 public IMySessionDerived _mySession = MySession.Instance();

 public SomeClassDerived ()
 {
  _mySession.connect();
 }

 public void doSomethingElse()
 {    
  _mySession.doSomethingElse();
 }
}

One more thing, IMySessionDerived implements IMySessionBase.

Answer 1

Pasting @Selvin answer instead of the link buried in the comments:
The trick here is to use the keyword "base()"

using System;
using System.Runtime.CompilerServices;

public class Program
{
    public static void Main()
    {
        var o1 = new O1();
        o1.DS1();
        var o2 = new O2();
        o2.DS1();
        o2.DS2();
    }

    public class Session1
    {
        protected readonly Type ownerType;
        public Session1(Type type)
        {
            ownerType = type;
        }

        public virtual void DS1([CallerMemberName] string functionName = "")
        {
            Console.WriteLine(ownerType.Name + ":" +  GetType().Name + ":" + functionName);
        }       
    }

    public class Session2 : Session1
    {
        public Session2(Type type):base(type) { }

        public virtual void DS2([CallerMemberName] string functionName = "")
        {
            Console.WriteLine(ownerType.Name + ":" +  GetType().Name + ":" + functionName);
        }   
    }

    public class O1
    {
        private readonly Session1 t;

        public O1() : this(new Session1(typeof(O1))) { }
        protected O1(Session1 t)
        {
            this.t = t;
        }
        public void DS1()
        {
            t.DS1();
        }
    }

    public class O2 : O1
    {
        private readonly Session2 t;

        public O2() : this(new Session2(typeof(O2))) { }
        protected O2(Session2 t) : base(t)
        {
            this.t = t;
        }

        public void DS2()
        {
            t.DS2();
        }
    }
}