using System;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var response = new RestResponse<ResponseData>() {

            Data = new ResponseData() { 

                reply = new DerivedReply() {

                    AdditionalData = "", 

                    error = null

                } 

            } 

        };

        Log(response);

    }

    public class Error {}

    public interface IReply {

        Error error {get;set;}

    }

    public class BaseReply : IReply {

        public Error error {get; set;}

    }

    public class DerivedReply : BaseReply {

        public string AdditionalData{get; set;}

    }

    public interface IResponseData {

        IReply reply {get;}

    }

    public class ResponseData : IResponseData 

    {

        public DerivedReply reply {get; set;}

        IReply IResponseData.reply {get { return this.reply;}}

    }

​

    public interface IRestResponse

    {

    }

​

    public interface IRestResponse<T> : IRestResponse

    {

        T Data

        {

            get;

            set;

        }

    }

​

    public class RestResponse<T> : IRestResponse<T>

    {

        public T Data

        {

            get;

            set;

        }

    }

​

    public static void Log(IRestResponse response)

    {

        var data = (response as IRestResponse<IResponseData>).Data; // Here I want do get data as IResponseData, but NullRef occurs

        Console.WriteLine(data.ToString());

    }

}