C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Test_Inheritance
{
    class Program
    {
        static void Main(string[] args)
        {
            B test = new B();
            test.PrintString();
        }
    }

    class A
    {
        string x = "Hello1";

        public void PrintString()
        {
            Console.WriteLine(x);
        }

    }

    class B:A
    {
        string x = "Hello2";
    }
}

class BaseFileClass
{
    protected void ProcessFile();
}

class XMLClass : BaseFileClass
{
    public override void ProcessFile()
    {
        //Process XML File
    }
}

class CSVCLass: BaseFileClass
{
    public override void ProcessFile()
    {
        //Process CSV File
    }
}


class ExcelClass: BaseFileClass
{
    public override void ProcessFile()
    {
        //Process Excel File
    }
}

Base b = null;
if( isXMl)
{
    b = new XMLCLass();
}
else if( isExcel)
{
    b = new ExcelCLass();
}
else if( isCSV)
{
    b = new CSVCLass();
}

b.ProcessFile();