Base to Derived is possible(Boxing), But Derived to base gives run time exception, however that is equivalent to Unboxing

class SampleClass
{
   public int i = 10;
}
class MainClass
{
   static void Main()
   {
      object a;
      a = 1;   // an example of boxing
      Console.WriteLine(a);
      Console.WriteLine(a.GetType());
      Console.WriteLine(a.ToString());
      a = new SampleClass();
      SampleClass classRef;
      classRef = (SampleClass)a; //Example of Unboxing
      Console.WriteLine(classRef.i);
   }
}

All classes in .net implicitly inherited from Object Class.In above code

classRef = (SampleClass)a; //Example of Unboxing is possible.

 class BaseSample
    {
        public int ii = 10;
    }
    class SampleClass:BaseSample
    {
        public int i = 10;
    }

    class MainClass
    {
        static void Main()
        {

  SampleClass classRef;

    BaseSample b = new BaseSample();
            b.ii = 90;

   classRef = (SampleClass)b; //Getting Run time Exception

}

Why I am gettting run time exception,

Boxing and UnBoxing is possible

 public int ConvertToINT(object obj)
        {
            int i = (int)obj;
            return Convert.ToInt32(obj);
        }
        public string CovertToString(object obj)
        {
            string s = (string)obj;
            //return Convert.ToString(s);
            return s;
        }

Convert.ToInt32(obj); This is Base to derived and possible,

Butwhen

Base b=new Base();

Child c=(Child)b;//Gives run time exception

Why is it So.

Please help