Orice este un obiect

using System;

namespace Namespace
{
    public interface IObject
    {
        void MyToString();
    }
    public class MyObject : IObject
    {
        public void MyToString(){}
    }
    public class Program
        {
            static void SomeFunction(object a, object b, IObject c, MyObject d)
            {
                Int32       i           = (int)a;            
                MyObject    obj_b       = b as MyObject;
                MyObject    obj_bb      = (MyObject)b;
                IObject     iobj_b      = (IObject)b;
                
                MyObject    obj_c       = (MyObject)c;
                MyObject    obj_cc      = c as MyObject;

                IObject     iobj_d      = d as MyObject;
                IObject     iobj_dd     = d;
                IObject     iobj_ddd    = (MyObject)d;
            }
            static void Main(string[] args)
            {
                Int32 i = 3;            
                MyObject myobj = new MyObject();
                
                SomeFunction(i, myobj, myobj, myobj);
            }
        }
}

Clasa de baza a fiecarei instante este System.Object. Orice instanta a clasei System.Object este capabila sa referentieze orice alta instanta a unei clase.
Mecanismul insa difera de la caz la caz si se poate observa foarte bine in codul IL:

.method private hidebysig static void  SomeFunction(object a,
                                                    object b,
                                                    class Namespace.IObject c,
                                                    class Namespace.MyObject d) cil managed
{
  // Code size       55 (0x37)
  .maxstack  1
  .locals init ([0] int32 i,
           [1] class Namespace.MyObject obj_b,
           [2] class Namespace.MyObject obj_bb,
           [3] class Namespace.IObject iobj_b,
           [4] class Namespace.MyObject obj_c,
           [5] class Namespace.MyObject obj_cc,
           [6] class Namespace.IObject iobj_d,
           [7] class Namespace.IObject iobj_dd,
           [8] class Namespace.IObject iobj_ddd)
//000019:             {
  IL_0000:  nop
//000020:                 Int32       i           = (int)a;            
  IL_0001:  ldarg.0
  IL_0002:  unbox.any  [mscorlib]System.Int32
  IL_0007:  stloc.0
//000021:                 MyObject    obj_b       = b as MyObject;
  IL_0008:  ldarg.1
  IL_0009:  isinst     Namespace.MyObject
  IL_000e:  stloc.1
//000022:                 MyObject    obj_bb      = (MyObject)b;
  IL_000f:  ldarg.1
  IL_0010:  castclass  Namespace.MyObject
  IL_0015:  stloc.2
//000023:                 IObject     iobj_b      = (IObject)b;
  IL_0016:  ldarg.1
  IL_0017:  castclass  Namespace.IObject
  IL_001c:  stloc.3
//000024:                 
//000025:                 MyObject    obj_c       = (MyObject)c;
  IL_001d:  ldarg.2
  IL_001e:  castclass  Namespace.MyObject
  IL_0023:  stloc.s    obj_c
//000026:                 MyObject    obj_cc      = c as MyObject;
  IL_0025:  ldarg.2
  IL_0026:  isinst     Namespace.MyObject
  IL_002b:  stloc.s    obj_cc
//000027: 
//000028:                 IObject     iobj_d      = d as MyObject;
  IL_002d:  ldarg.3
  IL_002e:  stloc.s    iobj_d
//000029:                 IObject     iobj_dd     = d;
  IL_0030:  ldarg.3
  IL_0031:  stloc.s    iobj_dd
//000030:                 IObject     iobj_ddd    = (MyObject)d;
  IL_0033:  ldarg.3
  IL_0034:  stloc.s    iobj_ddd
//000031:             }
  IL_0036:  ret
} // end of method Program::SomeFunction