Windows Phone variable class type -

i want create a user class type object, , maybe later change class type. eg.

there 2 custom class:

class a{} class b{} 

then later want declare object changes it's type accordingly;

var obj;  if(x=0)     obj=new a(); else     obj=new b(); 

is possible in windows phone visual studio environment? note: want obj global variable, can use anywhere.

it possible, not great design. should have more typesafe, either having base class , b , variable object base type or have classes interfaced.

example:

class baseclass { }  class a: baseclass{} class b: baseclass{}  baseclass obj; 

or

interface iclass{}  class : iclass{} class b: iclass{}  iclass obj;