How to Set the on Object NUll in C#

How to Set the on Object NUll in C#

i have an Object X. In this X x1=new X(); Here x1 has 3 properties say
x1.a; x1.b; x1.c; Here a b and c are objects of other class. Irrespective
of these all i want to set the whole object x1 to Null. or set each of
x1.a=null; x1.b=null;
Is there any solution to accomplish.