use of virtual/override on class variables as I can

Question

Why can't I use virtual/override on class variables as I can on methods?

Like for example we have 2 classes

 

class A

{

public virtual int x;

}

class B : A

{

public override  int x;

}

  when i compile this above code it display error like "The modifier 'virtual' is not valid for this item"???

why display this type of error?? please help me in details .