Question about method with ref argument

[Grofit]

Hey,

Hopefully a simple question, heres some code:

class A { public int Var1; }
class B : A { public int Var2; }

class C
{
   public static A GetA() 
   {
      A Var = new A();
      generateA(Var);
      return Var;
   }

   public static A GetB()
   {
      A Var = new B();
      generateA(ref Var);
      generateB(ref Var); // Error
      return Var;
   }

   protected static generateA(ref A Var)
   { Var.Var1 = 1; }

   protected static generateB(ref B Var)
   { Var.Var2 = 2; }

}

Now thats just a simple example of what im trying to do, but it seems to kick up a fuss when i try to pass over a base type that is acctually a subclass. In my real code i have a few methods that build up classes internal members, so one method for each sub class, that way i can reuse the code to build up the subclasses by populating in order...

Anyway hopefully that shows what im trying to do, any help or advice would be great!