Consider the following two class definitions:

class ClassX { public virtual void f() { WriteLine("f in X"); } public void g() { WriteLine("g in X"); } } class ClassY : ClassX { public override void f() { WriteLine("f in Y"); } public void h() { WriteLine("h in Y"); } } 

For each of the following declarations and initializations, tell which are syntactically valid and which are not.

ClassX x1 = new ClassX();

Valid

Invalid

ClassX x2 = new ClassY();

Valid

Invalid

ClassY y1 = new ClassX();

Valid

Invalid

ClassY y2 = new ClassY();

Valid

Invalid