Simple demonstration of how polymorphism through method overriding and hiding works in C#

MethodOverridingDemo.cs

using System;

namespace PolymorphismDemo
{
    public class A
    {
        public virtual void Foo() => Console.WriteLine("A.Foo()");
    }

    public class B : A
    {
        public new void Foo() => Console.WriteLine("B.Foo()");              // Static binding (hiding)
    }

    public class C : A
    {
        public override void Foo() => Console.WriteLine("C.Foo()");         // Dynamic binding (overriding)
    }
    
    /*public class D : A                                                    // WARNING: new or override required
    {
        public void Foo() => Console.WriteLine("D.Foo()");
    }*/

    public class E : C
    {
        public sealed override void Foo() => Console.WriteLine("E.Foo()");  // C.Foo() is implicitly virtual
    }

    /*public class F : E                                                    // ERROR: sealed method cannot be overridden
    {
        public override void Foo() => Console.WriteLine("F.Foo()");
    }*/

    public class G : E
    {
        public new void Foo() => Console.WriteLine("G.Foo()");              // ... but it can be hidden
    }

    class MethodOverridingDemo
    {
        static void Main(string[] args)
        {
            A thing = new A();
            B thing2 = new B();
            C thing3 = new C();

            A thing4 = thing2;
            A thing5 = thing3;

            A thing6 = new E();
            A thing7 = new G();

            thing.Foo();     // A.Foo()
            thing2.Foo();    // B.Foo()
            thing3.Foo();    // C.Foo()

            thing4.Foo();    // A.Foo()
            thing5.Foo();    // C.Foo()

            thing6.Foo();    // E.Foo()
            thing7.Foo();    // E.Foo()
        }
    }
}