Sealing an interface after implementing it

Question

I am working on a small project and I came across that problem.

The project output is a library containing an interface. I would like to implement that interface and

Answer 1

Your understanding of sealed keyword is incorrect. As a method modifier, sealed is used to prevent a virtual method(defined in the base class) to be override in the next generation of derived classes. For example:

class Base
{
   public virtual void M() { }
}

class Derived : Base
{
   public sealed override void M() { }
}

class A : Derived
{
   public override void M() { } //compile error, M is sealed in Derived
}

Developers can always use new modifier to define a method with the same name in the derived class, that hides the one defined in the base class.

if someone create a specialized class of type A, he/she won't be able to change the method's behavior.

If "specialized class" means a class derived from A, the answer is: he can always hide the method in A, but he can't change the method's behavior.