Inheritance : does not contain a definition for and no extension method accepting a first argument

Question

abstract class Parent
{
        protected string attrParent;

        public AttrParent { get; protected set }

        public Parent(string sParent)
        {
            AttrParent = sParent;
        }
}

class Child : Parent
{
        private string attrChild;

        public AttrChild { get; private set }

        public Child(string sParent, string sChild) : base(sParent)
        {
            AttrChild = sChild;
        }
}

class Program
{
        static void Main(string[] args)
        {
            Parent p = new Child();

            p.AttrChild = "hello";
        }
}

When I run this program, I get the following error :

'Example.Parent' does not contain a definition for 'AttrChild' and no extension method 'AttrChild' accepting a first argument of type 'Example.Parent'"

Can anybody explain why this is?

Answer 1

The moment you assign Child instance to variable typed as Parent you can only access members declared on Parent.

You'd have to downcast it back to Child to access Child-only members:

Parent p = new Child();

Child c = (Child)p;
c.AttrChild = "hello";

That cast might fail at runtime, because there might be a different class that inherits Parent.