How specialize member type for a derived class in C#?

Question

I would like to do something like this:

Where Plan has a Contact Property and there is a specialization of Plan and Contact.

I tried

abstract class Contact { }

class SpecialContact : Contact { }

abstract class Plan {
   public virtual Contact contact;
}

class SpecialPlan : Plan {
   public override SpecialContact contact;
}

But the type override are forbidden in C#.

How can I model this and specialize the sub property of a sub class?

I Just need to enforce that the SpecialPlan will only have SpecialContact.. not any other Contact

Answer 1

void Main()
{
    var sp = new SpecialPlan();
    sp.Contact = new SpecialContact(); // new Contact(); won't compile
}

abstract class Contact
{
}

class SpecialContact : Contact
{
}

abstract class Plan<T> where T: Contact
{
    public T Contact { get; set; }
}

class SpecialPlan : Plan<SpecialContact>
{
}

If you want to override the get/set property.

void Main()
{
    var sp = new SpecialPlan();
    sp.Contact = new SpecialContact(); // new Contact(); won't compile
}

abstract class Contact
{
}

class SpecialContact : Contact
{
}

abstract class Plan<T> where T: Contact
{
    protected T contact;
    public virtual T Contact { get { return contact; } set { contact = value; } }
}

class SpecialPlan : Plan<SpecialContact>
{
    public override SpecialContact Contact { get { return contact; } set { contact = value; } }
}