Using Interface to add a private setter
===

I have an Interface that has only one property like below

    public interface IExtendable
    {
          Hashtable ExtendedProperties { get; set;}
    }

I want setter to be private but it doesnt allow because its coming from an Interface and everyhing should be public in it as I know.

So what can be the best practice to allowing private setter ? 

- Dont use Interface in this scenario or what ?

Thanks in advance,