>I wrote an interface that defines a property, then a class to implment it.
>
>I tried to define the property in the interface with an internal set, but the compiler won't allow assesors in an interface. Does this mean that any property that stems from an interface must be public?

Yes. All interface properties are, by definition, public.

> How do I keep another class from from a different assembly from setting this property?

If you don't want classes in another assembly to access the properties via the interface then make the interface itself internal.