>I wrote an interface that defines a property, then a class to implment it.
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>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? How do I keep another class from from a different assembly from setting this property?

when you define an Interface it is just the contract which defines what must be implemented. In your concrete class that implements that interface, you can set the property to something other than public when you implement the property.
Tim