Question

I am looking for a visibility modifier for attributes of an inner class that would allow the outer class to modify/set a value but external classes could only get/read the value.

public class Outer {
    public class Inner {
        // I want this to be editable by Outer instances
        // but read-only to other external classes.
        public string attribute;
    }
}

Answer 1

You don’t have an access modifier for that, but you can get away with something like this:

public class Outer
{
    private static Action<Inner, string> InnerAttributeSetter;

    public class Inner
    {
        static Inner()
        {
            Outer.InnerAttributeSetter = (inner, att) => inner.Attribute = att;
        }

        public string Attribute { get; private set; }
    }

    public Outer()
    {
        var inner = new Inner();

        InnerAttributeSetter(inner, "Value");

        Console.WriteLine(inner.Attribute);
    }
}

Basically you taking advantage of the fact that nested classes have access to private members of the enclosing class and providing for the enclosing class a proxy to set the attribute property for a given Inner instance. Since external classes do not have access to this proxy you satisfied your requirement.