C# nested class/struct visibility

I'm trying to figure out what the proper syntax is to achieve a certain API goal, however I am struggling with visibility.

I want to be able to access a Messenger instance's member like msgr.Title.ForSuccesses.

However, I do not want to be able to instantiate Messenger.Titles from outside my Messenger class.

I'm also open开发者_开发技巧 to making Messenger.Titles a struct.

I'm guessing I need some sort of factory pattern or something, but I really have no idea how I'd go about doing that.

See below:

class Program {
    static void Main(string[] args) {
        var m = new Messenger { Title = { ForErrors = "An unexpected error occurred ..." } }; // this should be allowed
        var t = new Messenger.Titles(); // this should NOT be allowed
    }
}

public class Messenger {
    // I've tried making this private/protected/internal...
    public class Titles {
        public string ForSuccesses { get; set; }
        public string ForNotifications { get; set; }
        public string ForWarnings { get; set; }
        public string ForErrors { get; set; }

        // I've tried making this private/protected/internal as well...
        public Titles() {}
    }

    public Titles Title { get; private set; }
    public Messenger() {
        Title = new Titles();
    }
}

You just need to make Titles private and expose an interface instead of it.

class Program {
    static void Main(string[] args) {
        var m = new Messenger { Title = { ForErrors = "An unexpected error occurred ..." } }; // this is allowed
        var t = new Messenger.Titles(); // this is NOT allowed
    }
}

public class Messenger {
    public interface ITitles {
        string ForSuccesses { get; set; }
        string ForNotifications { get; set; }
        string ForWarnings { get; set; }
        string ForErrors { get; set; }
    }

    private class Titles : ITitles {
        public string ForSuccesses { get; set; }
        public string ForNotifications { get; set; }
        public string ForWarnings { get; set; }
        public string ForErrors { get; set; }
    }

    public ITitles Title { get; private set; }
    public Messenger() {
        Title = new Titles();
    }
}

If you make the Titles constructor internal you will be able to create instances of it within your assembly only. If it is an API, perhaps that will be protected enough? You can see this pattern within the BCL (such as HttpWebRequest that can be created only through calls to WebRequest.Create).

Why Would I Ever Need to Use C# Nested Classes Nested type is never intended to be initialized from external type.

Well, you could make Titles a struct and make the constructor either public or internal. In that way, every time a client gets a copy of the Titles instance through the Title property, they will be getting the value, not the reference. They could modify that value, but to apply that change to the internal state of your object, they would need to be able to set the value back again through the Title property. They can't, because you have the Title setter marked private.

You will have to do the same when you change a value internally. For example:

// Your constructor...
public Messenger()
{
    Titles t = new Titles();
    t.ForSuccesses = "blah";
    Title = t;
}

You can do this internally because you have access to the private setter for the Title property.

The main downside is that it might confuse the clients of your framework a bit because it looks like you can set the values of the Titles instance, but there is no real way for them to commit that change back to the Messenger class.