Setting a get-only property on an object in a collection

Question

I have a similar issue to this other post that I have class

public class MyClass
{
    // other properties
    public bool IsDefault { get; internal set; }
}

and I have a collection class defined like

public class MyCollection : ICollection<MyClass>
{
    private List<MyClass> _itemList = new List<MyClass>();

    public void SetAsDefault(MyClass item)
    {
        // Make sure no other items are default.
        _itemList.ForEach(x => x.IsDefault = false);

        // Set specified item as default.
        _itemList.Find(x => x.Equals(item)).IsDefault = true;
    }
}

I want to make sure that only that single item in MyCollection is flagged as the default, while the same time being able to check directly on MyClass whether a particular instance is the default. Marking the setter for MyClass.IsDefault as internal developers outside the assembly can't set IsDefault . However, in my case, the developers I want to avoid faultily setting the IsDefault property has access inside the assembly, so using internal isn't enough.

Are there any clever solutions to this conundrum? I'm completely at a loss here...

Answer 1

I think this is really hard to cleanly get right. Since every other developer could just set the Property somewhere else, I would try and turn the problem arround.

You could remove the Objects property and make it a property on the collection. Since someone can retrieve the MyClass instance and set it anyway, you could add a method to your MyCollection that checks wether a MyClass instance is the default in that collection.

Otherwise I don't see any good way to really forbid what you want.

Answer 2

If you want to get or set instance that is default for specific collection you need property:

public MyClass Default { get; set; }

defined on MyCollection class.

If you want to know whether given instance of MyClass is default for specific collection you need method:

public bool IsDefaultFor(MyCollection collection) {
    return collection.Default == this;
}

If you require default instance to be member of the collection, add such assertion in setter of Default property and Remove method.