Question

Why I can implement a read-only property like so…

public IList<object> SelectedItems { get; }

…and still be able to use its members, like Add, Remove, etc. when I use the property ?

Isn’t readonly supposed to be readonly “all the way down” ?

Answer 1

isn’t readonly supposed to be readonly “all the way down” ?

1. This isn’t a “readonly” type. It’s a property where you can retrieve the value (the IList<object>), but can’t set the value to a different instance of IList<object> as there is no property setter.

2. No. It’s not readonly “all the way down” – the fact that the property only has a getter means you can get the value, but you can do anything you wish with it.

Note that I’d be careful using the term “readonly”. C# has a different, distinct meaning and dedicated readonly keyword, which applies only to fields. This isn’t “readonly” in the C# meaning, but a property which only provides a get operation.

If you want to have a “read only” list, a good option is to return a ReadOnlyCollection<T>. For example, if your internal class is a List<T>, you can use:

public IList<object> SelectedItems { get { return this.selected.AsReadOnly(); } }

If you are going to do this, it might be better to actually return the ReadOnlyCollection<T> as well:

public ReadOnlyCollection<object> SelectedItems { get { return this.selected.AsReadOnly(); } }

This way, users won’t expect that it’s safe to call .Add() on the resulting list.