What does immutable and readonly mean in C#?

Question

Is it correct that it is not possible to change the value of an immutable object?

I have two scenarios regarding readonly that I want to understand:

1. What if I have a collection and mark it as readonly, like the following. Can I still call _items.Add?

   private readonly ICollection<MyItem> _items;
   

2. And also for the following variable, if later on I call _metadata.Change which will change the internal values of a couple member variable in the Metadata instance. Is _metadata still immutable?

   private readonly Metadata _metadata;
   

For both variables above, I totally understand that I can't directly assign new values to them outside of initializer and constructors.

Answer 1

I suggest you to read the series of blog posts by Eric Lippert. The first part is Immutability in C# Part One: Kinds of Immutability. Very informative and helpful, as always. The series describes what does it mean for a variable to be readonly, immutable etc. in details.

Generally, readonly means only that you can't re-assign a field outside the constructor. The field itself can be modified as long as it stays the same instance. So yes, you can add elements to the collection stored in readonly field.

About mutability, this is more complex and it depends a bit what kind of mutability you consider. When Metadata internal values are references and those references itself (the instances it point to) doesn't change, you could say Metadata stays not mutated. But it is mutated logically. See Eric's posts for more insights.

Answer 2

Marking a field as read-only only means that you cannot change the value of that field. It has no bearing on the internal state of the object there. In your examples, while you would not be able to assign a new metadata object to the _metadata field, nor a new ICollection to the _items field (outside of a constructor that is), you can change the internal values of the existing objects stored in those fields.