Need help understanding "readonly" in C#

Question

I'm trying to get my head around this. What's the difference between these 4?

 1. private ObservableCollection<T> _myObservableCollestionOfTs
 2. private readonly ObservableCollection<T> _myObservableCollestionOfTs
 3. private ReadOnlyObservableCollection<T> _myObservableCollestionOfTs
 4. private readonly ReadOnlyObservableCollection<T> _myObservableCollestionOfTs

Thank you for any help :)

Answer 1

1. A mutable reference to a mutable collection. The collection's elements can be changed, and the reference pointing to the collection can be reassigned to a different collection or set to null.

2. An immutable reference to a mutable collection. The collection's elements can be changed, but the reference pointing to the collection cannot.

3. A mutable reference to an immutable collection. The collection's elements cannot be changed, but the reference can be pointed to a different collection.

4. An immutable reference to an immutable collection. Neither the collection's elements nor the reference pointing to the collection can be changed.

Read-only collections are simply wrapper classes around ordinary collections that prevent modification of the collection's elements; see the remarks here .