Q:

Safeguarding against user error when saving a list of information

I have a private List<Experience> experiences; that tracks generic experiences and experience specific information. I am using Json Serialize and Deserialize to save and load my list. When you start the application the List populates itself with the current saved information automatically and when a new experience is added to the list it saves the new list to file. 
A concern that is popping into my head that I would like to get ahead of is, there is nothing that would stop the user from at any point doing something like experiences = new List<Experience>(); and then adding new experiences to it. Saving this would result in a loss of all previous data as right now the file is overwritten with each save. In an ideal world, this wouldn't happen, but I would like to figure out how to better structure my code to guard against it. Essentially I want to disallow removing items from the List or setting the list to a new list after the list has already been populated from load.
I have toyed with the idea of just appending the newest addition to the file, but I also want to cover the case where you change properties of an existing item in the List, and given that the list will never be all that large of a file, I thought overwriting would be the simplest approach as the cost isn't a concern.
Any help in figuring out the best approach is greatly appreciated.
Edit* Looked into the repository pattern https://www.infoworld.com/article/3107186/application-development/how-to-implement-the-repository-design-pattern-in-c.html and this seems like a potential approach.

A:

I'm making an assumption that your user in this case is a code-level consumer of your API and that they'll be using the results inside the same memory stack, which is making you concerned about reference mutation.
In this situation, I'd return a copy of the list rather than the list itself on read-operations, and on writes allow only add and remove as maccettura recommends in the comments. You could keep the references to the items in the list intact if you want the consumer to be able to mutate them, but I'd think carefully about whether that's appropriate for your use case and consider instead requiring the consumer to call an update function (which could be the same as your add function a-la HTTP PUT).

