Properties with separate types when referenced internally and externally

[ekolis]

I find myself coding properties like so:

// private backing field that can be manipulated inside my class
private IList<string> names = new List<string>();

// expose it publicly but make it read only, so you can't go adding your own items or clearing it or whatever
public IEnumerable<string> Names => names;

It would be nice if there were some way to combine the backing field into the property somehow, so that from a public context (outside the class) the property appears to be IEnumerable<string>, but from a private context (inside the class) it appears to be IList<string>.

I'm not sure what the syntax for this would look like, maybe someone has an idea?

Of course the external type of a property would have to be a supertype of the property's internal type (or maybe an implicitly convertible type?), if they're not the same; it wouldn't make sense to declare a string property that appears to be an int from the outside!

[CyrusNajmabadi]

private readonly List<string> _names = new();
public IEnumerable<string> Names => _names;

Note: your pattern isn't safe here at all. A consumer can still cast the result back to a List and mutate it.

[HaloFour]

I don't think this use case warrants having a new form of auto-property. I just don't think it's common enough. Syntactically I don't think it makes sense to try to jam two members with different accessibility levels, different names and different types into a single member. And auto-properties also don't allow the class to access the backing field directly, except in the one very specific case of read-only auto-properties initialized within the constructor.