Is there a difference between having a private setter OR only defining a getter?

Question

I need to make a class that wraps two dictionaries together, so that their values can be retrieved by a key of either an int or a string.

Properties seem to be the best approach here, but is there a difference between these two implementations?

public class Atlas<TValue>
{
    private Dictionary<int, TValue> _byIndex;
    private Dictionary<string, TValue> _byName;

    public Dictionary<int, TValue> ByIndex
    {
        get { return _byIndex; }
    }

    public Dictionary<string, TValue> ByName
    {
        get { return _byName; }
    }
}

And

public class Atlas<TValue>
{
    public Dictionary<int, TValue> ByIndex { get; private set; }
    public Dictionary<string, TValue> ByName { get; private set; }
}

In either case, the dictionary object is immutable and the elements can freely be changed, which is what I want. However, trying to change the dictionary object will result in either a ~ cannot be assigned to -- it is read only or a ~ cannot be used in this context because the set accessor is inaccessible. I realize the compiler will fluff out my auto properties into something similar to the top block of code anyways...

Does it actually matter which compiler error is raised?

Answer 1

The only difference is that in the second case the setter is inaccessible, but it is there, while in the firs case there is no accessor at all. This means that a program that uses reflection could potentially access the properties of the second example, while in case of the first example you would need to access fields instead.

As far as non-reflective use is concerned, there is no difference between the two code snippets: outside classes will not be able to set the dictionaries.

You may want to go further and hide the presence of the dictionaries from the users of your classes. Rather than providing two properties of Dictionary type, you may want to hide this detail of implementation from the users of your class by hiding it behind a pair of method:

public class Atlas<TValue> {
    public bool TryGetByIndex(int index, out TValue val);
    public void Add(int index, TValue val);
    public bool TryGetByName(string name, out TValue val);
    public void Add(string name, TValue val);
    public TValue this[string name] { get ... set ...}
    public TValue this[int index] { get ... set ...}
    // You may want to add more methods or properties here, for example to iterate atlas elements
}