How do I expose a private dictionary<K, V> as a read-only collection?

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I suspect there is no quick answer to this, but I'll ask anyway.

I have a class that has:

C#

private Dictionary<string,string> _records = new Dictionary<string,string>();

I would like to expose this to another class but don't want this other class to be able to modify the dictionary in any way - i.e. I don't want it to be able to assign a different dictionary and I don't want it to be able to modify the contents of the existing dictionary.

The number of entries in the dictionary is low - never more than the low tens - so performance is not an issue.

I'm not sure how to get the compiler version, but I am targeting .NET 4.5.2. I seem to have all modern syntax I read about with some exceptions, such as named tuple fields, so I think that puts me on C# 5 (or I could be talking rubbish)?

What is the best way of doing this?

Incidentally, in this particular case, I have a fair amount of flexibility about how these two classes interact. In particular, it would be fine if I could just return an unmodifiable List<string> of the keys in the dictionary. Then I could just add another function to return the value for a specified key.

I could also ditch the Dictionary<string,string> altogether.

What I have tried:

I have done quite a bit of Googling on this subject, but there just seems to be a tonne of contradictory information.

Allowing some form of iteration over a privately maintained collection seems like such a common requirement that I am surprised not to have found an accepted pattern or set of best practices for doing this.

Posted 27-Apr-17 23:50pm

Patrick Skelton

Updated 28-Apr-17 3:20am

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Richard MacCutchan 28-Apr-17 6:07am

You could either add some methods to your class to allow the other class access to the content. Or, create a get method that returns a temporary copy of the dictionary.

CHill60 28-Apr-17 6:09am

Beat me to it! A virtual 5*

Richard MacCutchan 28-Apr-17 6:12am

Ha, my guessing is faster than yours. :)

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Richard MacCutchan · Accepted Answer · 2017-04-28T00:09:00

Solution 1

See Object.MemberwiseClone Method (System)[^].

Posted 28-Apr-17 0:09am

Richard MacCutchan

Richard Deeming · Accepted Answer · 2017-04-28T03:20:00

Quote:
In particular, it would be fine if I could just return an unmodifiable List<string> of the keys in the dictionary. Then I could just add another function to return the value for a specified key.

That sounds like the way to go, since it will avoid creating a copy of the dictionary on every access.

I'd be inclined to have the container class implement IReadOnlyDictionary(TKey, TValue)[^]:

C#

public class Foo : IReadOnlyDictionary<string, string>
{
    private readonly Dictionary<string,string> _records = new Dictionary<string,string>();
    
    public int Count => _records.Count;
    
    // The Dictionary.Keys and Dictionary.Values collections are read-only, so this is OK:
    
    public IEnumerable<string> Keys => _records.Keys;
    
    public IEnumerable<string> Values => _records.Values;
    
    public string this[string key] => _records[key];
    
    public bool ContainsKey(string key) => _records.ContainsKey(key);
    
    public bool TryGetValue(string key, out string value) => _records.TryGetValue(key, out value);
    
    public IEnumerator<KeyValuePair<string, string>> GetEnumerator() => _records.GetEnumerator();
    
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

NB: You might be tempted to just cast the _records field to an IReadOnlyDictionary<string, string> and return that. But that wouldn't prevent the calling code from casting it back to a Dictionary<string, string> and modifying it.

Depending on your requirements, you might also want to take a look at the Immutable Collections[^] package.