Inline instantiation of a constant List

Question

I try to do something like this:

public const List<String> METRICS = new List<String>()
        {
            SourceFile.LOC,
            SourceFile.MCCABE,
            SourceFile.NOM,
            SourceFile.NOA,
            SourceFile.FANOUT,
            SourceFile.FANIN,
            SourceFile.NOPAR,
            SourceFile.NDC,
            SourceFile.CALLS
        };

But unfortunately this doesn't work:

FileStorer.METRICS' is of type 'System.Collections.Generic.List<string>'. A const field of a reference type other than string can only be initialized with null.

How can I solve this problem?

http://stackoverflow.com/questions/1109805/declaring-a-const-double-in-c — Artem Koshelev, Jan 12 '11 at 11:51
NOTE: Closing this question as a duplicate was a mistake. The linked question is about **constant arrays**, this question is about **constant lists**. Although both approaches provide roughly the same functionality, they are different and the answers are different. Only answers to this question mention `ReadOnlyCollection`. Questions are related, but neither is duplicate of another. — Athari, Mar 05 '14 at 21:35

score 219 · Accepted Answer · edited Mar 05 '14 at 19:44

const is for compile-time constants. You could just make it static readonly, but that would only apply to the METRICS variable itself (which should typically be Metrics instead, by .NET naming conventions). It wouldn't make the list immutable - so someone could call METRICS.Add("shouldn't be here");

You may want to use a ReadOnlyCollection<T> to wrap it. For example:

public static readonly IList<String> Metrics = new ReadOnlyCollection<string>
    (new List<String> { 
         SourceFile.LoC, SourceFile.McCabe, SourceFile.NoM,
         SourceFile.NoA, SourceFile.FanOut, SourceFile.FanIn, 
         SourceFile.Par, SourceFile.Ndc, SourceFile.Calls });

ReadOnlyCollection<T> just wraps a potentially-mutable collection, but as nothing else will have access to the List<T> afterwards, you can regard the overall collection as immutable.

(The capitalization here is mostly guesswork - using fuller names would make them clearer, IMO.)

Whether you declare it as IList<string>, IEnumerable<string>, ReadOnlyCollection<string> or something else is up to you... if you expect that it should only be treated as a sequence, then IEnumerable<string> would probably be most appropriate. If the order matters and you want people to be able to access it by index, IList<T> may be appropriate. If you want to make the immutability apparent, declaring it as ReadOnlyCollection<T> could be handy - but inflexible.

i came here looking for the `new List{ ... }` part. Thanks Jon :) — Jason, Aug 23 '12 at 05:18
I suppose there's a reason, but to me it's odd that `ReadOnlyCollection` implements `IList<>` as that exposes `.Add` — AaronLS, Oct 28 '14 at 21:34

LukeH · Answer 2 · 2011-01-12T11:51:55.283

You'll need to use a static readonly list instead. And if you want the list to be immutable then you might want to consider using ReadOnlyCollection<T> rather than List<T>.

private static readonly ReadOnlyCollection<string> _metrics =
    new ReadOnlyCollection<string>(new[]
        {
            SourceFile.LOC,
            SourceFile.MCCABE,
            SourceFile.NOM,
            SourceFile.NOA,
            SourceFile.FANOUT,
            SourceFile.FANIN,
            SourceFile.NOPAR,
            SourceFile.NDC,
            SourceFile.CALLS
        });

public static ReadOnlyCollection<string> Metrics
{
    get { return _metrics; }
}

score -11 · Answer 3 · answered Mar 28 '19 at 14:43

-11

You are looking for a simple code, like this:

    List<string> tagList = new List<string>(new[]
    {
         "A"
        ,"B"
        ,"C"
        ,"D"
        ,"E"
    });

answered Mar 28 '19 at 14:43

Agnaldo Junior

51
5

1

This does not answer the question since instantiation of a property is the need of the questioner. – vchan Jul 08 '20 at 09:19

Inline instantiation of a constant List

3 Answers3