Declare a const array

Question

Is it possible to write something similar to the following   public const string   Titles      German    Spanish    Corrects    Wrongs

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Since C  6 you can write it like   public static string   Titles   gt  new string      German    Spanish    Corrects    Wrongs       See also  C    The New and Improved C  6 0  specifically the chapter  Expression Bodied Functions and Properties    This will make a read-only static property  but it will still allow you to alter the content of the array returned  but when you call the property again  you will get the original  unaltered array again   For clarification  this code is the same as  or actually a shorthand for    public static string   Titles       get   return new string      German    Spanish    Corrects    Wrongs           Please note that there is a downside to this approach  A new array is actually instantiated on each and every reference  so if you are using a very large array  this might not be the most efficient solution  But if you re-use the same array  by putting it in a private attribute for instance  it will again open up the possibility to change the contents of the array   If you want to have an immutable array  or list  you could also use   public static IReadOnlyList lt string gt  Titles   get      new string      German    Spanish    Corrects    Wrongs       But  this still has a risk for changes  as you can still cast it back to a string   and alter the contents  as such     string    Titles  1     French

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As an alternative  to get around the elements-can-be-modified issue with a readonly array  you can use a static property instead   The individual elements can still be changed  but these changes will only be made on the local copy of the array    public static string   Titles        get               return new string      German    Spanish    Corrects    Wrongs              Of course  this will not be particularly efficient as a new string array is created each time

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Arrays are probably one of those things that can only be evaluated at runtime  Constants must be evaluated at compile time  Try using  readonly  instead of  const

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Yes  but you need to declare it readonly instead of const   public static readonly string   Titles      German    Spanish    Corrects    Wrongs       The reason is that const can only be applied to a field whose value is known at compile-time  The array initializer you ve shown is not a constant expression in C   so it produces a compiler error   Declaring it readonly solves that problem because the value is not initialized until run-time  although it s guaranteed to have initialized before the first time that the array is used    Depending on what it is that you ultimately want to achieve  you might also consider declaring an enum   public enum Titles   German  Spanish  Corrects  Wrongs

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A  NET Framework v4 5  solution that improves on tdbeckett s answer   using System Collections ObjectModel           public ReadOnlyCollection lt string gt  Titles   get      new ReadOnlyCollection lt string gt     new string      German    Spanish    Corrects    Wrongs         Note  Given that the collection is conceptually constant  it may make sense to make it static to declare it at the class level   The above    Initializes the property s implicit backing field once with the array    Note that   get    - i e   declaring only a property getter - is what makes the property itself implicitly read-only  trying to combine readonly with   get    is actually a syntax error   Alternatively  you could just omit the   get    and add readonly to create a field instead of a property  as in the question  but exposing public data members as properties rather than fields is a good habit to form   Creates an array-like structure  allowing indexed access  that is truly and robustly read-only  conceptually constant  once created   both with respect to    preventing modification of the collection as a whole  such as by removing or adding elements  or by assigning a new collection to the variable   preventing modification of individual elements   Even indirect modification isn t possible - unlike with an IReadOnlyList lt T gt  solution  where a  string    cast can be used to gain write access to the elements  as shown in mjepsen s helpful answer  The same vulnerability applies to the IReadOnlyCollection lt T gt  interface  which  despite the similarity in name to class ReadOnlyCollection  does not even support indexed access  making it fundamentally unsuitable for providing array-like access

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This is the only correct answer  You cannot currently do this     All the other answers are suggesting using static read-only variables which are similar to  but not the same as a constant  A constant is hard coded into the assembly  A static read only variable is settable once  probably as an object is is initialized   These are sometimes interchangeable  but not always

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This is a way to do what you want   using System  using System Collections ObjectModel  using System Collections Generic   public ReadOnlyCollection lt string gt  Titles   get   return new List lt string gt     German    Spanish    Corrects    Wrongs    AsReadOnly        It is very similar to doing a readonly array

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You could take a different approach  define a constant string to represent your array and then split the string into an array when you need it  e g   const string DefaultDistances    5 10 15 20 25 30 40 50   public static readonly string   distances   DefaultDistances Split         This approach gives you a constant which can be stored in configuration and converted to an array when needed

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You can declare array as readonly  but keep in mind that you can change element of readonly array   public readonly string   Titles      German    Spanish    Corrects    Wrongs         Titles 0     bla     Consider using enum  as Cody suggested  or IList   public readonly IList lt string gt  ITitles   new List lt string gt    German    Spanish    Corrects    Wrongs    AsReadOnly

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If you declare an array behind an IReadOnlyList interface you get a constant array with constant values that is declared at runtime   public readonly IReadOnlyList lt string gt  Titles   new      German    Spanish    Corrects    Wrongs       Available in  NET 4 5 and higher

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Best alternative   public static readonly byte   ZeroHash     0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

User · Answer

You can t create a  const  array because arrays are objects and can only be created at runtime and const entities are resolved at compile time   What you can do instead is to declare your array as  readonly   This has the same effect as const except the value can be set at runtime  It can only be set once and it is thereafter a readonly  i e  const  value

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I believe you can only make it readonly

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For the sake of completeness  now we also have ImmutableArrays at our disposal  This should be truly immutable   public readonly static ImmutableArray lt string gt  Tiles   ImmutableArray Create new      German    Spanish    Corrects    Wrongs        Requires System Collections Immutable NuGet reference  https   msdn microsoft com en-us library mt452182 v vs 111  aspx

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For my needs I define static array  instead of impossible const and it works  public static string   Titles      German    Spanish    Corrects    Wrongs

[c#] Declare a const array

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