Are immutable arrays possible in .NET?

Question

Is it possible to somehow mark a System.Array as immutable. When put behind a public-get/private-set they can't be added to, since it requires re-allocation and re-assignment, but a consumer can still set any subscript they wish:

public class Immy {     public string[] { get; private set; } } 

I thought the readonly keyword might do the trick, but no such luck.

Answer 1

The Framework Design Guidelines suggest returning a copy of the Array. That way, consumers can't change items from the array.

// bad code // could still do Path.InvalidPathChars[0] = 'A'; public sealed class Path {    public static readonly char[] InvalidPathChars =        { '\"', '<', '>', '|' }; } 

these are better:

public static ReadOnlyCollection<char> GetInvalidPathChars(){    return Array.AsReadOnly(InvalidPathChars); }  public static char[] GetInvalidPathChars(){    return (char[])InvalidPathChars.Clone(); } 

The examples are straight from the book.

Answer 2

ReadOnlyCollection<T> is probably what you are looking for. It doesn't have an Add() method.