AsReadOnlyExtension.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication17
{
    class Program
    {
        static readonly ReadOnlyCollection<string> hoge = new []{"1", "2", "3"}.AsReadOnly(); 
        static readonly ReadOnlyCollection<string> fuga = Array.AsReadOnly(new[] {"1", "2"});
        static void Main(string[] args)
        {
        }
    }

    static class ArrayReadOnlyExtensions
    {
        public static ReadOnlyCollection<T> AsReadOnly<T>(this T[] array)
        {
            return Array.AsReadOnly(array);
        }
    }
}