using System;

using System.Collections.Immutable;

​

public class Program

{

    public static void Main()

    {

        var arr1 = new [] {1,2,3}.ToImmutableArray();

        var arr2 = ImmutableArray.Create(new [] {1,2,3});

        var arr3 = ImmutableArray.Create(1,2,3);

        Console.WriteLine(string.Join(",", arr1));

        Console.WriteLine(string.Join(",", arr2));

        Console.WriteLine(string.Join(",", arr3));

    }

}