对象数组中相等性的字符串比较：意外的false

Question

Consider the following:

using System;

namespace ObjectArrayWeirdness
{
    public class Program
    {
        public static object[] Change(object[] set)
        {
            object[] result = new object[set.Length];

            // If the following two lines are replaced with:
            // result[0] = "ab";
            // then ArrayEquals will evaluate true.
            result[0] = "a";
            result[0] += "b";
            return result;
        }

        public static bool ArrayEquals(object[] a, object[] b)
        {
            if (a.Length != b.Length) return false;
            for (int i = 0; i < a.Length; i++)
            {
                if (a[i] != b[i])
                {
                    return false;
                }
            }
            return true;
        }


        static void Main(string[] args)
        {
            object[] expected = { "ab" };
            object[] actual = Change(new object[] { 1 });
            Console.WriteLine(ArrayEquals(expected, actual));
        }
    }
}

Dotnetfiddle for convenience

Output:

false

Output of result[0].GetType() :

System.String

My questions:

1. Does the comparison in ArrayEquals fail because it's comparing object reference and not value?

2. If that's the case, why does it succeed when the correct string is assigned in one step (without using += ) into result[0] ?

I must admit I expected normal string comparison given that GetType() identifies both a[0] and b[0] as strings.

Answer 1

Does the comparison in ArrayEquals fail because it's comparing object reference and not value?

Yes

If that's the case, why does it succeed when the correct string is assigned in one step (without using +=) into result[0]?

This is a compiler optimization. When you declare two literal strings that are the same at compile time, the compiler collapses them into a single object instance to save space. But if the string is constructed at run time, you'll end up with two separate string objects.

Answer 2

That exactly what happens. You are comparing object reference. Note however that string overrides the == and != operators to compare literals. So your comparison will work as you expect it if you cast the array items to string:

if ((string)a[i] != (string)b[i])
{
    return false;
}