Sample code demonstrating the difference between value types and reference types regarding the mutability of their readonly fields

Program.cs

using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var valueType = new ValueType(1);
            Console.WriteLine(valueType.ReadOnlyField);

            // was expecting maybe an error
            valueType.GetType().GetField("ReadOnlyField").SetValue(valueType, 666);
            Console.WriteLine(valueType.ReadOnlyField);

            Console.WriteLine();

            var referenceType = new ReferenceType(2);
            Console.WriteLine(referenceType.ReadOnlyField);

            // but here, readonly is simply ignored
            referenceType.GetType().GetField("ReadOnlyField").SetValue(referenceType, 666);
            Console.WriteLine(referenceType.ReadOnlyField);
        }
    }

    class ReferenceType
    {
        public readonly int ReadOnlyField;
        public ReferenceType(int value) => ReadOnlyField = value;
    }

    struct ValueType
    {
        public readonly int ReadOnlyField;
        public ValueType(int value) => ReadOnlyField = value;
    }
}

C#'s readonly field modifer is ignored by Reflection.
What I thought were differences in behavior between value- and reference-types was in fact a bug in the code above due to valueType being boxed (thus copied) on line 13 when passed to SetValue. As a result, the new value gets assigned to the copied object, not the original one.