c# – Switch use of readonly variables in case

Question:

I have the following code

static void Main(string[] args)
        {
            string value = "value1";
            // switch с const переменными работает
            switch(value)
            {
                case ConstVariable.VALUE1:
                    Console.WriteLine(ConstVariable.VALUE1);
                    break;
                case ConstVariable.VALUE2:
                    Console.WriteLine(ConstVariable.VALUE2);
                    break;
                case ConstVariable.VALUE3:
                    Console.WriteLine(ConstVariable.VALUE3);
                    break;
            }

            // ЗДЕСЬ ОШИБКА!!! switch c readonly переменными не работает
            switch (value)
            {
                case ReadOnlyVariable.VALUE1:
                    Console.WriteLine(ConstVariable.VALUE1);
                    break;
                case ReadOnlyVariable.VALUE2:
                    Console.WriteLine(ConstVariable.VALUE2);
                    break;
                case ReadOnlyVariable.VALUE3:
                    Console.WriteLine(ConstVariable.VALUE3);
                    break;
            }
        }


        static class ReadOnlyVariable
        {
            public static readonly string VALUE1 = "value1";

            public static readonly string VALUE2 = "value2";

            public static readonly string VALUE3 = "value3";

        }

        static class ConstVariable
        {
            public const string VALUE1 = "value1";

            public const string VALUE2 = "value2";

            public const string VALUE3 = "value3";

        }

In a switch block I can use const variables, but if I use readonly variables I get an error

A constant value is expected

Why is that, aren't readonly variables defined at compile time?

Answer:

readonly means that the field must be initialized in the constructor and subsequent modification of this field is prohibited. Those. it is a guarantee that once the field is initialized in the constructor, it can no longer be changed in other parts of the class. But that doesn't make it a compile-time entity – no, it's protected from reinitialization, but it's not const.