C# const / readonly / static

/* It's good behaviour to declare variables as readonly/const if they are not supposed to change (mutable).
- Const is used for variables that get initialized at compile-time, basically while the program loads
- Readonly is used for variables that get initialized at run-time, well... while the program runs already
- Any variable declared as const will also, implicitly, be declared static
- Static methods/variables don't need an object to be called
- non-static methods can only be called on objects, that's why my method is static in this example
Quite sure there are exceptions, but round-about every initialization that calls a method can't be const -> happens at run-time
Lots of programing language have all variables const by default and need a keyword for it to be mutable/changeable
*/

//this works:
        readonly int bla = AddFive(4); //used with myObject.bla

        static int AddFive(int number) //used with MyClass.AddFive()
        {
            const int numberFive = 5;
            number += numberFive;
            return number;
        }

//this works:
        static readonly int bla = AddFive(4); //used with MyClass.bla

         static int AddFive(int number) //used with MyClass.AddFive()
        {
            int numberFive = 5;
            number += numberFive;
            return number;
        }

//this doesn't:
        const int bla = AddFive(4); //error: cannot be marked static

        static int AddFive(int number)
        {
            const int numberFive = 5;
            number += numberFive;
            return number;
        }

//this works:
        const int bla = 9;
//this also works:
        const int yada = 4 + 5;