Some of my global variables need to be initiated only once. I do it by loading a file and setting them to whatever. Now I want when I try to set a new value to this variable that an exception is thrown.
public class Foo { public static int MIN; private static loadConstants() { MIN = 18; } public static void Main() { loadConstants(); MIN = 15; // this must throw an exception // edit: at least mustn't set the new value } } How can I do that ?
(probably very easy and I'm sorry)
Create a static constructor, and mark the variable readonly. Then set the value in the constructor.
public static class Foo { public static readonly int MIN; static Foo() { MIN = 18; } public static void Main() { } } 
public class Foo { public readonly static int MIN; static Foo() { MIN = 18; } public static void Main() { } } 
MIN = 18. I strongly recommend to not do any file I/O in static type constructors. Or any complex logic at all. They are a real pain to work with when debugging and its hard to control the exact time when they are executed.If you can't or don't want to use the static-constructor from the other answers (for example because you have lots of things to do with the type before actually initializing the variables or because you realize that static constructors are a real pain to debug..) you could to other things:
One compile-time solution is to pack the variables in your own type as non-static readonly and hold a static reference to this type
public class Constants { public readonly int MIN; public Constants() { MIN = 18; } } public class Foo { public static Constants GlobalConstants { get; private set; } public static void Main() { // do lots of stuff GlobalConstants = new GlobalConstants(); } } Or you can make the constant into a property, only providing the getter for anyone outside your class. Note, that the declaring class will still be able to change the property.
public class Foo { public static int MIN { get; private set; } } public static void Main() { MIN = 18; MIN = 23; // this will still work :( } } Or - if for some strange reason - you really want an exception instead of a compile error, you can make a property out of the constant and throw your exception in the setter.
public class Foo { static int _min; public static int MIN { get { return _min; } set { throw new NotSupportedException(); } } public static void Main() { _min = 18; } } Rather than have a public member variable, you could create a public property, and then manage your CONST logic in your implementation.
private static int? _min; public static int MIN { set { if (!_min.HasValue()) { _min = value; } else { throw; } } get { return _min.ValueOrDefault(); } } 
MIN = 15;line doesn't even compile, so it doesn't throw an exception either.readonlymeans just that. The value can only be set in the constructor, and after that it is constant.