1. Constants

2. 2 Objectives Describe ways to create constants –const –readonly –enum

3. 3 Motivation Idea of constant is useful –makes programs more readable –allows more compile time error checking

4. 4 Const Keyword const used to indicate compile time constant –applicable to local variables –applicable to fields

5. 5 Const local variable Local variable can be declared constant –must be initialized inline –value cannot be changed later double Area(double radius) { const double pi = 3.14; return pi * radius * radius; } constant local variable

6. 6 Const field Field can be declared constant –must be initialized inline –value cannot be changed later public class Math { public const double PI = 3.14159265358979323846; public const double E = 2.7182818284590452354;... } constant fields

7. 7 Const and static Field cannot be both declared both const and static –not needed –const field automatically static –saves memory by generating single copy public class Math { public const double PI = 3.14159265358979323846; public const double E = 2.7182818284590452354;... } implicitly static double radius = 1.5; double area = Math.PI * radius * radius; access using type name

8. 8 Compile time const const provides restrictive notion of constant –value must be computable at compile time void Process(int width, int height) { const int area = width * height;... } error, value must be compile time constant

9. 9 Limited const references Value for const reference must be compile time constant –references can only be null –too limited to be truly useful Works for strings since compile time literals exist const Person p = null; const Person ann = new Person("Ann"); ok error const string greeting = "hello"; ok

10. 10 No const parameters Parameters cannot be const –not value type parameters –not reference type parameters –parameter value is not determined at compile time double Process(const Person p) {... } error, const parameters not supported double Average(const int a, const int b) {... } error, const parameters not supported

11. 11 Readonly Keyword readonly used to indicate runtime constant –applicable only to fields

12. 12 Readonly field Field can be declared readonly –can only be initialized using variable initializer or in constructor –value cannot be changed later –compiler warning if not set –value used for initialization can be determined at runtime class Person { readonly DateTime dob; public Person(DateTime dob) { this.dob = dob; }... } readonly field

13. 13 ok Readonly and static Fields often made both readonly and static –more flexible than const –prevents write access –allows runtime determination of value –saves memory by generating single copy struct Point { public static readonly Point origin = new Point(0, 0);... }

14. 14 Enum Keyword enum used to create new type –with corresponding set of symbolic constants

15. 15 Enum definition Can create enumeration value type –use keyword enum –specify type name –give list of named constants enum Day { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday } define enum

16. 16 Enum use Can create variables of enum type –use enum name as type name Can use values listed in enum –values are scoped inside enum –prefix with enum name for access Day d; d = Day.Monday; if (d == Day.Saturday || d == Day.Sunday) Console.WriteLine("weekend"); create variable

17. 17 Enum underlying type Enum uses an underlying type for representation –default is int –can use any integral type except char represented using int enum ErrorCode : byte { DomainError, RangeError } represented using short enum Day { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday }

18. 18 Enum values Enum constants have values –default is sequential starting at 0 –can explicitly specify any value and others follow in sequence enum Day { Sunday, // 0 Monday, // 1 Tuesday, // 2 Wednesday, // 3 Thursday, // 4 Friday, // 5 Saturday // 6 } default values enum Day { Sunday = 1, Monday, // 2 Tuesday, // 3 Wednesday, // 4 Thursday = 10, Friday, // 11 Saturday // 12 } specify value

19. 19 Enum and casting Must cast to convert between enum and underlying type –cast needed in either direction –helps reduce chance of assigning invalid value Day d = (Day)3; int i = (int)d; convert to Day convert to int

20. 20 Enum inheritance hierarchy enum is part of type hierarchy –ultimately descends from Object like all types –automatically derived from the value type Enum –Enum class provides many convenient utility methods ValueType Object Enum user enum

21. 21 Boxing Enums are value types –boxed when used as object –value copied into box Day d = Day.Monday; object o = d;... boxed o Monday d

22. 22 Unboxing Can extract enum from box –cast required –System.InvalidCastException thrown if cast fails Day d = Day.Monday; object o = d; Day e = (Day)o;... unbox o Monday e

23. 23 Summary Three different techniques to create constants –const –readonly –enum static often combined with readonly on field –gives more flexible notion of constant than const keyword