Constants & Readonly

Constants & Readonly

   Because a constant value never changes, constants are always considered to be part of the defining type. In other words, constants are always considered to be static members, not instance members. Defining a constant causes the creation of metadata. When code refers to a constant symbol, compilers look up the symbol in the metadata of the assembly that defines the constant, extract the constant’s value, and embed the value in the emitted Intermediate Language (IL) code Because a constant’s value is embedded directly in code, constants don’t require any memory to be allocated for them at runtime. In addition, you can’t get the address of a constant and you can’t pass a constant by reference. These

constraints also mean that constants don’t have a good cross-assembly versioning story, so you should use them only when you know that the value of a symbol will never change

Const instance fields

·         Constants are static by default

·         They must have a value at compilation-time (you can have e.g. 3.14 * 2, but cannot call methods)

·         Could be declared within functions

·         Are copied into every assembly that uses them (every assembly gets a local copy of values)

·         Can be used in attributes

·         The value of your const property is set at compile time and can't change at runtime

·         A const field can only be initialized at the declaration of the field

Readonly instance fields

·         Are evaluated when instance is created

·         A readonly field can be initialized either at the declaration or in a constructor Therefore, readonly fields can have different values depending on the constructor used

Const	Readonly
The value of your const property is set at compile time and can't change at runtime	The value of your const property is set at run time.
They Can not be static	They Can be static
Value of const in given in the declaration time only. As given in below example if you assignee the value to const variable you will get the  compile time error.	Value of const in given in the declaration time as well as in the constructor. You can assign the value to the read only variable in the constrctor.

When to use what

·         Use const when you have a variable of a type you can know at runtime (string literal, int, double, enums,...) that you want all instances or consumers of a class to have access to where the value should not change.

·         Use static when you have data that you want all instances or consumers of a class to have access to where the value can change.

·         Use static readonly when you have a variable of a type that you cannot know at runtime (objects) that you want all instances or consumers of a class to have access to where the value should not change.

·         Use readonly when you have an instance level variable you will know at the time of object creation that should not change.