Monday, February 23, 2009

const / read-only / static in C#

  • It is defined at compile time and cannot be changed at runtime and must be initialized as they are declared.
    E.g. public const double PI = 3.14159;
    Changing the value elsewhere this will cause a compiler error.
  • Constants must be of an integral type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, or string), an enumeration, or a reference to null.
  • Since classes or structures are initialized at run time with the new keyword, and not at compile time, you can't set a constant to a class or structure.
  • Constants can be marked as public, private, protected, internal, or protected internal.
  • Constants are accessed as if they were static fields, although they cannot use the static keyword.
  • To use a constant outside of the class that it is declared in, you must fully qualify it using the class name.
  • Can be used in attributes
  • Are copied into every assembly that uses them (every assembly gets a local copy of values)
  • Could be declared within functions
  • The compiler performs some optimization by not declaring any stack space for the field
  • It is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor as well being able to be initialized as they are declared.


    public class MyClass
    public readonly double PI = 3.14159;


    public class MyClass
    public readonly double PI;

    public MyClass()
    PI = 3.14159;
  • Because a readonly field can be initialized either at the declaration or in a constructor, readonly fields can have different values depending on the constructor used.
  • A readonly field can also be used for runtime constants.
    E.g. public static readonly uint l1 = (uint)DateTime.Now.Ticks;
  • readonly members are not implicitly static, and therefore the static keyword can be applied to a readonly field explicitly if required.
  • A readonly member can hold a complex object by using the new keyword at initialization.
  • readonly members cannot hold enumerations.
  • Use of the static modifier to declare a static member, means that the member is no longer tied to a specific object.
  • This means that the member can be accessed without creating an instance of the class.
  • Only one copy of static fields and events exists, and static methods and properties can only access static fields and static events. E.g. public static int NumberOfWheels = 4;
  • The static modifier can be used with classes, fields, methods, properties, operators, events and constructors, but cannot be used with indexers, destructors, or types other than classes.
  • static members are initialized before the static member is accessed for the first time, and before the static constructor, if any is called.
  • To access a static class member, use the name of the class instead of a variable name to specify the location of the member. E.g., int i = Car.NumberOfWheels;
Note: There are some small performance gains to be realized from using const instead of readonly, but you give up quite a bit of flexibility. You’ll need to recompile every assembly that uses a const value. In the case of readonly you need only update the definition. This flexibility greatly overrides the minimal performance gains from using const as the key. Minimize your use of const to attribute parameters and enums. Everything else should be declared readonly instead.