Const, Read Only, Static in C#

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Within a class, const, static and readonly members are special in comparison to the other
modifiers.

const vs. readonly

const and readonly perform a similar function on data members, but they have a few important differences.


const

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const keyword and must be initialized as they are declared. For example;
public class MyClass
{
  public const double PI = 3.14159;
}
PI cannot be changed in the application anywhere else in the code as this will cause a compiler error.

Constants must be a value type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool), an enumeration, a string literal, 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.

readonly

A read only member 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. For example: 
public class MyClass
{
  public readonly double PI = 3.14159;
}
or
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. Areadonly field can also be used for runtime constants as in the following example:
public static readonly uint l1 = (uint)DateTime.Now.Ticks;
Notes
  • 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.


static

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 andstatic events. For example:
public class Car
{
  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. For example:
int i = Car.NumberOfWheels;


MSDN references

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