Here are the differences between C# .NET const, readonly and static readonly fields.


  • Static by default
  • Must have compilation-time value (i.e.: you can have "A"+"B" but cannot have method calls)
  • 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

Readonly instance fields:

  • Are evaluated when instance is created
  • Must have set value by the time constructor exits

Static readonly fields:

  • Are evaluated when code execution hits class reference (i.e.: new instance is created or static method is executed)
  • Must have evaluated value by the time static constructor is done
  • You really do not want to put ThreadStaticAttribute on these (since static constructor will be executed in one thread only and it will set value for its thread; all other threads will have this value uninitialized)

Class that has only constant or readonly instance fields is considered to be prepared for the concurrency challenges of parallel computing, since it has no mutable state.

BTW, there is a simple unit-test to enforce that your classes are immutable.