.Net Tips – using readonly vs const in C#

In C#, there are two ways for you to declare a constant variable, you can either declare the variable as readonly, or const:

readonly

A variable declared with the readonly modifier can only be assigned as part of the declaration or in the class’s constructor:

private static readonly string _defaultString = "Hello World";

const

A variable declared with the const modifier must be initialized as part of the declaration, and its value cannot be modified:

private const string DefaultString = "Hello World";

readonly vs const

Here’s a quick glance of how the two modifiers differ:

Evaluation Time Performance Type Restrictions Scope
readonly Runtime Slow None Instance or Static
const Compile-Time Fast Primitive types, enums or strings Static

As you can see, though const is faster, readonly offers much more flexibility. As Bill Wagner’s stated in his first Effective C# book:

A slower, correct program is better than a faster, broken program

which is why he’s recommended you should prefer readonly to const. But you must be wondering how using the const modifier can ‘break’ your program!?

Well, with const, the value of the variable is evaluated at compile-time and as I mentioned in my post on the volatile modifier here, the compiler will apply constant propagation and replace any reference to the constant variable with its value in the generated IL code, and this happens across assemblies! This behaviour introduces a subtle bug when you update the value of a constant variable and the updated value is not reflected in other assemblies (because the constant variable is not evaluated at runtime) until you rebuild the assemblies which references this constant variable.