Const and ReadOnly

What is the difference between const and read only fields?

Const is evaluated at compile time, readonly is evaluated at runtime. const must be initialized at declaration time only where as readonly field can be initialized in the declaration or constructor.

Const must be used very carefully especially when you try to make them public. Compiler will do something like find-and-replace for consts. So if a 3rd party assembly takes a dependency on your assembly that exposes const as public, then the const value will be there in 3rd party assebly's IL. So, if you change the value of the const later, then the 3rd party assembly has to be re-compiled (and possibly re-packaged and re-shipped).