Const and readonly

There is an article in this web site that is written to answer this question. I would look for it myself but if you look for it then you will find namy more useful articles. If the article does not include everything that Vulpes says then you should post a comment to the article.

A simplified explanation of the difference is that the value of a const is determined at compile time whereas the value of a readonly item is determined during execution.

There are several differences:

1. Const can be applied to fields or local variables; readonly can only be applied to fields.

2. When applied to a field, const is implicitly static (i.e. it applies to all instances); readonly can be either instance or static.

3. Const values can only be set where the constant is defined; readonly values can be set either where the field is defined or from within a constructor.

4. The C# compiler must be able to fully compute the value of a const at compile time because the constant will actually be replaced by that value. This means that constants are limited to those fundamental types which support literals, such as int, double, decimal, bool, char and string.

In contrast the value of a readonly field need not be computed until runtime, which is the reason why a static readonly field is sometimes  preferred to a const field.