Numerous books and articles explain the difference between
public const string Foo = "foo"; and
public static readonly string Foo = "foo";
The former is treated as true constant that never ever changes, and it may be baked verbatim into the code of any caller:
const string CarthageFate = "Carthago delenda est";
The latter is treated as a field that might actually change between assembly versions, program invocations, or even in different app domains. You can actually do things like
public static readonly string InitTime = DateTime.Now.ToString();
So, I read about all that, but I never tested it. Until now that is. Since this fact was material for my current project, I wrote a little test that I offer for you enjoyment: StringConstant.zip.
We have two versions of a
DefiningLib library defining some constants and readonly fields, and a
UsingApp that uses it.
public const string VersionConst = "v1"; public static readonly string VersionField = "v1"; public static readonly string InitTime = DateTime.Now.ToString();
Version 1 is compiled by the standard “Debug” configuration and produces the following output:
DefiningLib version: 126.96.36.199 Init time: 1/11/2013 9:51:38 AM VersionConst: v1 Versionfield: v1
Then we compile version 2 of the defining lib by switching to “Debug.v2” solution configuration. Version 2 looks like this:
public const string VersionConst = "v2"; public static readonly string VersionField = "v2"; public static readonly string InitTime = DateTime.Now.ToString();
UsingApp stays the same. We then manually copy
DefiningLib\bin\Debug.v2 folder to
UsingApp\bin\Debug and invoke
UsingApp.exe. The output is as follows:
DefiningLib version: 188.8.131.52 Init time: 1/11/2013 9:54:06 AM VersionConst: v1 Versionfield: v2
Voila, the theory is indeed right. The constant was baked in into
UsingApp.exe and stayed “v1”. The field reference was updated to “v2” as expected.
Lesson learned: if there is even a remote possibility that your “constant” value might change in the next version, make it a readonly field.