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 a 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 your 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: 1.0.0.0
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();
Only DefiningLib changes, UsingApp stays the same. We then manually copy DefiningLib.dll from DefiningLib\bin\Debug.v2 folder to UsingApp\bin\Debug and invoke UsingApp.exe. The output is as follows:
DefiningLib version: 2.0.0.0
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.
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