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: 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.