small efficiency improvement
Solution wasn't building because i18n.PostBuild config was set to not build. Enabled it and set it to Any CPU. Hopefully this will also resolve a problem importing this project into other projects that are set to "Any CPU", where this project was causing a warning about it being built for "Mixed Platforms" (thus a mismatch between importer and importee).
Given that the SessionKey constant is accessed from both static members and virtual ones which you presumably intend to be overridable, seems reasonable for this member to be accessible from those overrides.