.Net Framework has a feature called Shadow Copy. When shadow copy is enabled on an appdomain, assemblies loaded in that appdomain will be copied to a shadow copy cache directory, and will be used from there. This is great because the original file is not locked. So it can be changed at will. ASP.NET uses this extensively. Actually, shadow copy is enabled on every appdomain created by ASP.NET.

 

This feature is implemented by fusion.

 

Unfortunately the documentation for shadow copy is rather poor. MSDN has documentation on various things affecting shadow copy. But it does not discuss the detail of shadow copy feature. This post is an attempt to discuss in detail how shadow copy works.

 

All the discussion is based on AppDomainSetup class unless specified.

 

There are three things affecting shadow copy:

  1. How to enable shadow copy?
  2. What will be shadow copied?
  3. Where do shadow copied bits go? 

1. How to enable shadow copy?

MSDN says it all. Set AppDomainSetup.ShadowCopyFiles to true will enable shadow copy.

 

2. What will be shadow copied?

AppDomainSetup.ShadowCopyDirectories controls what will be shadow copied. It is a list of directory names, separated by a semicolon. Each directory name should be absolute directory path. Assemblies loaded from those directories will be shadow copied. If ShadowCopyDirectories is null, all assemblies will be shadow copied. (An interesting corner case is what if ShadowCopyDirectories is empty string. I’ll leave this as an exercise for the readers. What do you think the behavior will be?)

 

3. Where do shadow copied bits go?

AppDomainSetup.CachePath + AppDomainSetup.ApplicationName control where shadow copied bits go. If both CachePath and ApplicationName are set, shadow copied bits will go to CachePath\ApplicationName. Otherwise shadow copied bits will go to your download cache (which is stored in %userprofile%\local settings\application data\assembly). The combination of CachePath and ApplicationName is really handy for ASP.NET, because they can set a common CachePath, and have each application shadow copy to a different location.

You are responsible to clean up the shadow copy cache if you set CachePath+ApplicationName. In the case of download cache as shadow copy cache, it is automatically managed by fusion.

 

On the default appdomain you cannot change its AppDomainSetup. But you can use AppDomain’s API to change the properties. The relevant APIs are:

SetCachePath, SetShadowCopyFiles, SetShadowCopyPath (the name of this one is not consistent with AppDomainSetup.ShadowCopyDirectories). The ApplicationName is set to the name of the application on default domain.