Web.config with app.config in .dll

Any configuration that might differ across environments should be stored at the machine level, not the application level. (More info on configuration levels.)

These are the kinds of configuration elements that I typically store at the machine level:

• Application settings
• Connection strings
• retail=true
• Smtp settings
• Health monitoring
• Hosting environment
• Machine key

When each environment (developer, integration, test, stage, live) has its own unique settings in the c:\Windows\Microsoft.NET\Framework64\v2.0.50727\CONFIG directory, then you can promote your application code between environments without any post-build modifications.

And obviously, the contents of the machine-level CONFIG directory get version-controlled in a different repository or a different folder structure from your app. You can make your .config files more source-control friendly through intelligent use of configSource.

I've been doing this for 7 years, on over 200 ASP.NET applications at 25+ different companies. (Not trying to brag, just want to let you know that I've never seen a situation where this approach doesn't work.)

In Visual Studio 2010 and above, you now have the ability to apply a transformation to your web.config depending on the build configuration.

When creating a web.config, you can expand the file in the solution explorer, and you will see two files:

• Web.Debug.Config
• Web.Release.Config

They contain transformation code that can be used to

• Change the connection string
• Remove debugging trace and settings
• Register error pages

See Web.config Transformation Syntax for Web Application Project Deployment on MSDN for more information.

It is also possible, albeit officially unsupported, to apply the same kind of transformation to an non web application app.config file. See Phil Bolduc blog concerning how to modify your project file to add a new task to msbuild.

This is a long withstanding request on the Visual Studio Uservoice.

An extension for Visual Studio 2010 and above, "SlowCheetah," is available to take care of creating transform for any config file. Starting with Visual Studio 2017.3, SlowCheetah has been integrated into the IDE and the code base is being managed by Microsoft. This new version also support JSON transformation.