Multiple AppSettings.config files in a hierarchy - asp.net-mvc

Question

I have a question that has struck me a couple of times when creating ASP.Net MVC applications: Say you have one application that you want to deploy to multiple customers. The application code is identical, but you want to be able to have one appSettings.config file for each customer, so that you are able to deploy to different customers by just changing the configSource of the appSettings tag in web.config ( a bit simplified, but still).
Then you realize that 50% of the content in appSettings.config is common for all customers, and only 50% is customer dependent. What you may end up doing is having duplicated entries in all the appSettings files, which is a major pitfall as you then need to remember to update all of them if you want to do an application-wide change to the config.
In a case like this I would really like to have some sort of hierarchical system where you are able to have a "base config" and a "customer config" in separate files. Then I would like the ConfigurationManager to first check for a value in the customer config, and if it is not defined there it will go to the base config instead.
I haven't found a straight-forward way of solving this with the out-of-the-box functionality in ASP.Net MVC4. Does it exits, or do I need to work my way around the default ConfigurationManager class somehow? I could potentially create my own class and replace all calls to ConfigurationManager.AppSettings[key] with a call to my own implementation, but I'd rather avoid that if I could. I want to be able to take use of some of the basic functionality that the built-in ConfigurationManager takes care of, like caching, etc.
Anyone who has solved a similar problem like this before? I keep thinking that it seems like a common scenario..

Answer 1

It is a common scenario, and there are different ways to solve it. One way would be to use config transforms. You could have a Web.Customer1.config, Web.Customer2.config, etc, just like you have Web.Debug.config and Web.Release.config. In the customer-specific transform files, you could "override" only the appSettings that your customer wants to customize.
To create the different transforms, first create different project platforms. Go to the Visual Studio Configuration Manager, and in the Configuration column for your web project (or any project that needs customized configuration settings), click the dropdown and then click <New...>. Name the New Project Configuration Customer1 or whatever you want, check the box for Copy settings from, and pick Release from that dropdown. Also check the Create new solution configurations checkbox.
Finally, right click your web.config file and click Add config transform. This will generate a template Web.Customer1.config file for you. Edit it to override the appSettings it needs to, using the xdt: config transform attributes. Then you can publish the project using the Customer1 solution build configuration. As part of the build, the web.config will be transformed and you will end up with a different web.config file for each customer. You can also use this to customize projects for different deployments, i.e. changing db connection strings, smtp servers, literally anything in the XML configuration file.
As a last thought, make sure you right click each Web.Xyx.config file, choose properties, and set its Build Action to None.
Example:
base web.config
<appSettings>
<add key="CommonProperty1" value="[for all customers]" />
<add key="CommonProperty2" value="[for all customers]" />
<add key="CommonProperty3" value="[for all customers]" />
<add key="CustomProperty1" value="[for one customer]" />
<add key="CustomProperty2" value="[for one customer]" />
<add key="CustomProperty3" value="[for one customer]" />
<appSettings>
web.Customer1.config
<appSettings>
<add key="CustomProperty1" value="The Ohio State University" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<add key="CustomProperty2" value="Scarlet" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<add key="CustomProperty3" value="Gray" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<appSettings>
web.Customer2.config
<appSettings>
<add key="CustomProperty1" value="Michigan University" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<add key="CustomProperty2" value="Blue" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<add key="CustomProperty3" value="Maize" xdt:Transform="SetAttributes" xdt:Locator="Match(key)" />
<appSettings>