Using the File attribute of the appSettings element of a .NET config file

Introduction

If you need to share configuration settings among multiple .NET assemblies, the practice of maintaining separate config files for each assembly can quickly become tedious. For instance, if you have multiple executables within a directory that all need a 'ConnectionString' entry in their config file, the traditional method in .NET would be for each executable to have its own config file. This can become a burden in an enterprise environment when you need to change the connection string, as you would be forced to change each individual config file. Fortunately, there is a better approach. This better approach involves using the File attribute of the .NET config file's appSettings element. This attribute should contain the relative path to a custom config file, which all other applications can share. The description from MSDN on the appSettings File attribute follows:

Specifies a relative path to an external file containing custom application configuration settings. The specified file contains the same kind of settings that are specified in the <add>, <remove>, and <clear> elements and uses the same key/value pair format as those elements. The path specified is relative to the main configuration file. For a Windows Forms application, this would be the binary folder (such as /bin/debug), not the location of the application configuration file. For Web Forms applications, the path is relative to the application root, where the web.config file is located.

Note that the runtime ignores the attribute if the specified file can not be found.

Essentially, each executable's config file will contain an entry such as:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
 <appSettings file="settings.config">
 </appSettings>
</configuration>

where "settings.config" is a custom config file which looks something like this:

<appSettings>
 <add key="Setting1" value="This is Setting 1 from settings.config" />
 <add key="Setting2" value="This is Setting 2 from settings.config" />
 <add key="ConnectionString" value="ConnectString from settings.confg" />
</appSettings>

When you run your application, you simply use the AppSettings property from System.Configuration.ConfigurationSettings to reference the configuration setting, such as:

Dim s As String = _
   System.Configuration.ConfigurationSettings.AppSettings("ConnectionString")

In this case, if there is a 'ConnectionString' key in the individual config file, that value will be used. Otherwise, the value will be retrieved from the shared "settings.config" file.

As indicated above, if you choose, each individual config file can also define the keys which are present in the shared config file. In that case, the individual config file settings will take precedence over the shared config file. That could be useful in a situation where you want to quickly test a new setting without disrupting the other applications which are using the shared config file.

Hopefully, this article has shed some light on the sometimes confusing world of .NET configuration files. I hope many of you find this method useful when dealing with multiple configuration files. Good luck.