Variables within app.config/web.config

Question

Is it is possible to do something like the following in the app.config or web.config files?

<appSettings>
 <add key="MyBaseDir" value="C:\MyBase" />
 <add key="Dir1" value="[MyBaseDir]\Dir1"/>
 <add key="Dir2" value="[MyBaseDir]\Dir2"/>
</appSettings>

I then want to access Dir2 in my code by simply saying:

 ConfigurationManager.AppSettings["Dir2"]

This will help me when I install my application in different servers and locations wherein I will only have to change ONE entry in my entire app.config. (I know I can manage all the concatenation in code, but I prefer it this way).

Answer 1

A slightly more complicated, but far more flexible, alternative is to create a class that represents a configuration section. In your app.config / web.config file, you can have this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <!-- This section must be the first section within the <configuration> node -->
    <configSections>
        <section name="DirectoryInfo" type="MyProjectNamespace.DirectoryInfoConfigSection, MyProjectAssemblyName" />
    </configSections>

    <DirectoryInfo>
        <Directory MyBaseDir="C:\MyBase" Dir1="Dir1" Dir2="Dir2" />
    </DirectoryInfo>
</configuration>

Then, in your .NET code (I'll use C# in my example), you can create two classes like this:

using System;
using System.Configuration;

namespace MyProjectNamespace {

    public class DirectoryInfoConfigSection : ConfigurationSection {

        [ConfigurationProperty("Directory")]
        public DirectoryConfigElement Directory {
            get {
                return (DirectoryConfigElement)base["Directory"];
            }
    }

    public class DirectoryConfigElement : ConfigurationElement {

        [ConfigurationProperty("MyBaseDir")]
        public String BaseDirectory {
            get {
                return (String)base["MyBaseDir"];
            }
        }

        [ConfigurationProperty("Dir1")]
        public String Directory1 {
            get {
                return (String)base["Dir1"];
            }
        }

        [ConfigurationProperty("Dir2")]
        public String Directory2 {
            get {
                return (String)base["Dir2"];
            }
        }
        // You can make custom properties to combine your directory names.
        public String Directory1Resolved {
            get {
                return System.IO.Path.Combine(BaseDirectory, Directory1);
            }
        }
    }
}

Finally, in your program code, you can access your app.config variables, using your new classes, in this manner:

DirectoryInfoConfigSection config =
  (DirectoryInfoConfigSection)ConfigurationManager.GetSection("DirectoryInfo");
String dir1Path = config.Directory.Directory1Resolved;  // This value will equal "C:\MyBase\Dir1"