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" 

Answer 2

You can accomplish using my library Expansive. Also available on nuget here.

It was designed with this as a primary use-case.

Moderate Example (using AppSettings as default source for token expansion)

In app.config:

<configuration>     <appSettings>         <add key="Domain" value="mycompany.com"/>         <add key="ServerName" value="db01.{Domain}"/>     </appSettings>     <connectionStrings>         <add name="Default" connectionString="server={ServerName};uid=uid;pwd=pwd;Initial Catalog=master;" provider="System.Data.SqlClient" />     </connectionStrings> </configuration> 

Use the .Expand() extension method on the string to be expanded:

var connectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString; connectionString.Expand() // returns "server=db01.mycompany.com;uid=uid;pwd=pwd;Initial Catalog=master;" 

or

Use the Dynamic ConfigurationManager wrapper "Config" as follows (Explicit call to Expand() not necessary):

var serverName = Config.AppSettings.ServerName; // returns "db01.mycompany.com"  var connectionString = Config.ConnectionStrings.Default; // returns "server=db01.mycompany.com;uid=uid;pwd=pwd;Initial Catalog=master;" 

Advanced Example 1 (using AppSettings as default source for token expansion)

In app.config:

<configuration>     <appSettings>         <add key="Environment" value="dev"/>         <add key="Domain" value="mycompany.com"/>         <add key="UserId" value="uid"/>         <add key="Password" value="pwd"/>         <add key="ServerName" value="db01-{Environment}.{Domain}"/>         <add key="ReportPath" value="\\{ServerName}\SomeFileShare"/>     </appSettings>     <connectionStrings>         <add name="Default" connectionString="server={ServerName};uid={UserId};pwd={Password};Initial Catalog=master;" provider="System.Data.SqlClient" />     </connectionStrings> </configuration> 

Use the .Expand() extension method on the string to be expanded:

var connectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString; connectionString.Expand() // returns "server=db01-dev.mycompany.com;uid=uid;pwd=pwd;Initial Catalog=master;"