MessageBox.Show("The current environment is " + Config.Name);
MessageBox.Show("The connection timeout is " + Config.ConnectionTimeout);

<?xml version="1.0"?>
<configuration>
 <appSettings>
 
   <!-- ENVIRONMENT
        Use the "Environment" key below to select the environment (DEV | TEST | PROD). 
 
        For settings common to all environments, add a normal appSetting key (e.g. "Name").
        To change a setting for a specific environment, add an appSetting key with
        the environment name (e.g. "DEV.Name"). An environment-specific setting
        will override a common setting.
 
        In the example below, the DEV environment will have a connection timeout of 
        2 minutes while the TEST and PROD environments will use 90 minutes.
   -->
 
   <add key="Environment" value="DEV"/>
 
   <add key="DEV.Name"  value="Development"/>
   <add key="TEST.Name" value="Test"/>
   <add key="PROD.Name" value="Production"/>
 
   <add key="ConnectionTimeout"     value="90"/>
   <add key="DEV.ConnectionTimeout" value="2"/>
 
 </appSettings>
</configuration>

using System;
using System.Configuration;
using System.Collections.Generic;
using System.Text;
 
/// <summary>
/// A type-safe wapper around the project's config file. This class ensures 
/// all appSetting keys are located in one place and should provide more
/// readable code. This also allow settings to be easily moved to a database
/// or other store.
/// </summary>
public class Config
{
   /// <summary>
   /// Retrieve a value from the config file. The "environment" appsetting
   /// configures the environment (e.g. Fort Hills development, Edmonton test).
   /// Prefix appsetting with this environment value to specifiy an appsetting
   /// that is specific to the environment. 
   /// 
   /// Note: Do NOT make this method public as we want to provide a 
   /// type-safe wrapper and ensure all  constants (the appSetting keys) 
   /// are located in one place (this class).
   /// </summary>
   private static string GetValue(string key)
   {
       string env = ConfigurationManager.AppSettings["environment"];
       string fullKey = env + "." + key;
       string value = ConfigurationManager.AppSettings[fullKey];
 
       if (value == null || value == "")
           value = ConfigurationManager.AppSettings[key];
 
       if (value == null)
           value = "";
 
       return value;
   }
 
   /// <summary>
   /// Name of the environment
   /// </summary>
   public static string Name
   {
       get { return GetValue("Name").Trim(); }
   }
 
   /// <summary>
   /// Note we convert this to a TimeSpan so that users of this class
   /// don't need to know if the timeout is specified in seconds, 
   /// minutes or hours. When adding new settings convert them 
   /// to a data type that will be most useful to users of this class.
   /// </summary>
   public static TimeSpan ConnectionTimeout
   {
        get 
        { 
             return TimeSpan.FromMinutes(
                 Convert.ToDouble(GetValue("ConnectionTimeout"))); 
        }
   }
}