single app.config multi-project c#

Let's say you have this folder structure:

• Solution
  • Project1
  • Project2
  • Project3

Do this:

1. Create the App.config file in the Solution level folder. You won't find an option to add an App.config file from the templates, so just create a new empty text file with the name App.config, and paste in the contents of a regular App.config file.

2. For each project in Solution Explorer:

   1. Right click and select Add > Existing Item
   2. Locate the file

   3. Select Add as link from the drop down box next to the Add button.

      

Edited to add:

You correctly state that the above method only shared the file up to build-time. To use a shared file at run-time, see the answers to this question.

The common config file

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section 
            name="appSettings" 
            type="System.Configuration.AppSettingsSection, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" 
            />
    </configSections>
    <appSettings>
        <add key="key1" value="value1"/>
    </appSettings>
</configuration>

To access mapped config file

ConfigurationFileMap fileMap = new ConfigurationFileMap(file); //Path to your config file
Configuration configuration = ConfigurationManager.OpenMappedMachineConfiguration(fileMap);
string value = configuration.AppSettings.Settings["key1"].Value;

Here's the "Add existing item" dialog in VS 2008:

Click on the little dropdown indicator on the "Add" button and pick "Add as Link" from the context menu.

Marc