Change default app.config at runtime

Question

I have the following problem:
We have an application that loads modules (add ons). These modules might need entries in the app.config (e.g. WCF configuration). Because t

Answer 1

@Daniel solution works OK. A similar solution with more explanation is in c-sharp corner. For completeness I'd like to share my version: with using, and the bit flags abbreviated.

using System;//AppDomain
using System.Linq;//Where
using System.Configuration;//app.config
using System.Reflection;//BindingFlags

    /// 

    /// Use your own App.Config file instead of the default.
    /// 
    /// 
    public static void ChangeAppConfig(string NewAppConfigFullPathName)
    {
        AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", NewAppConfigFullPathName);
        ResetConfigMechanism();
        return;
    }

    /// 

    /// Remove cached values from ClientConfigPaths.
    /// Call this after changing path to App.Config.
    /// 
    private static void ResetConfigMechanism()
    {
        BindingFlags Flags = BindingFlags.NonPublic | BindingFlags.Static;
        typeof(ConfigurationManager)
            .GetField("s_initState", Flags)
            .SetValue(null, 0);

        typeof(ConfigurationManager)
            .GetField("s_configSystem", Flags)
            .SetValue(null, null);

        typeof(ConfigurationManager)
            .Assembly.GetTypes()
            .Where(x => x.FullName == "System.Configuration.ClientConfigPaths")
            .First()
            .GetField("s_current", Flags)
            .SetValue(null, null);
        return;
    }