So I've been using this pattern for a while and promising to blog it for almost as long.

Code is on GitHub; package is on NuGet. Here you go :)

How application settings should look:

Here's a class that needs some configuration settings:

public class EmailSender : IEmailSender
{
    private readonly SmtpHostConfigurationSetting _smtpHost;
    private readonly SmtpPortConfigurationSetting _smtpPort;

    public EmailSender(SmtpHostConfigurationSetting smtpHost,
                        SmtpPortConfigurationSetting smtpPort)
    {
        _smtpHost = smtpHost;
        _smtpPort = smtpPort;
    }

    public void Send(MailMessage message)
    {
        // NOTE the way we can use our strongly-typed settings directly as
        // a string and int respectively
        using (var client = new SmtpClient(_smtpHost, _smtpPort))
        {
            client.Send(message);
        }
    }
}

Here's how we declare the settings:

// This will give us a strongly-typed string setting.
public class SmtpHostConfigurationSetting : ConfigurationSetting<string>
{
}

// This will give us a strongly-typed int setting.
public class SmtpPortConfigurationSetting : ConfigurationSetting<int>
{
    protected override IEnumerable<string> ValidationErrors(int value)
    {
        if (value <= 0) yield return "TCP port numbers cannot be negative.";
        if (value > 65535) yield return "TCP port numbers cannot be greater than 65535.";
    }
}

Here's how we set them in our [web|app].config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="SmtpHostConfigurationSetting" value="localhost" />
    <add key="SmtpPortConfigurationSetting" value="25" />
  </appSettings>
</configuration>

... and here's how we provide mock values for them in our unit tests:

var smtpHost = new SmtpHostConfigurationSetting {Value = "smtp.example.com"};
var smtpPort = new SmtpPortConfigurationSetting {Value = 25};

var emailSender = new EmailSender(smtpHost, smtpPort);

emailSender.Send(someTestMessage);

Getting started

In the NuGet Package Manager Console, type:

Install-Package ConfigInjector

then run up the configurator like this:

ConfigurationConfigurator
    .RegisterConfigurationSettings()
    .FromAssemblies(/* TODO: Provide a list of assemblies to scan for configuration settings here  */)
    .RegisterWithContainer(configSetting => /* TODO: Register this instance with your container here */ )
    .DoYourThing();

You can pick your favourite container from the list below or roll your own.

Getting started with Autofac

var builder = new ContainerBuilder();

builder.RegisterType<DeepThought>();

ConfigurationConfigurator
    .RegisterConfigurationSettings()
    .FromAssemblies(typeof (DeepThought).Assembly)
    .RegisterWithContainer(configSetting => builder.RegisterInstance(configSetting)
                                                   .AsSelf()
                                                   .SingleInstance())
    .DoYourThing();

return builder.Build();

Getting started with Castle Windsor

var container = new WindsorContainer();

container.Register(Component.For<DeepThought>());

ConfigurationConfigurator
    .RegisterConfigurationSettings()
    .FromAssemblies(typeof (DeepThought).Assembly)
    .RegisterWithContainer(configSetting => container.Register(Component.For(configSetting.GetType())
                                                                        .Instance(configSetting)
                                                                        .LifestyleSingleton()))
    .DoYourThing();

return container;

Getting started with Ninject

var kernel = new StandardKernel();

kernel.Bind<DeepThought>().ToSelf();

ConfigurationConfigurator
    .RegisterConfigurationSettings()
    .FromAssemblies(typeof (DeepThought).Assembly)
    .RegisterWithContainer(configSetting => kernel.Bind(configSetting.GetType())
                                                  .ToConstant(configSetting))
    .DoYourThing();

return kernel;