NServiceBus.Override to easily override configuration sections, and sources at runtime.

NServiceBus.Override.cs

#region Copyright
//The MIT License (MIT)
//
//Copyright (c) 2013 Adam Tybor (adam.tybor@gmail.com)
//
//Permission is hereby granted, free of charge, to any person obtaining a copy
//of this software and associated documentation files (the "Software"), to deal
//in the Software without restriction, including without limitation the rights
//to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//copies of the Software, and to permit persons to whom the Software is
//furnished to do so, subject to the following conditions:
//
//The above copyright notice and this permission notice shall be included in
//all copies or substantial portions of the Software.
//
//THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//THE SOFTWARE.
#endregion

namespace NServiceBus
{
  using System;
  using System.Configuration;

  using global::NServiceBus.Config.ConfigurationSource;
  using global::NServiceBus.Override;

  public static class OverrideExtensions
  {
    /// <summary>
    /// Easily override configuration section values in code.  This will also preserve any custom configuration sources that were set.
    /// If the current config source is null, then it will instantiate a new DefaultConfigurationSource
    /// so make sure to call config.CustomConfigurationSource first if you don't want the default configuration
    /// source being used.
    /// </summary>
    /// <typeparam name="T">The type of of ConfigurationSection</typeparam>
    /// <param name="config">The config instance</param>
    /// <param name="action">An action to customize the configuration values.</param>
    /// <returns>The configure instance</returns>
    public static Configure OverrideConfigSection<T>(this Configure config, Action<IAllowOverriding<T>> action)
      where T : ConfigurationSection, new()
    {
      var settings = new OverrideSettings<T>(Configure.ConfigurationSource ?? new DefaultConfigurationSource());
      action(settings);
      config.CustomConfigurationSource(((IProvideConfigurationSource)settings).Create());
      return config;
    }

    /// <summary>
    /// Wraps the current configuration source with an outer configuration source.  If the outer
    /// source returns a null value, then the wrapper will delegate to calling the innter source.
    /// If the current config source is null, then it will instantiate a new DefaultConfigurationSource
    /// so make sure to call config.CustomConfigurationSource first if you don't want the default configuration
    /// source being used.
    /// </summary>
    /// <param name="config">The configure instance</param>
    /// <param name="outer">The IConfigurationSource to wrap the current source with.</param>
    /// <returns>The current configuration instance.</returns>
    public static Configure WrapConfigurationSource(this Configure config, IConfigurationSource outer)
    {
      config.CustomConfigurationSource(
        new WrappingConfigurationSource(Configure.ConfigurationSource ?? new DefaultConfigurationSource(), outer));
      return config;
    }
  }
}

namespace NServiceBus.Override
{
  using System;
  using System.Configuration;

  using global::NServiceBus.Config.ConfigurationSource;

  public interface IProvideConfigurationSource
  {
    IConfigurationSource Create();
  }

  public interface IAllowOverriding<out T>
  {
    void Override(Action<T> action);
  }

  class OverrideSettings<T> : IProvideConfigurationSource, IAllowOverriding<T>
    where T : ConfigurationSection, new()
  {
    readonly IConfigurationSource _innerSource;

    T _current;

    public OverrideSettings(IConfigurationSource innerSource)
    {
      _innerSource = innerSource ?? new NullConfigurationSource();
    }

    public void Override(Action<T> action)
    {
      _current = _current ?? _innerSource.GetConfiguration<T>() ?? new T();
      action(_current);
    }

    IConfigurationSource IProvideConfigurationSource.Create()
    {
      return new OverrideConfigurationSource(_innerSource, _current);
    }
  }

  class WrappingConfigurationSource : IConfigurationSource
  {
    readonly IConfigurationSource _inner;
    readonly IConfigurationSource _current;

    public WrappingConfigurationSource(IConfigurationSource inner, IConfigurationSource current)
    {
      _inner = inner;
      _current = current;
    }

    public T GetConfiguration<T>() where T : class, new()
    {
      return _current.GetConfiguration<T>() ?? _inner.GetConfiguration<T>();
    }
  }

  class OverrideConfigurationSource : IConfigurationSource
  {
    readonly IConfigurationSource _innerSource;
    readonly ConfigurationSection _overrideSection;

    public OverrideConfigurationSource(IConfigurationSource innerSource, ConfigurationSection overrideSection)
    {
      if (overrideSection == null)
      {
        throw new ArgumentNullException("overrideSection");
      }

      _innerSource = innerSource ?? new NullConfigurationSource();
      _overrideSection = overrideSection;
    }

    public T GetConfiguration<T>() where T : class, new()
    {
      if (typeof(T).Equals(_overrideSection.GetType()))
        return _overrideSection as T;
      return _innerSource.GetConfiguration<T>();
    }
  }

  class NullConfigurationSource : IConfigurationSource
  {
    public T GetConfiguration<T>() where T : class, new()
    {
      return null;
    }
  }
}