浅谈BlogEngine扩展机制的实现

在Blogengine里，每一个扩展的Class都必须有Extension标记
ExtensionAttribute是一个密封类，继承自System.Attribute,System.Attribute没有任何实现，只是一个标记而已。
ExtensionAttribute类的代码:(完整代码可以从BlogEngine.NET.zip下载)

[AttributeUsage(AttributeTargets.Class)]  //指明该Attrubite只能应用于Class上
  public sealed class ExtensionAttribute : System.Attribute
  {
    /// <summary>
    /// Creates an instance of the attribute and assigns a description.
    /// </summary>
    public ExtensionAttribute(string description, string version, string author)
    {
      _Description = description;
      _Version = version;
      _Author = author;
    }

    private string _Description;
    /// <summary>
    /// Gets the description of the extension.
    /// </summary>
    public string Description
    {
      get { return _Description; }
    }

    private string _Version;

    /// <summary>
    /// Gets the version number of the extension
    /// </summary>
    public string Version
    {
      get { return _Version; }
    }

    private string _Author;

    /// <summary>
    /// Gets the author of the extension
    /// </summary>
    public string Author
    {
      get { return _Author; }
    }

  }

[AttributeUsage(AttributeTargets.Class)]  //指明该Attrubite只能应用于Class上
public sealed class ExtensionAttribute : System.Attribute
{
/// <summary>
/// Creates an instance of the attribute and assigns a description.
/// </summary>
public ExtensionAttribute(string description, string version, string author)
{
_Description = description;
_Version = version;
_Author = author;
}
private string _Description;
/// <summary>
/// Gets the description of the extension.
/// </summary>
public string Description
{
get { return _Description; }
}
private string _Version;
/// <summary>
/// Gets the version number of the extension
/// </summary>
public string Version
{
get { return _Version; }
}
private string _Author;
/// <summary>
/// Gets the author of the extension
/// </summary>
public string Author
{
get { return _Author; }
}
}

这个类非常之简单，只是一些描述性的信息，如作者、版本、描述等。
到这里，ExtensionAttribute类就可以应用在任何一个Class上了
看看mp3player.cs

/// <summary>
/// 增加flashMp3播放器
/// </summary>
[Extension("mp3 player", "1.0.0.0", "lemongtree.com")]  //此处给mp3player类打上Extension标记
public class mp3player
{
    public mp3player()
    {
        Post.Serving += new EventHandler<ServingEventArgs>(Post_Serving);
    }
    private void Post_Serving(object sender, ServingEventArgs e)
    {
        //此处略过
    }
}

/// <summary>
/// 增加flashMp3播放器
/// </summary>
[Extension("mp3 player", "1.0.0.0", "lemongtree.com")]  //此处给mp3player类打上Extension标记
public class mp3player
{
public mp3player()
{
Post.Serving += new EventHandler<ServingEventArgs>(Post_Serving);
}
private void Post_Serving(object sender, ServingEventArgs e)
{
//此处略过
}
}

mp3player就是一个针对于Post的扩展了。
有朋友可能会问了:mp3player这个在什么时候实例化的呢？或是我如何得知这个扩展是不是Enable的呢?
整个实例化的过程在global.asax的Application_Start事件中

Assembly a = Assembly.Load(assemblyName);
      Type[] types = a.GetTypes();   //获取当前程序集中的所有类型

      foreach (Type type in types)
      {
        object[] attributes = type.GetCustomAttributes(typeof(ExtensionAttribute), false);   //获取含有ExtensionAttribute标记的类
        foreach (object attribute in attributes)
        {
          if (ExtensionManager.ExtensionEnabled(type.Name))
          {
            a.CreateInstance(type.FullName);  //创建实例
          }
        }
      }

Assembly a = Assembly.Load(assemblyName);
Type[] types = a.GetTypes();   //获取当前程序集中的所有类型
foreach (Type type in types)
{
object[] attributes = type.GetCustomAttributes(typeof(ExtensionAttribute), false);   //获取含有ExtensionAttribute标记的类
foreach (object attribute in attributes)
{
if (ExtensionManager.ExtensionEnabled(type.Name))
{
a.CreateInstance(type.FullName);  //创建实例
}
}
}

到这里很明了了，扩展类决定于它有没有打Extension标记，是否创建实例取决于该扩展有没有被启用.
BlogEngine有很多地方是值得我们学习的.