Mono Addin是一个开源的插件框架，其主要支持的特性如下：
The main features of Mono.Addins are:
Supports descriptions of add-ins using custom attributes (for simple and common extensions) or using an xml manifest (for more complex extensibility needs).
Support for add-in hierarchies, where add-ins may depend on other add-ins.
Lazy loading of add-ins.
Provides an API for accessing to add-in descriptions, which will allow building development and documentation tools for handling add-ins.
Dynamic activation / deactivation of add-ins at run time.
Allows sharing add-in registries between applications, and defining arbitrary add-in locations.
Allows implementing extensible libraries.
Supports add-in localization.
In addition to the basic add-in engine, it provides a Setup library to be used by applications which want to offer basic add-in management features to users, such as enabling/disabling add-ins, or installing add-ins from on-line repositories.
（来自mono官方网站的解释，http://www.mono-project.com/Mono.Addins）
大概意思是：
1. 支持用自定义属性（Attribute，简单项目)和xml定义（大型项目）；
2. 支持插件层次结构，允许插件之间相互依赖（插件本身可以定义扩展点供其他插件进行扩展）
3. 支持插件懒加载；
4. 提供了访问插件描述的API接口；
5. 支持运行期激活/关闭插件；
6. 主持在应用程序之间共享插件，也允许插件私有；
7. 支持可扩展的类库；
8. 本地化支持；
9. 除了提供插件引擎外，还提供了安装工具，用于提供安装/卸载插件、启用/禁用插件
下面分析用Attribute声明的方式来进行插件定义
定义扩展点如下：
[TypeExtensionPoint] //定义扩展点的属性
public interface ISnippetProvider
{
string GetText (string shortcut);
}
下面定义基于该扩展点的扩展如下：
[Extension] // 扩展点属性
class StockSnippetProvider: ISnippetProvider
{
public string GetText (string shortcut)
{
foreach (ExtensionNode<SnippetAttribute> node in AddinManager.GetExtensionNodes("/SnippetsAddinNode/StockSnippets"))
{
if (node.Data.Shortcut == shortcut)
return node.Data.Text;
}
return null;
}
}
//定义扩展属性如下：
[AttributeUsage (AttributeTargets.Assembly, AllowMultiple=true)]
public class SnippetAttribute : CustomExtensionAttribute
{
public SnippetAttribute ()
{ }
public SnippetAttribute ([NodeAttribute ("Shortcut")] string shortcut, [NodeAttribute ("Text")] string text)
{
Shortcut = shortcut;
Text = text;
}
[NodeAttribute]
public string Shortcut { get; set; }
[NodeAttribute]
public string Text { get; set; }
}
到目前位置，扩展点和扩展都已经定义好了，现在使用属性来生命扩展点，并定义几个扩展：
声明扩展点如下：
[assembly: ExtensionPoint("/SnippetsAddinNode/StockSnippets", ExtensionAttributeType = typeof(SnippetAttribute))]
声明扩展如下：
[assembly: Snippet("foreach", "foreach (var item in col)\n{\n\t<|>\n}")]
[assembly: Snippet("for", "for (int n=0; n<len; n++)\n{\n\t<|>\n}")]
[assembly: Snippet("c", "hello,world!")]
使用扩展点的代码如下：
foreach (ISnippetProvider provider in AddinManager.GetExtensionObjects <ISnippetProvider>())
{
string fullText = provider.GetText (word);
}
下一篇将分析用xml描述文件的方式来定义插件
