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动态加载与插件系统的初步实现（二）：AppDomain卸载与代理 - Jusfr - 博客园

Excerpt

前一篇文章简单展示了类型发现和MEF使用，本文初步进入AppDomain相关内容。CLR程序运行时会创建默认程序集容器即AppDomain，默认AppDomain不支持卸载其程序集，但CLR支持创建和卸载AppDomain，这意味着我们可以间接地通过额外的AppDomain实现插件的热插拔。代理Ap

前一篇文章简单展示了类型发现和MEF使用，本文初步进入AppDomain相关内容。

CLR程序运行时会创建默认程序集容器即AppDomain，默认AppDomain不支持卸载其程序集，但CLR支持创建和卸载AppDomain，这意味着我们可以间接地通过额外的AppDomain实现插件的热插拔。

代理AppDomain创建PluginProvider实例，该实例及其发现的IPlugin的实现需要被被默认AppDomain访问，于是发生了跨AppDomain边界的访问，PluginProvider及IPlugin的具体实现需要由MarshalByRefObject派生（更多相关内容仍然需要自行MSDN）。

为Plugin项目添加PluginProxy类，该类负责维护上述的额外AppDomain、将PluginProvider封送回默认AppDomain。本例仅设计了单个插件容器的场景，故以单例模式实现：

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<span>public</span> <span>class</span><span> PluginProxy
{
    </span><span>private</span> <span>readonly</span> <span>static</span> PluginProxy instance = <span>new</span><span> PluginProxy();

    </span><span>private</span><span> PluginProxy()
    {
    }

    </span><span>public</span> <span>static</span><span> PluginProxy Instance
    {
        </span><span>get</span> { <span>return</span><span> instance; }
    }

    </span><span>private</span> AppDomain pluginDomain = <span>null</span><span>;
    </span><span>private</span> PluginProvider pluginProvider = <span>null</span><span>;

    </span><span>public</span><span> PluginProvider Provider
    {
        </span><span>get</span><span>
        {
            </span><span>if</span> (pluginDomain == <span>null</span><span>)
            {
                pluginDomain </span>= AppDomain.CreateDomain(<span>"</span><span>PluginDomain</span><span>"</span><span>);
                Type pluginProviderType </span>= <span>typeof</span><span>(PluginProvider);
                pluginProvider </span>=<span> (PluginProvider)pluginDomain.CreateInstanceAndUnwrap(pluginProviderType.Assembly.FullName, pluginProviderType.FullName);
            }
            </span><span>return</span><span> pluginProvider;
        }
    }

    </span><span>public</span> <span>void</span><span> Unload()
    {
        </span><span>if</span> (pluginDomain != <span>null</span><span>)
        {
            AppDomain.Unload(pluginDomain);
            pluginDomain </span>= <span>null</span><span>;
        }
    }
}</span>

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AppDomain的创建与跨边界访问对象的成本很高，后文中默认AppDomain与插件的交互将以代理PluginProxy通知PluginProvider的方式实现。Plugin中PluginProvider小幅修改：

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<span>public</span> <span>class</span><span> PluginProvider : MarshalByRefObject
{
    [ImportMany]
    </span><span>public</span> IEnumerable&lt;Lazy&lt;IPlugin&gt;&gt; Plugins { <span>get</span>; <span>private</span> <span>set</span><span>; }

    </span><span>public</span><span> PluginProvider()
    {
        AggregateCatalog catalog </span>= <span>new</span><span> AggregateCatalog();
        catalog.Catalogs.Add(</span><span>new</span> DirectoryCatalog(<span>"</span><span>.</span><span>"</span><span>));
        CompositionContainer container </span>= <span>new</span><span> CompositionContainer(catalog);
        container.ComposeParts(</span><span>this</span><span>);
    }
}</span>

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MyPlugin1中Plugin1小幅修改：

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[Export(<span>typeof</span><span>(IPlugin))]
</span><span>public</span> <span>class</span><span> Plugin1 : MarshalByRefObject, IPlugin
{
    </span><span>public</span><span> String DoStuff()
    {
        </span><span>return</span> <span>"</span><span>MyPlugin1 Plugin1.DoStuff</span><span>"</span><span>;
    }
}</span>

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主程序PluginProxy由静态类属性访问，同时加入逻辑检验DLL可否在AppDomain卸载后删除，WinForm与WCFRestService示例在后续给出。代码文件

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<span>class</span><span> Program
{
    </span><span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
    {
        PluginProvider pluginProvider </span>=<span> PluginProxy.Instance.Provider;
        </span><span>foreach</span> (Lazy&lt;IPlugin&gt; plugin <span>in</span><span> pluginProvider.Plugins)
        {
            Console.WriteLine(plugin.Value.DoStuff());
        }
        PluginProxy.Instance.Unload();

        String filename </span>= <span>"</span><span>MyPlugin1.dll</span><span>"</span><span>;
        </span><span>if</span><span> (File.Exists(filename))
        {
            File.Delete(filename);
            Console.WriteLine(</span><span>"</span><span>File deleted</span><span>"</span><span>);
        }
        </span><span>else</span><span>
        {
            Console.WriteLine(</span><span>"</span><span>File not exist</span><span>"</span><span>);
        }
        Console.WriteLine(</span><span>"</span><span>Press Enter to exit</span><span>"</span><span>);
        Console.ReadLine();
    }
}</span>

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