聊一聊 .NET 的 AssemblyLoadContext 可插拔程序集
<h2 id="一背景">一：背景</h2><h3 id="1-讲故事">1. 讲故事</h3>
<p>最近在分析一个崩溃dump时，发现祸首和<code>AssemblyLoadContext</code>有关，说实话这东西我也比较陌生，后来查了下大模型，它主要奔着替代 .NetFrameWork 时代的 AppDomain 的，都是用来做晚期加卸载，实现对宿主程序的可插拔，AppDomain.Create 是在AppDomain级别上，后者是在 Assembly 级别上。</p>
<h2 id="二assembly-插拔分析">二：Assembly 插拔分析</h2>
<h3 id="1-一个简单的案例">1. 一个简单的案例</h3>
<p>简单来说这东西可以实现 Assembly 的可插拔，这个小案例有三个基本元素。</p>
<ol>
<li>IPlugin 组件接口</li>
</ol>
<p>这块比较简单，新建一个类库，里面主要就是组件需要实现的接口。</p>
<pre><code class="language-C#">
namespace MyClassLibrary.Interfaces
{
public interface IPlugin
{
string Name { get; }
string Version { get; }
void Execute();
string GetResult();
}
}
</code></pre>
<ol start="2">
<li>SamplePlugin 组件实现</li>
</ol>
<p>新建一个组件，完成这些接口方法的实现。</p>
<pre><code class="language-C#">
public class SamplePlugin : IPlugin
{
public string Name => "Sample Plugin";
public string Version => "1.0.0";
public void Execute()
{
Console.WriteLine("SamplePlugin is executing...");
}
public string GetResult()
{
return "Hello from SamplePlugin!";
}
}
</code></pre>
<ol start="3">
<li>自定义的 CustomAssemblyLoadContext 上下文</li>
</ol>
<p>最后就是在调用处自定义下 AssemblyLoadContext 以及简单调用，参考代码如下：</p>
<pre><code class="language-C#">
namespace Example_1_6
{
internal class Program
{
static void Main(string[] args)
{
Console.WriteLine("=== 插件系统启动 ===");
// 设置插件目录
string pluginsPath = @"D:\sources\woodpecker\Test\MyClassLibrary\bin\Debug\net8.0\";
Console.WriteLine($"插件路径: {pluginsPath}");
var dllFile = Directory.GetFiles(pluginsPath, "MyClassLibrary.dll").FirstOrDefault();
var _loadContext = new CustomAssemblyLoadContext("MyPluginContext", pluginsPath);
var assembly = _loadContext.LoadAssembly(dllFile);
var type = assembly.GetType("MyClassLibrary.SamplePlugin");
IPlugin plugin = (IPlugin)Activator.CreateInstance(type);
Console.WriteLine($"- {plugin.Name} v{plugin.Version}");
Console.WriteLine($"\n执行插件: {plugin.Name} v{plugin.Version}");
plugin.Execute();
string result = plugin.GetResult();
Console.WriteLine($"插件返回: {result}");
Console.ReadKey();
}
}
public class CustomAssemblyLoadContext : AssemblyLoadContext
{
private readonly string _dependenciesPath;
public CustomAssemblyLoadContext(string name, string dependenciesPath)
: base(name, isCollectible: true)
{
_dependenciesPath = dependenciesPath;
}
public Assembly LoadAssembly(string assemblyPath)
{
return LoadFromAssemblyPath(assemblyPath);
}
public new void Unload()
{
base.Unload();
}
}
}
</code></pre>
<p>将代码运行起来，可以看到插件代码得到执行。</p>
<p><img src="https://img2024.cnblogs.com/blog/214741/202508/214741-20250830121855353-2022459207.png" alt="" loading="lazy"></p>
<h3 id="2-组件已经插上了吗">2. 组件已经插上了吗</h3>
<p>plugin中的方法都已经执行了，那 <code>MyClassLibrary.dll</code> 自然就插上去了，接下来如何验证呢？可以使用 windbg 的 <code>!dumpdomain</code> 命令即可。</p>
<pre><code class="language-C#">
0:015> !dumpdomain
--------------------------------------
System Domain: 00007ff8e9d4b150
LowFrequencyHeap: 00007FF8E9D4B628
HighFrequencyHeap:00007FF8E9D4B6B8
StubHeap: 00007FF8E9D4B748
Stage: OPEN
Name: None
--------------------------------------
Domain 1: 00000211d617dc80
LowFrequencyHeap: 00007FF8E9D4B628
HighFrequencyHeap:00007FF8E9D4B6B8
StubHeap: 00007FF8E9D4B748
Stage: OPEN
Name: clrhost
Assembly: 00000211d613e560
ClassLoader: 00000211D613E5F0
Module
00007ff889d54000 C:\Program Files\dotnet\shared\Microsoft.NETCore.App\8.0.16\System.Private.CoreLib.dll
...
Assembly: 000002118052b0d0
ClassLoader: 000002118052B160
Module
00007ff88a11c060 D:\sources\woodpecker\Test\MyClassLibrary\bin\Debug\net8.0\MyClassLibrary.dll
</code></pre>
<p>从卦中可以清晰的看到 <code>MyClassLibrary.dll</code> 已经成功的送入。</p>
<h3 id="3-组件如何卸载掉">3. 组件如何卸载掉</h3>
<p>能不能卸载掉，其实取决于你在 <code>new AssemblyLoadContext()</code> 时塞入的 isCollectible 字段决定的，如果为true就是一个可卸载的程序集，参考代码如下：</p>
<pre><code class="language-C#">
public CustomAssemblyLoadContext(string name, string dependenciesPath)
: base(name, isCollectible: true)
{
_dependenciesPath = dependenciesPath;
}
</code></pre>
<p>其次要知道的是<code>卸载程序集</code>是一个异步操作，不要以为调用了 <code>UnLoad()</code> 就会立即卸载，它只是起到了一个标记删除的作用，只有程序集中的实例无引用根了，即垃圾对象的时候，再后续由 GC 来实现卸载。</p>
<p>这一块我们可以写段代码来验证下，我故意将逻辑包装到 DoWork() 方法中，然后处理完之后再次触发GC，修改后的代码如下：</p>
<pre><code class="language-C#">
internal class Program
{
static void Main(string[] args)
{
DoWork();
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("GC已触发，请再次观察 Assembly 是否被卸载...");
Console.ReadLine();
}
static void DoWork()
{
Console.WriteLine("=== 插件系统启动 ===");
// 设置插件目录
string pluginsPath = @"D:\sources\woodpecker\Test\MyClassLibrary\bin\Debug\net8.0\";
Console.WriteLine($"插件路径: {pluginsPath}");
var dllFile = Directory.GetFiles(pluginsPath, "MyClassLibrary.dll").FirstOrDefault();
var _loadContext = new CustomAssemblyLoadContext("MyPluginContext", pluginsPath);
var assembly = _loadContext.LoadAssembly(dllFile);
var type = assembly.GetType("MyClassLibrary.SamplePlugin");
IPlugin plugin = (IPlugin)Activator.CreateInstance(type);
Console.WriteLine($"- {plugin.Name} v{plugin.Version}");
Console.WriteLine($"\n执行插件: {plugin.Name} v{plugin.Version}");
plugin.Execute();
string result = plugin.GetResult();
Console.WriteLine($"插件返回: {result}");
_loadContext.Unload();
Console.WriteLine("程序集已标记为卸载... 请观察 Assembly 是否被卸载...");
Console.ReadKey();
}
}
</code></pre>
<p><img src="https://img2024.cnblogs.com/blog/214741/202508/214741-20250830121855365-571617532.png" alt="" loading="lazy"></p>
<p>从卦中可以看到确实已经不再有 <code>MyClassLibrary.dll</code> 程序集了，但托管堆上还有 CustomAssemblyLoadContext 死对象，当后续GC触发时再回收，用windbg验证如下：</p>
<pre><code class="language-C#">
0:014> !dumpobj /d 238e9c464c8
Name: Example_1_6.CustomAssemblyLoadContext
MethodTable: 00007ff88a06f098
EEClass: 00007ff88a079008
Tracked Type: false
Size: 88(0x58) bytes
File: D:\sources\woodpecker\Test\Example_1_6\bin\Debug\net8.0\Example_1_6.dll
Fields:
MT Field Offset Type VT Attr Value Name
00007ff889e870a04001116 30 System.IntPtr1 instance 000002388042A8F0 _nativeAssemblyLoadContext
00007ff889dd5fa84001117 8 System.Object0 instance 00000238e9c46520 _unloadLock
00000000000000004001118 10 0 instance 0000000000000000 _resolvingUnmanagedDll
00000000000000004001119 18 0 instance 0000000000000000 _resolving
0000000000000000400111a 20 0 instance 0000000000000000 _unloading
00007ff889e8ec08400111b 28 System.String0 instance 0000023880006a30 _name
00007ff889e3a5f0400111c 38 System.Int641 instance 0 _id
00007ff889f2f108400111d 40 System.Int321 instance 1 _state
00007ff889ddd070400111e 44 System.Boolean1 instance 1 _isCollectible
00007ff88a0ed1204001114 a00 ...Private.CoreLib]]0 static 00000238e9c46550 s_allContexts
00007ff889e3a5f04001115 bc0 System.Int641 static 1 s_nextId
0000000000000000400111f a08 ...yLoadEventHandler0 static 0000000000000000 AssemblyLoad
00000000000000004001120 a10 ...solveEventHandler0 static 0000000000000000 TypeResolve
00000000000000004001121 a18 ...solveEventHandler0 static 0000000000000000 ResourceResolve
00000000000000004001122 a20 ...solveEventHandler0 static 0000000000000000 AssemblyResolve
00000000000000004001123 a28 0 static 0000000000000000 s_asyncLocalCurrent
00007ff889e8ec084000001 48 System.String0 instance 0000023880006938 _dependenciesPath
0:014> !gcroot 238e9c464c8
Caching GC roots, this may take a while.
Subsequent runs of this command will be faster.
Found 0 unique roots.
</code></pre>
<h2 id="三总结">三：总结</h2>
<p>有时候感叹 <code>知识无涯人有涯</code>，在 dump分析中不断的螺旋式提升，理论指导实践，实践反哺理论。<br>
<img src="https://images.cnblogs.com/cnblogs_com/huangxincheng/345039/o_210929020104最新消息优惠促销公众号关注二维码.jpg" width="700" height="300" alt="图片名称" align="center"></p><br><br>
来源：https://www.cnblogs.com/huangxincheng/p/19065688
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