调试 .NET 5 中的程序集可卸载性

Question

我有一个需要动态卸载的程序集。我已按照 https://learn.microsoft.com/ 中的步骤操作en-us/dotnet/standard/ assembly/unloadability 用于我的加载程序的开发以及使用 WinDbg 的调试。我有“内存泄漏”，因为我的卸载命令不起作用。我的问题是如何使卸载工作正常进行？

最初，我注意到使用卸载命令后内存并没有减少，因此我创建了一个指向程序集的弱引用字典。每次执行操作时，我都会动态加载和卸载程序集，这会在内存中创建程序集的多个副本。我可以使用 Dictionary查看组件是否已按照上述指南中的方式卸载。无论我等多久，或者垃圾被收集多少次，它们都不会被释放。这对我来说有一些出色的参考资料，但我一生都找不到它。

我没有犯他们在示例代码中犯的任何错误 - 我保留的唯一参考是弱引用。一旦我完成了程序集和程序集加载器，我什至将它们分配为空，尽管它们无论如何都会立即超出范围。我没有任何回调，或者据我所知没有跨界类型，因为我跨界进行的唯一通信是通过文件。大会正在顺利结束，没有出现任何问题。所以我启动了 WinDbg 并尝试按照他们的指示进行操作。一切都很顺利，直到我尝试获取 AssemblyLoader 的 GC 根：

0:000> !gcroot -all 0x00007ff7b943f9c8
Found 0 roots.

当我单步执行应用程序中的线程时，我没有看到任何可疑的东西，尽管老实说，我没有足够的直觉这个领域意义重大。输出如下。

0:021> ~*e !clrstack
OS Thread Id: 0x3510 (0)
        Child SP               IP Call Site
000000FEA5D7E6E0 00007ff8813111c4 [InlinedCallFrame: 000000fea5d7e6e0] Interop+User32.WaitMessage()
000000FEA5D7E6E0 00007ff7b8f38a15 [InlinedCallFrame: 000000fea5d7e6e0] Interop+User32.WaitMessage()
000000FEA5D7E6B0 00007ff7b8f38a15 ILStubClass.IL_STUB_PInvoke()
000000FEA5D7E770 00007ff813db015e System.Windows.Forms.Application+ComponentManager.Interop.Mso.IMsoComponentManager.FPushMessageLoop(UIntPtr, msoloop, Void*) [/_/src/System.Windows.Forms/src/System/Windows/Forms/Application.ComponentManager.cs @ 398]
000000FEA5D7E860 00007ff813db28f5 System.Windows.Forms.Application+ThreadContext.RunMessageLoopInner(msoloop, System.Windows.Forms.ApplicationContext) [/_/src/System.Windows.Forms/src/System/Windows/Forms/Application.ThreadContext.cs @ 1111]
000000FEA5D7E8F0 00007ff813db2568 System.Windows.Forms.Application+ThreadContext.RunMessageLoop(msoloop, System.Windows.Forms.ApplicationContext) [/_/src/System.Windows.Forms/src/System/Windows/Forms/Application.ThreadContext.cs @ 975]
000000FEA5D7E950 00007ff813a89c86 System.Windows.Forms.Application.Run(System.Windows.Forms.Form) [/_/src/System.Windows.Forms/src/System/Windows/Forms/Application.cs @ 1198]
000000FEA5D7E990 00007ff7b8bd9f4a ConfigTool.Program.Main() [C:\Users\isherman\Documents\Projects\twmp\FormsConfigTool\Program.cs @ 42]
<snipping out unmanaged threads>
OS Thread Id: 0xa2dc (6)
        Child SP               IP Call Site
000000FEA667FC30 00007ff88338d8c4 [DebuggerU2MCatchHandlerFrame: 000000fea667fc30] 
OS Thread Id: 0xc5fc (7)
        Child SP               IP Call Site
<snip>
OS Thread Id: 0x900c (12)
        Child SP               IP Call Site
<snip>
OS Thread Id: 0x9b88 (16)
        Child SP               IP Call Site
<snip>
OS Thread Id: 0xa358 (18)
        Child SP               IP Call Site
OS Thread Id: 0x9a8c (19)
        Child SP               IP Call Site
<snip>

这是我用来执行和卸载程序集加载上下文的代码：

        private static Dictionary<int, WeakReference> references;
        private static int refCount =0;
        [MethodImpl(MethodImplOptions.NoInlining)]
        private static int ExecuteAndUnload(string assemblyPath, 
                           XmlDocument doc, out WeakReference alcWeakRef)
        {
            references ??= new Dictionary<int, WeakReference>();
            assemblyPath = Path.GetFullPath(assemblyPath);
            var alc = new SequesteredTagBrowser(assemblyPath);
            Assembly a = alc.LoadFromAssemblyPath(assemblyPath);
            Debug.Assert(a.EntryPoint !=null);
            alcWeakRef = new WeakReference(alc, true);
            references.Add(refCount++, alcWeakRef);
            //I pass some xml in as an argument to the Program.Main function
            var args = new object[] {new[] {doc.OuterXml}};
            a.EntryPoint.Invoke(null, args);
            a = null;

            alc.Unload();
            alc = null;
            GC.Collect();

            return 0;
        }

这是 SequesteredTagBrowser 类的代码：

    public class SequesteredTagBrowser:AssemblyLoadContext
    {
        private AssemblyDependencyResolver _resolver;

        public SequesteredTagBrowser(string mainAssemblyToLoadPath) : base(isCollectible: true)
        {
            
            _resolver = new AssemblyDependencyResolver(mainAssemblyToLoadPath);
        }

        protected override Assembly Load(AssemblyName name)
        {
            string assemblyPath = _resolver.ResolveAssemblyToPath(name);
            if (assemblyPath != null)
            {
                return LoadFromAssemblyPath(assemblyPath);
            }

            return null;
        }

    }