新建 .NET 类库项目TestLib引用Microsoft.AspNetCore.App框架。using Microsoft.AspNetCore.Hosting; using Microsoft.AspNetCore.Builder; using Microsoft.AspNetCore.Http; using Microsoft.Extensions.Hosting; // 标记为 HostingStartup 程序集 [assembly: HostingStartup(typeof(TestLib.TestHostingStartup))] namespace TestLib; public class TestHostingStartup : IHostingStartup { public void Configure(IWebHostBuilder builder) { // 观测点 1记录 Configure 被调用证明 TestLib 被成功加载为 HostingStartup Console.WriteLine([TestLib.HostingStartup] Configure() invoked.); // 观测点 2检查当前程序集归属的 ALC var currentAlc AssemblyLoadContext.GetLoadContext( typeof(TestHostingStartup).Assembly); Console.WriteLine($[TestLib.HostingStartup] Loaded in ALC: {currentAlc?.Name ?? Default}); Console.WriteLine($[TestLib.HostingStartup] Is Default ALC: {currentAlc AssemblyLoadContext.Default}); // 观测点 3注册一个诊断中间件验证 HostingStartup 注入是否生效 builder.Configure(app { app.Use(async (ctx, next) { ctx.Response.Headers[X-HostingStartup] TestLib-Injected; await next(); }); }); Console.WriteLine([TestLib.HostingStartup] Diagnostic middleware registered.); } }TestHostingStartup本身也是一个观测手段——如果Configure()被调用说明 ASP.NET Core 在 Default ALC 中成功定位并加载了 TestLib反之如果只有FileNotFoundException日志而Configure()从未触发则证实了加载失败。public static class Initializer { public static void Initialize() { Console.WriteLine([TestLib] Called from custom ALC context.); // 模拟被注入到环境变量的场景 Environment.SetEnvironmentVariable( ASPNETCORE_HOSTINGSTARTUPASSEMBLIES, TestLib); var host Host.CreateDefaultBuilder() .ConfigureWebHostDefaults(webBuilder { webBuilder.UseUrls(http://localhost:5002); webBuilder.Configure(app app.Run(async ctx await ctx.Response.WriteAsync(Hello from inner host!))); }) .Build(); host.Start(); Console.WriteLine([TestLib] Inner host started on http://localhost:5002); } }第三步启动控制台程序TestLib in Default ALC: False —— 插件确实只存在于自定义 ALC异常来自GenericWebHostBuilder.ExecuteHostingStartups()→Assembly.Load()—— Default ALC 中找不到 TestLib解决方案在插件的ConfigureWebHostDefaults中排除自身程序集Host.CreateDefaultBuilder() .ConfigureWebHostDefaults(webBuilder { // 在 Assembly.Load 发生之前就排除 TestLib webBuilder.UseSetting( WebHostDefaults.HostingStartupExcludeAssembliesKey, TestLib); webBuilder.UseUrls(http://localhost:5002); webBuilder.Configure(app app.Run(async ctx await ctx.Response.WriteAsync(Hello from inner host!))); }) .Build();WebHostOptions在构建待加载列表时先执行assemblies.Except(excludeAssemblies)再交给ExecuteHostingStartups。排除发生在Assembly.Load()之前加载尝试根本不会触发。令人疑惑的API直觉上我们可能想直接清空HostingStartupAssemblieswebBuilder.UseSetting( WebHostDefaults.HostingStartupAssembliesKey, ); // 无效但在WebHostBuilder内部配置会按以下优先级合并AddInMemoryCollection(_settings) ← UseSetting 写入低优先级 .AddConfiguration(hostConfig) ← 环境变量在此高优先级_settings中的空值被hostConfig中的环境变量覆盖因此UseSetting无法清空由环境变量注入的值。而HostingStartupExcludeAssemblies没有对应的环境变量UseSetting写入的值不会被覆盖。总结FileNotFoundException: Could not load file or assembly TestLib——ASP.NET Core 尝试在 Default ALC 中加载 TestLib 作为 HostingStartup 程序集但没找到。这个失败的后果仅仅是一