.NET Core自定义 ALC 中启动WebHost的HostingStartup解析异常

var assembly = Assembly.Load(new AssemblyName(assemblyName));

private void ExecuteHostingStartups()
{
    var webHostOptions = new WebHostOptions(
        _config, Assembly.GetEntryAssembly()?.GetName().Name);

    if (webHostOptions.PreventHostingStartup)
        return;

    var exceptions = new List<Exception>();

    var assemblyNames = webHostOptions.HostingStartupAssemblies
        .Except(webHostOptions.HostingStartupExcludeAssemblies,
                StringComparer.OrdinalIgnoreCase)
        .Distinct(StringComparer.OrdinalIgnoreCase);

    // 1) 先 Except(excludeAssemblies)，再 Distinct 去重
    // 2) 逐个程序集独立 try/catch，异常不中断循环

    foreach (var assemblyName in assemblyNames)
    {
        try
        {
            var assembly = Assembly.Load(new AssemblyName(assemblyName));
            foreach (var attr in
                assembly.GetCustomAttributes<HostingStartupAttribute>())
            {
                var hostingStartup = (IHostingStartup)Activator
                    .CreateInstance(attr.HostingStartupType);
                hostingStartup.Configure(_hostingStartupWebHostBuilder);
            }
        }
        catch (Exception ex)
        {
            exceptions.Add(ex);  // 收集异常，继续处理后续程序集
        }
    }

    if (exceptions.Count > 0)
        _hostingStartupErrors = exceptions;
}

using System.Reflection;
using System.Runtime.Loader;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Hosting;

// 1. 在 Default ALC 中启动第一个 WebHost
Console.WriteLine("[ConsoleApp] Starting first web host...");
var mainHost = Host.CreateDefaultBuilder(args)
    .ConfigureWebHostDefaults(webBuilder =>
    {
        webBuilder.UseUrls("http://localhost:5001");
        webBuilder.Configure(app => app.Run(async ctx =>
            await ctx.Response.WriteAsync("Hello from main host!")));
    })
    .Build();
await mainHost.StartAsync();
Console.WriteLine("[ConsoleApp] First host listening on http://localhost:5001");

// 2. 通过自定义 ALC 加载插件程序集
var pluginPath = Path.Combine(
    AppContext.BaseDirectory, "Plugins", "TestLib.dll");
var alc = new CustomAssemblyLoadContext(pluginPath);
var asm = alc.LoadFromAssemblyPath(pluginPath);

// 验证隔离：TestLib 不在 Default ALC 中
Console.WriteLine($"[ConsoleApp] TestLib in Default ALC: " +
    $"{AssemblyLoadContext.Default.Assemblies.Any(a => a.GetName().Name == "TestLib")}");
// 输出: False

// 3. 调用插件初始化方法
asm.GetType("TestLib.Initializer")!
   .GetMethod("Initialize")!
   .Invoke(null, null);

Console.WriteLine("Press ENTER to shut down...");
Console.ReadLine();
await mainHost.StopAsync();

// 自定义 ALC：仅加载 Plugins 目录下的程序集，其余交给 Default ALC
internal sealed class CustomAssemblyLoadContext : AssemblyLoadContext
{
    private readonly AssemblyDependencyResolver _resolver;

    public CustomAssemblyLoadContext(string pluginDir) : base(isCollectible: true)
    {
        _resolver = new AssemblyDependencyResolver(pluginDir);
    }

    protected override Assembly? Load(AssemblyName assemblyName)
    {
        var path = _resolver.ResolveAssemblyToPath(assemblyName);
        return path is not null ? LoadFromAssemblyPath(path) : null;
    }
}

<Target Name="CopyTestLibToPlugins" AfterTargets="Build">
    <MakeDir Directories="$(OutputPath)Plugins" />
    <Copy SourceFiles="..\TestLib\bin\$(Configuration)\net10.0\TestLib.dll"
          DestinationFolder="$(OutputPath)Plugins" />
</Target>

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.");
    }
}

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");
    }
}

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();

webBuilder.UseSetting(
    WebHostDefaults.HostingStartupAssembliesKey, "");  // 无效

AddInMemoryCollection(_settings)     ← UseSetting 写入（低优先级）
   .AddConfiguration(hostConfig)     ← 环境变量在此（高优先级）