如何动态加载程序集及其依赖项且避免文件锁定？

我创建了一个包含三个项目的解决方案，场景如下：

1. Entities项目 #

包含Person类：

namespace Entities
{
    public class Person
    {
        public string Name { get; set; }
    }
}

2. Business项目 #

引用Entities项目，包含PersonBusiness类：

using Entities;

namespace Business
{
    public class PersonBusiness
    {
        public string GetPersonName(string name)
        {
            return name;
        }

        public Person GetPerson(string name)
        {
            Person person = new Person();
            person.Name = name;
            return person;
        }
     }
}

3. Runner项目 #

通过自定义AssemblyLoadContext动态加载Business.dll，代码如下：

using System.Runtime.Loader;

namespace Runner
{
    public class AssemblyLoader : AssemblyLoadContext
    {
        internal AssemblyLoader() : base(isCollectible: true)
        {

        }
    }

    internal class Assembly
    {
        private dynamic Instance { get; set; }
        private AssemblyLoader AssemblyLoader { get; set; }
        private Type AssemblyType { get; set; }

        public void Load()
        {
        
            AssemblyLoader = new AssemblyLoader();

            using (var fs = new FileStream(@"C:\temp\assemblies\business.dll", FileMode.Open, FileAccess.Read))
            {

                System.Reflection.Assembly assembly = AssemblyLoader.LoadFromStream(fs);

                AssemblyType = assembly.GetType("Business.PersonBusiness");

                Instance = Activator.CreateInstance(AssemblyType);

                Console.WriteLine(Instance.GetPersonName("Person name"));
                
                // 启用下方代码可运行但会锁定程序集
                //AssemblyLoadContext.Default.LoadFromAssemblyPath(@"C:\temp\assemblies\Entities.dll");

                Console.WriteLine(Instance.GetPerson("Person name 2").Name);

            }

        }

        public void Unload()
        {
            AssemblyLoader.Unload();

        }
    }
}

运行时错误 #

System.IO.FileNotFoundException: 'Could not load file or assembly 'Entities, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

尝试用AssemblyLoadContext.Default.LoadFromAssemblyPath加载Entities.dll可正常运行，但会锁定该程序集文件，无法在运行时修改或删除。

解决方案 #

问题根源是自定义AssemblyLoader未处理依赖项加载逻辑：当Business.dll依赖Entities.dll时，默认会到AssemblyLoadContext.Default中查找，但自定义上下文未加载该依赖，因此抛出找不到的异常；而直接用Default上下文加载会因保持文件句柄导致锁定。

要实现无锁定加载，需让自定义上下文通过文件流加载依赖项，重写AssemblyLoadContext.Load方法即可实现：

1. 修改自定义AssemblyLoader类

using System.Linq;
using System.IO;

public class AssemblyLoader : AssemblyLoadContext
{
    private readonly string _assemblyDir;

    internal AssemblyLoader(string assemblyDir) : base(isCollectible: true)
    {
        _assemblyDir = assemblyDir;
    }

    protected override Assembly? Load(AssemblyName assemblyName)
    {
        // 优先检查当前上下文是否已加载该程序集
        var loadedAssembly = Assemblies.FirstOrDefault(a => a.GetName().Name == assemblyName.Name);
        if (loadedAssembly != null)
        {
            return loadedAssembly;
        }

        // 构建依赖程序集的路径
        string assemblyPath = Path.Combine(_assemblyDir, $"{assemblyName.Name}.dll");
        if (File.Exists(assemblyPath))
        {
            // 通过文件流加载，避免锁定文件
            using var fs = new FileStream(assemblyPath, FileMode.Open, FileAccess.Read);
            return LoadFromStream(fs);
        }

        // 找不到则返回null，让系统尝试其他上下文加载
        return null;
    }
}

2. 修改Runner项目的Load方法

public void Load()
{
    string assemblyDir = @"C:\temp\assemblies";
    AssemblyLoader = new AssemblyLoader(assemblyDir);

    string businessDllPath = Path.Combine(assemblyDir, "business.dll");
    using (var fs = new FileStream(businessDllPath, FileMode.Open, FileAccess.Read))
    {
        System.Reflection.Assembly assembly = AssemblyLoader.LoadFromStream(fs);

        AssemblyType = assembly.GetType("Business.PersonBusiness");
        Instance = Activator.CreateInstance(AssemblyType);

        Console.WriteLine(Instance.GetPersonName("Person name"));
        Console.WriteLine(Instance.GetPerson("Person name 2").Name);
    }
}

核心原理 #

• 重写Load方法后，自定义上下文会自行处理依赖项加载，通过文件流读取程序集，不会持有文件句柄，因此不会锁定文件；
• 以isCollectible: true创建的上下文，调用Unload方法后，只要无残留引用，就能释放相关资源，程序集文件可随时修改或删除。

内容的提问来源于stack exchange，提问作者Eric Boeing