c# - MEF 无属性方法 : lazy initialization by condition

Question

我之前的帖子包含尝试使用无属性(基于约定)的方法来配置 MEF:MEF 2: import many .
但它包含按条件(特定名称、版本)延迟初始化插件所需的类 PluginMetadataAttribute 中的导出元数据属性用法。
如何摆脱对 ExportAttribute 的依赖？

Answer 1

我找到了三个解决方案。
解决方案一(使用类常量字段，较差的解决方案):

public class Plugin1 : IPlugin
{
    public const string Name = "Plugin1";
    public const string Version = "1.0.0.0";

    public void Run()
    {
        Console.WriteLine("Plugin1 runed");
    }
}
// ...
var builder = new RegistrationBuilder();
builder
    .ForTypesDerivedFrom<IPlugin>()
    .Export<IPlugin>(exportBuilder => {
        exportBuilder.AddMetadata("Name", t => t.GetField("Name").GetRawConstantValue());
        exportBuilder.AddMetadata("Version", t => t.GetField("Version").GetRawConstantValue());
    });

解决方案2(使用类属性，较差的解决方案):

public interface IPluginMetadata
{
    string Name { get; }
    string Version { get; }
}

public interface IPlugin : IPluginMetadata
{
    void Run();
}

public class Plugin1 : IPlugin
{
    public string Name { get { return "Plugin 1"; } }
    public string Version { get { return "1.0.0.0"; } }

    public void Run()
    {
        Console.WriteLine("Plugin1 runed");
    }
}

并通过以下描述的方法获取属性值:https://stackoverflow.com/a/11162876/1986524

解决方案 3(使用属性，更好但不是很满意):

using System;
using System.ComponentModel.Composition.Hosting;
using System.ComponentModel.Composition.Registration;
using System.Reflection;

namespace MEF2
{
    public interface IPluginMetadata
    {
        string Name { get; }
        string Version { get; }
    }

    public interface IPlugin
    {
        void Run();
    }

    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
    public class PluginMetadataAttribute : Attribute, IPluginMetadata
    {
        public string Name { get; set; }
        public string Version { get; set; }

        public PluginMetadataAttribute(string name, string version)
        {
            Name = name;
            Version = version;
        }
    }

    [PluginMetadata("Plugin1", "1.0.0.0")]
    public class Plugin1 : IPlugin
    {
        public void Run()
        {
            Console.WriteLine("Plugin1 runed");
        }
    }

    [PluginMetadata("Plugin2", "2.0.0.0")]
    public class Plugin2 : IPlugin
    {
        public void Run()
        {
            Console.WriteLine("Plugin2 runed");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var builder = new RegistrationBuilder();
            builder
                .ForTypesDerivedFrom<IPlugin>()
                .Export<IPlugin>(exportBuilder => {
                    exportBuilder.AddMetadata("Name", t => t.GetCustomAttribute<PluginMetadataAttribute>().Name);
                    exportBuilder.AddMetadata("Version", t => t.GetCustomAttribute<PluginMetadataAttribute>().Version);
                });

            var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly(), builder);

            using (var container = new CompositionContainer(catalog, CompositionOptions.DisableSilentRejection)) {
                var plugins = container.GetExports<IPlugin, IPluginMetadata>();

                foreach (var plugin in plugins) {
                    Console.WriteLine("{0}, {1}", plugin.Metadata.Name, plugin.Metadata.Version);
                    plugin.Value.Run();
                }
            }
        }
    }
}

解决方案 3 包含此代码中的问题:

.Export<IPlugin>(exportBuilder => {
    exportBuilder.AddMetadata("Name", t => t.GetCustomAttribute<PluginMetadataAttribute>().Name);
    exportBuilder.AddMetadata("Version", t => t.GetCustomAttribute<PluginMetadataAttribute>().Version);
})

问题:

1. 无法在缺少元数据的情况下取消添加元数据
2. 重复代码 t.GetCustomAttribute<PluginMetadataAttribute>()
3. Export<>不提供过滤器

如果有人知道其他解决方案，请写信。