C# 外掛架構實戰：MEF 與 MAF 深度解析與程式碼範例

為什麼需要外掛架構？

現代應用程式常需支援第三方擴充、熱插拔模組、多團隊平行開發。硬編碼相依會導致部署耦合、測試困難、版本衝突。外掛架構透過「契約介面」與「發現/組合機制」實現鬆耦合擴充。

MEF (Managed Extensibility Framework) - 輕量級組合

MEF 核心概念：Export (提供)、Import (需求)、Catalog (目錄)、Container (容器)。支援屬性/建構函式/方法注入，自動解析相依圖。

定義契約介面

namespace Contracts {
    public interface ICalculator {
        IList GetOperations();
        double Operate(IOperation op, double[] operands);
    }
    public interface IOperation {
        string Name { get; }
        int NumberOperands { get; }
    }
    public interface ICalculatorExtension {
        string Title { get; }
        string Description { get; }
        FrameworkElement GetUI();
    }
}

實作外掛 (不同組件)

[Export(typeof(ICalculator))]
public class BasicCalculator : ICalculator {
    public IList GetOperations() => new List {
        new Operation { Name = "+", NumberOperands = 2 },
        new Operation { Name = "-", NumberOperands = 2 },
        new Operation { Name = "*", NumberOperands = 2 },
        new Operation { Name = "/", NumberOperands = 2 }
    };

    public double Operate(IOperation op, double[] operands) {
        return op.Name switch {
            "+" => operands[0] + operands[1],
            "-" => operands[0] - operands[1],
            "*" => operands[0] * operands[1],
            "/" => operands[1] != 0 ? operands[0] / operands[1] : throw new DivideByZeroException(),
            _ => throw new InvalidOperationException("Unknown operation")
        };
    }
}

public class Operation : IOperation {
    public string Name { get; set; }
    public int NumberOperands { get; set; }
}

方法層級 Export

public class Bomb {
    [Export("Bomb")]
    public void Fire() => Console.WriteLine("you are dead!!!");
}

主程式組合與使用

class Program {
    [ImportMany(typeof(ICalculator))]
    public IEnumerable Calculators { get; set; }

    [Import("Bomb")]
    public Action Bomb { get; set; }

    public void Run() {
        var catalog = new DirectoryCatalog(@"c:\plugins");
        var container = new CompositionContainer(catalog);
        container.ComposeParts(this);

        var calc = Calculators.ElementAt(1); // 取得第二個計算機
        var ops = calc.GetOperations().ToDictionary(o => o.Name);

        while (true) {
            Console.Write("Operation? (+,-,*,/,exit): ");
            var input = Console.ReadLine();
            if (input == "exit") break;
            if (!ops.TryGetValue(input, out var op)) continue;

            double[] args = new double[op.NumberOperands];
            for (int i = 0; i < op.NumberOperands; i++) {
                Console.Write($"Operand {i+1}: ");
                args[i] = double.Parse(Console.ReadLine());
            }
            Console.WriteLine($"Result: {calc.Operate(op, args)}");
        }
    }

    public void RunBomb() {
        var catalog = new DirectoryCatalog(@"c:\plugins");
        var container = new CompositionContainer(catalog);
        container.ComposeParts(this);
        Bomb?.Invoke();
    }
}

MAF (Managed Add-in Framework) - 隔離與版本控制

MAF 採用 7 組件管線架構，實現增益集隔離 (獨立 AppDomain/Process)、版本獨立部署、安全性沙箱。適合高可靠性、多版本共存、不信任程式碼執行場景。

7 組件管線架構

1. Add-in - 實際功能實作 (Addin_1, Addin_2)
2. Add-in View - 增益集端介面定義 (AddinSideView)
3. Add-in Adapter - 將 View 轉換為 Contract (AddinSideAdapter)
4. Contract - 核心介面，雙方共享 (IContract)
5. Host View - 宿主端介面定義 (HostSideView)
6. Host Adapter - 將 Contract 轉換為 Host View (HostSideAdapter)
7. Host - 宿主應用程式

關鍵程式碼片段

// Contract 介面
[AddInContract]
public interface IMyContract : IContract {
    string Say();
}

// Add-in 實作
[AddIn("Helloworld", Description = "this is helloworld addin", Publisher = "GhostBear", Version = "1.0")]
public class Addin_1 : AddinSideView.AddinSideView {
    public string Say() => "Helloworld";
}

// Adapter 範例 (Add-in 端)
[AddInAdapter]
public class AddinSideAdapter : ContractBase, IContract.IMyContract {
    private AddinSideView.AddinSideView _handler;
    public AddinSideAdapter(AddinSideView.AddinSideView handler) => _handler = handler;
    public string Say() => _handler.Say();
}

// Host 啟動與隔離
var tokens = AddInStore.FindAddIns(typeof(HostSideView.HostSideView), path);
var token = ChooseCalculator(tokens);
var process = new AddInProcess(Platform.X64); // 隔離進程
process.Start();
var addin = token.Activate(process, AddInSecurityLevel.FullTrust);
Console.WriteLine($"PID: {process.ProcessId}");
Console.WriteLine(addin.Say());

MEF vs MAF 選擇指南

現代替代方案

.NET Core / .NET 5+ 建議使用 System.ComponentModel.Composition (MEF 移植版) 或 Autofac/StructureMap/DI 容器 搭配 AssemblyLoadContext 實現隔離載入。MAF 維護成本高，新專案較少採用。