.NET 設計模式實戰：模板方法模式完整解析與資料存取框架案例

模式簡介

模板方法模式定義演算法骨架，將特定步驟延遲至子類別實作。子類別可在不改變演算法結構下重新定義步驟。將不變行為上移至超類別，消除重複代碼，提供代碼復用平台。

適用場景

完成一致性流程，但個別步驟在詳細層面實作不同時。

結構角色

• AbstractClass：抽象類別，實作 TemplateMethod() 定義演算法框架，宣告抽象 PrimitiveOperation1/2()
• ConcreteClassA/B：具體子類別，實作抽象步驟

基礎實作

public abstract class AbstractClass {
    public abstract void PrimitiveOperation1();
    public abstract void PrimitiveOperation2();
    public void TemplateMethod() {
        PrimitiveOperation1(); PrimitiveOperation2(); Console.WriteLine("模板方法執行完");
    }
}
public class ConcreteClassA : AbstractClass {
    public override void PrimitiveOperation1() => Console.WriteLine($"具體實現類{GetType().Name}方法1實現");
    public override void PrimitiveOperation2() => Console.WriteLine($"具體實現類{GetType().Name}方法2實現");
}
public class ConcreteClassB : AbstractClass {
    public override void PrimitiveOperation1() => Console.WriteLine($"具體實現類{GetType().Name}方法1實現");
    public override void PrimitiveOperation2() => Console.WriteLine($"具體實現類{GetType().Name}方法2實現");
}

// Client
static void Main() {
    AbstractClass ac = new ConcreteClassA(); ac.TemplateMethod();
    ac = new ConcreteClassB(); ac.TemplateMethod();
}

實戰案例：資料存取物件框架

定義資料操作骨架：Connect → Select → Process → Disconnect。Connect/Disconnect 通用放基類，Select/Process 交由子類實作。

abstract class DataAccessObject {
    protected string connectionString; protected DataSet dataSet;
    public virtual void Connect() => connectionString = "provider=Microsoft.JET.OLEDB.4.0;data source=..\\..\\..\\db1.mdb";
    public abstract void Select(); public abstract void Process();
    public virtual void Disconnect() => connectionString = "";
    public void Run() { Connect(); Select(); Process(); Disconnect(); }
}

class Categories : DataAccessObject {
    public override void Select() {
        string sql = "select CategoryName from Categories";
        var da = new OleDbDataAdapter(sql, connectionString);
        dataSet = new DataSet(); da.Fill(dataSet, "Categories");
    }
    public override void Process() {
        Console.WriteLine("Categories ----");
        foreach (DataRow row in dataSet.Tables["Categories"].Rows) Console.WriteLine(row["CategoryName"]);
        Console.WriteLine();
    }
}
class Products : DataAccessObject {
    public override void Select() {
        string sql = "select ProductName from Products";
        var da = new OleDbDataAdapter(sql, connectionString);
        dataSet = new DataSet(); da.Fill(dataSet, "Products");
    }
    public override void Process() {
        Console.WriteLine("Products ----");
        foreach (DataRow row in dataSet.Tables["Products"].Rows) Console.WriteLine(row["ProductName"]);
        Console.WriteLine();
    }
}

// Client
static void Main() {
    new Categories().Run(); new Products().Run();
}

總結

模板方法模式是框架設計核心模式，透過「好萊塢原則」（Don't call us, we'll call you）實現控制反轉，廣泛應用於資料存取、演算法框架、流程引擎等領域。