學習C#

理解C#中的抽象類

Tim Corey

19m 59s

C#中的抽象類別是一個基本概念，經常讓開發者感到困惑。在他的影片中，"C#抽象類別 - 它們是什麼、如何使用和最佳實踐"，Tim Corey深入介紹了抽象類別的定義、使用方法及最佳實踐。本文總結了他影片中的重點，並使用時間標記作為參考。

介紹

Tim在(0:00)解釋了為什麼抽象類別在其目的、功能和重要性上經常受到質疑。他將抽象類別描述為介於完整基底類別和介面之間的一種綜合體，就其功能而言，介於兩者之間。

應用程式演示概述

在(0:59)，Tim演示了一個範例應用程式來展示抽象類別。這個應用程式由一個控制台應用程式和一個類別庫組成，其中包含兩個資料存取類別來模擬資料庫操作。這些類別包含用於載入和保存資料的方法，Tim用於說明抽象類別、基底類別和介面之間的相似性和差異。

以下是基底類別和衍生類別的初始程式碼：

// Base Class Definition
public class DataAccess
{
    // Method to load the connection string
    public string LoadConnectionString()
    {
        Console.WriteLine("Loading the connection string...");
        return "Test Connection String";
    }

    // Method to load data
    public void LoadData()
    {
        Console.WriteLine("Loading data...");
    }

    // Method to save data
    public void SaveData()
    {
        Console.WriteLine("Saving data...");
    }
}

// Derived class that inherits from DataAccess
public class SQLDataAccess : DataAccess
{
    // Overriding LoadData method to specify SQL data loading
    public new void LoadData()
    {
        Console.WriteLine("Loading SQL data...");
    }

    // Overriding SaveData method to specify SQL data saving
    public new void SaveData()
    {
        Console.WriteLine("Saving SQL data...");
    }
}

// Derived class that inherits from DataAccess
public class SQLiteDataAccess : DataAccess
{
    // Overriding LoadData method to specify SQLite data loading
    public new void LoadData()
    {
        Console.WriteLine("Loading SQLite data...");
    }

    // Overriding SaveData method to specify SQLite data saving
    public new void SaveData()
    {
        Console.WriteLine("Saving SQLite data...");
    }
}

// Base Class Definition
public class DataAccess
{
    // Method to load the connection string
    public string LoadConnectionString()
    {
        Console.WriteLine("Loading the connection string...");
        return "Test Connection String";
    }

    // Method to load data
    public void LoadData()
    {
        Console.WriteLine("Loading data...");
    }

    // Method to save data
    public void SaveData()
    {
        Console.WriteLine("Saving data...");
    }
}

// Derived class that inherits from DataAccess
public class SQLDataAccess : DataAccess
{
    // Overriding LoadData method to specify SQL data loading
    public new void LoadData()
    {
        Console.WriteLine("Loading SQL data...");
    }

    // Overriding SaveData method to specify SQL data saving
    public new void SaveData()
    {
        Console.WriteLine("Saving SQL data...");
    }
}

// Derived class that inherits from DataAccess
public class SQLiteDataAccess : DataAccess
{
    // Overriding LoadData method to specify SQLite data loading
    public new void LoadData()
    {
        Console.WriteLine("Loading SQLite data...");
    }

    // Overriding SaveData method to specify SQLite data saving
    public new void SaveData()
    {
        Console.WriteLine("Saving SQLite data...");
    }
}

' Base Class Definition
Public Class DataAccess
    ' Method to load the connection string
    Public Function LoadConnectionString() As String
        Console.WriteLine("Loading the connection string...")
        Return "Test Connection String"
    End Function

    ' Method to load data
    Public Sub LoadData()
        Console.WriteLine("Loading data...")
    End Sub

    ' Method to save data
    Public Sub SaveData()
        Console.WriteLine("Saving data...")
    End Sub
End Class

' Derived class that inherits from DataAccess
Public Class SQLDataAccess
    Inherits DataAccess

    ' Overriding LoadData method to specify SQL data loading
    Public Shadows Sub LoadData()
        Console.WriteLine("Loading SQL data...")
    End Sub

    ' Overriding SaveData method to specify SQL data saving
    Public Shadows Sub SaveData()
        Console.WriteLine("Saving SQL data...")
    End Sub
End Class

' Derived class that inherits from DataAccess
Public Class SQLiteDataAccess
    Inherits DataAccess

    ' Overriding LoadData method to specify SQLite data loading
    Public Shadows Sub LoadData()
        Console.WriteLine("Loading SQLite data...")
    End Sub

    ' Overriding SaveData method to specify SQLite data saving
    Public Shadows Sub SaveData()
        Console.WriteLine("Saving SQLite data...")
    End Sub
End Class

' Base Class Definition
Public Class DataAccess
    ' Method to load the connection string
    Public Function LoadConnectionString() As String
        Console.WriteLine("Loading the connection string...")
        Return "Test Connection String"
    End Function

    ' Method to load data
    Public Sub LoadData()
        Console.WriteLine("Loading data...")
    End Sub

    ' Method to save data
    Public Sub SaveData()
        Console.WriteLine("Saving data...")
    End Sub
End Class

' Derived class that inherits from DataAccess
Public Class SQLDataAccess
    Inherits DataAccess

    ' Overriding LoadData method to specify SQL data loading
    Public Shadows Sub LoadData()
        Console.WriteLine("Loading SQL data...")
    End Sub

    ' Overriding SaveData method to specify SQL data saving
    Public Shadows Sub SaveData()
        Console.WriteLine("Saving SQL data...")
    End Sub
End Class

' Derived class that inherits from DataAccess
Public Class SQLiteDataAccess
    Inherits DataAccess

    ' Overriding LoadData method to specify SQLite data loading
    Public Shadows Sub LoadData()
        Console.WriteLine("Loading SQLite data...")
    End Sub

    ' Overriding SaveData method to specify SQLite data saving
    Public Shadows Sub SaveData()
        Console.WriteLine("Saving SQLite data...")
    End Sub
End Class

建立基底類別

Tim在(3:21)解釋了如何建立基底類別。他重構了程式碼，將DataAccess的基底類別中。透過繼承此基底類別，像SQLiteDataAccess這樣的其他類別可以存取這些共享方法，減少程式碼重複。

將基底類別改為抽象類別

Tim在(5:56)將基底類別轉換成抽象類別以展示差異。他將DataAccess更改為抽象類別，以防止其直接實例化。相反，只有繼承此抽象類別的類別，例如SQLDataAccess，可以實現其方法並使用其共享功能。

以下是當DataAccess成為抽象類別時程式碼的變更方式：

// Abstract Base Class Definition
public abstract class AbstractDataAccess
{
    // Shared method to load connection string
    public string LoadConnectionString()
    {
        Console.WriteLine("Loading the connection string...");
        return "Test Connection String";
    }

    // Abstract methods that must be implemented by derived classes
    public abstract void LoadData();
    public abstract void SaveData();
}

// Abstract Base Class Definition
public abstract class AbstractDataAccess
{
    // Shared method to load connection string
    public string LoadConnectionString()
    {
        Console.WriteLine("Loading the connection string...");
        return "Test Connection String";
    }

    // Abstract methods that must be implemented by derived classes
    public abstract void LoadData();
    public abstract void SaveData();
}

' Abstract Base Class Definition
Public MustInherit Class AbstractDataAccess
    ' Shared method to load connection string
    Public Function LoadConnectionString() As String
        Console.WriteLine("Loading the connection string...")
        Return "Test Connection String"
    End Function

    ' Abstract methods that must be implemented by derived classes
    Public MustOverride Sub LoadData()
    Public MustOverride Sub SaveData()
End Class

' Abstract Base Class Definition
Public MustInherit Class AbstractDataAccess
    ' Shared method to load connection string
    Public Function LoadConnectionString() As String
        Console.WriteLine("Loading the connection string...")
        Return "Test Connection String"
    End Function

    ' Abstract methods that must be implemented by derived classes
    Public MustOverride Sub LoadData()
    Public MustOverride Sub SaveData()
End Class

抽象類別中的介面部分

Tim在(8:34)解釋了抽象類別如何融合介面和基底類別的特性。他在抽象類別內宣告了抽象方法，例如public abstract void SaveData();，但不實現它們。這保証了任何衍生類別必須實現這些方法，類似於介面。

在抽象類別中覆寫方法

Tim在(12:56)討論如何在抽象類別中覆寫方法。他展示了您可以在基底類別中宣告一個方法為virtual，允許衍生類別覆寫它。此方法提供了方法實現和擴展的靈活性。

以下是展示方法覆寫的衍生類別程式碼：

// Derived class from abstract base class
public class SQLDataAccessWithAbstract : AbstractDataAccess
{
    // Implementing the abstract LoadData method
    public override void LoadData()
    {
        Console.WriteLine("Loading SQL data...");
    }

    // Implementing the abstract SaveData method
    public override void SaveData()
    {
        Console.WriteLine("Saving SQL data...");
    }
}

// Derived class from abstract base class
public class SQLDataAccessWithAbstract : AbstractDataAccess
{
    // Implementing the abstract LoadData method
    public override void LoadData()
    {
        Console.WriteLine("Loading SQL data...");
    }

    // Implementing the abstract SaveData method
    public override void SaveData()
    {
        Console.WriteLine("Saving SQL data...");
    }
}

' Derived class from abstract base class
Public Class SQLDataAccessWithAbstract
    Inherits AbstractDataAccess

    ' Implementing the abstract LoadData method
    Public Overrides Sub LoadData()
        Console.WriteLine("Loading SQL data...")
    End Sub

    ' Implementing the abstract SaveData method
    Public Overrides Sub SaveData()
        Console.WriteLine("Saving SQL data...")
    End Sub
End Class

' Derived class from abstract base class
Public Class SQLDataAccessWithAbstract
    Inherits AbstractDataAccess

    ' Implementing the abstract LoadData method
    Public Overrides Sub LoadData()
        Console.WriteLine("Loading SQL data...")
    End Sub

    ' Implementing the abstract SaveData method
    Public Overrides Sub SaveData()
        Console.WriteLine("Saving SQL data...")
    End Sub
End Class

何時使用抽象類別

Tim在(16:01)建議抽象類別不應每天使用，而是在特定情境下很有價值。他提醒我們不要僅僅因為兩個類別共享類似程式碼，就使用抽象類別。相反，他強調保持"是一個"的關係，並建議在適當時考慮使用輔助方法或類別來處理共享程式碼。

以下是展示使用方式的主程式碼：

// Main Program
class Program
{
    static void Main(string[] args)
    {
        // Using Base Class
        SQLDataAccess sqlData = new SQLDataAccess();
        Console.WriteLine(sqlData.LoadConnectionString());
        sqlData.LoadData();
        sqlData.SaveData();

        Console.WriteLine("--------------------------");

        // Using Derived Class from Abstract Base Class
        SQLDataAccessWithAbstract sqlDataAbstract = new SQLDataAccessWithAbstract();
        Console.WriteLine(sqlDataAbstract.LoadConnectionString());
        sqlDataAbstract.LoadData();
        sqlDataAbstract.SaveData();

        Console.WriteLine("--------------------------");

        // You can't instantiate Abstract Base Class directly
        // AbstractDataAccess abstractData = new AbstractDataAccess(); 
        // Error: Cannot create an instance of the abstract class
    }
}

// Main Program
class Program
{
    static void Main(string[] args)
    {
        // Using Base Class
        SQLDataAccess sqlData = new SQLDataAccess();
        Console.WriteLine(sqlData.LoadConnectionString());
        sqlData.LoadData();
        sqlData.SaveData();

        Console.WriteLine("--------------------------");

        // Using Derived Class from Abstract Base Class
        SQLDataAccessWithAbstract sqlDataAbstract = new SQLDataAccessWithAbstract();
        Console.WriteLine(sqlDataAbstract.LoadConnectionString());
        sqlDataAbstract.LoadData();
        sqlDataAbstract.SaveData();

        Console.WriteLine("--------------------------");

        // You can't instantiate Abstract Base Class directly
        // AbstractDataAccess abstractData = new AbstractDataAccess(); 
        // Error: Cannot create an instance of the abstract class
    }
}

' Main Program
Class Program
    Shared Sub Main(ByVal args As String())
        ' Using Base Class
        Dim sqlData As New SQLDataAccess()
        Console.WriteLine(sqlData.LoadConnectionString())
        sqlData.LoadData()
        sqlData.SaveData()

        Console.WriteLine("--------------------------")

        ' Using Derived Class from Abstract Base Class
        Dim sqlDataAbstract As New SQLDataAccessWithAbstract()
        Console.WriteLine(sqlDataAbstract.LoadConnectionString())
        sqlDataAbstract.LoadData()
        sqlDataAbstract.SaveData()

        Console.WriteLine("--------------------------")

        ' You can't instantiate Abstract Base Class directly
        ' Dim abstractData As New AbstractDataAccess()
        ' Error: Cannot create an instance of the abstract class
    End Sub
End Class

' Main Program
Class Program
    Shared Sub Main(ByVal args As String())
        ' Using Base Class
        Dim sqlData As New SQLDataAccess()
        Console.WriteLine(sqlData.LoadConnectionString())
        sqlData.LoadData()
        sqlData.SaveData()

        Console.WriteLine("--------------------------")

        ' Using Derived Class from Abstract Base Class
        Dim sqlDataAbstract As New SQLDataAccessWithAbstract()
        Console.WriteLine(sqlDataAbstract.LoadConnectionString())
        sqlDataAbstract.LoadData()
        sqlDataAbstract.SaveData()

        Console.WriteLine("--------------------------")

        ' You can't instantiate Abstract Base Class directly
        ' Dim abstractData As New AbstractDataAccess()
        ' Error: Cannot create an instance of the abstract class
    End Sub
End Class