学习 C#

了解 C# 抽象类

Tim Corey

19m 59s

C# 中的抽象类是一个基本概念，经常会引起开发人员的疑问。在他的视频" C# 抽象类 - 它们是什么、如何使用它们以及最佳实践"中，Tim Corey 深入探讨了抽象类是什么、如何使用它们以及最佳实践。本文总结了他视频中的要点，并附有时间戳以供参考。

简介

Tim（0:00）解释说，抽象类的目的、功能和重要性经常受到质疑。他将抽象类描述为介于完整基类和接口之间的混合体，在功能上介于两者之间。

应用程序演示

在（0:59），Tim 通过演示应用程序来演示抽象类。该应用程序包含一个控制台应用程序和一个类库，其中包含两个模拟数据库操作的数据访问类。这些类具有加载和保存数据的方法，Tim 用这些方法来说明抽象类、基类和接口之间的异同。

以下是基类和派生类的初始代码：

// 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

创建基类

Tim 在 (3:21) 处解释了如何创建基类。他重构代码，将一些通用方法，如DataAccess的基类中。通过继承这个基类，其他类如SQLiteDataAccess能够访问这些共享的方法，从而减少代码重复。

将基类抽象化

Tim 在 (5:56) 将基类转换为抽象类，以演示两者之间的区别。他将DataAccess更改为抽象类，防止其被直接实例化。相反，只有继承自该抽象类的类，如SQLDataAccess，可以实现其方法并使用其共享功能。

当DataAccess变为抽象类时，代码更改如下：

// 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

抽象类中的接口部分

Tim 在 (8:34) 解释了抽象类如何融合接口和基类的特性。他在抽象类中声明抽象方法，如public abstract void SaveData();，而不实现它们。这样就确保了任何派生类都必须实现这些方法，类似于接口的工作方式。

抽象类中的方法重写

Tim 在 (12:56) 讨论了如何重写抽象类中的方法。他展示了可以在基类中声明一个方法为virtual，允许派生类重写它。这种方法为派生类中方法的实现和扩展提供了灵活性。

以下是派生类的代码，展示了方法重写：

// 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

何时使用抽象类

Tim 在 (16:01) 建议，抽象类不应该每天都使用，但在特定情况下很有价值。他告诫人们不要仅仅因为两个类共享相似的代码就使用抽象类。相反，他强调要保持"is a"关系，并建议在适当的时候考虑使用辅助方法或类来共享代码。

以下是演示用法的主要程序代码：

// 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