C＃中的抽象类和接口有什么区别？

在C#中，抽象类用于共享代码和提供默认行为，而接口用于定义无实现细节的契约。1. 当需要在多个相关类之间共享代码或提供默认实现时，使用抽象类；2. 当需要多个不相关的类实现相同行为或支持多继承时，使用接口；3. 抽象类可包含实现、字段和构造函数，接口仅定义成员签名（C# 8.0前），但从C# 8.0起接口可包含默认实现；4. 若需定义“能力”而非“种类”关系，或需解耦设计，应选择接口；5. 若需在未来添加方法而不破坏现有代码，抽象类更优；6. 现代C#中接口支持默认实现，但仍推荐接口用于能力定义，抽象类用于共享逻辑和状态。

In C#, both abstract classes and interfaces are used to achieve abstraction, but they serve different purposes and have distinct characteristics. Choosing between them depends on the design needs of your application.

When to Use an Abstract Class

An abstract class is a class that cannot be instantiated on its own and must be inherited by other classes. It can contain both implemented methods and abstract methods (which have no implementation).

You'd typically use an abstract class when:

• You want to share code among several closely related classes.
• You need to provide a common base class with some default behavior.
• You might want to add new methods in the future without breaking existing derived classes.

For example, if you're building a set of shapes like Circle, Rectangle, and Triangle, and they all share some basic logic for calculating area or perimeter, putting that shared logic into an abstract class makes sense.

public abstract class Shape
{
    public abstract double Area();

    public void PrintArea()
    {
        Console.WriteLine($"Area: {Area()}");
    }
}

Here, PrintArea provides shared functionality, while Area() forces derived classes to implement their own version.

When to Use an Interface

An interface defines a contract that implementing classes must follow. It contains only method signatures, properties, events, or indexers — no implementation details.

Interfaces are ideal when:

• You need multiple, unrelated classes to support the same behavior.
• You want to enable multiple inheritance of behavior (C# doesn’t allow multiple base classes, but allows multiple interface implementations).
• You’re designing for testability or dependency injection, where loose coupling is important.

A typical example is a logging system. Different logging services (like FileLogger, DatabaseLogger) may implement the same interface:

public interface ILogger
{
    void Log(string message);
}

Now, any class that implements ILogger must have a Log method. This lets you write generic code that works with any logger.

Key Differences Between Abstract Classes and Interfaces

Here's a quick comparison of the main differences:

• Implementation:
  Abstract classes can have method implementations; interfaces only define what methods should exist.

• Access Modifiers:
  Members of an abstract class can have access modifiers (private, protected, etc.), but interface members are always public.

• Fields and Constants:
  Abstract classes can have fields and constants; interfaces can't have fields (prior to C# 8.0), though they can have static members starting from C# 8.0.

• Constructors and Destructors:
  Abstract classes can have constructors and destructors; interfaces cannot.

• Multiple Inheritance:
  A class can implement multiple interfaces but inherit from only one abstract class.

So, if you need to enforce a structure across unrelated types, go with an interface. If you're modeling a family of related objects and want to share implementation, use an abstract class.

How to Decide Which One to Use

Ask yourself these questions:

• Do I need to share code and state between subclasses?
  → Yes → Use an abstract class.

• Are the implementing types not necessarily related by inheritance?
  → Yes → Use an interface.

• Do I want to define a capability rather than a "kind of" relationship?
  → Yes → Interface is likely better.

• Will this evolve over time, and do I want to avoid breaking changes?
  → Interfaces are trickier to change once published, because all implementers must adapt. Abstract classes allow adding non-abstract methods without breaking derived classes.

In modern C#, especially from C# 8.0 onward, interfaces can now include default implementations. This blurs the line a bit, but the general guidance still holds: use interfaces for capabilities and abstract classes for shared logic and state.

That’s the core of it — not too complicated, but easy to mix up if you don’t work with them daily.

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