C# class 可以从自己的实现中调用接口的默认接口方法吗？

Question

如果我有这样的默认接口方法：

public interface IGreeter
{
    void SayHello(string name) => System.Console.WriteLine($"Hello {name}!");
}

我可以让我的具体实现调用该默认方法吗？

public class HappyGreeter : IGreeter
{
    public void SayHello(string name)
    {
        // what can I put here to call the default method?
        System.Console.WriteLine("I hope you're doing great!!");
    }
}

所以调用：

var greeter = new HappyGreeter() as IGreeter;
greeter.SayHello("Pippy");

结果如下：

// Hello Pippy!
// I hope you're doing great!!

实际上，从实现 class 调用 C# 接口默认方法表明我可以调用我的 class未实现的方法，但正如预期的那样，添加对((IGreeter)this).SayHello(name); 在HappyGreeter.SaysHello内部会导致堆栈溢出。

Answer 1

据我所知，您不能在继承 class 时调用默认接口方法实现（尽管有建议）。 但是你可以从继承接口调用它：

public class HappyGreeter : IGreeter
{
    private interface IWorkAround : IGreeter
    {
        public void SayHello(string name)
        {
            (this as IGreeter).SayHello(name);
            System.Console.WriteLine("I hope you're doing great!!");
        }
    }

    private class WorkAround : IWorkAround {}

    public void SayHello(string name)
    {
        ((IWorkAround)new WorkAround()).SayHello(name);
    }
}

UPD

在我最初的答案中，我非常想表明您可以在继承接口中调用base ，但正如@Alexei Levenkov在评论中建议的那样，在这种特殊情况下，更简洁的方式将是这样的：

public class HappyGreeter : IGreeter
{
    private class WorkAround : IGreeter { }
    private static readonly IGreeter _workAround = new WorkAround();

    public void SayHello(string name)
    {
        _workAround.SayHello(name);
        System.Console.WriteLine("I hope you're doing great!!");
    }
} 

Answer 2

我知道这不是问题的答案，但下一种方法也可用于模拟base功能：

public interface IGreeter
{
    void SayHello(string name) => BaseSayHello(name);

    // This static method can be used in implementers of "IGreeter"
    // to emulate "base" functionality.
    protected static void BaseSayHello(string name) => System.Console.WriteLine($"Hello {name}!");
}

public class HappyGreeter : IGreeter
{
    public void SayHello(string name)
    {
        IGreeter.BaseSayHello(name);
        Console.WriteLine("I hope you're doing great!!");
    }
}

Answer 3

有一个非常简单的方法来处理这个：

1. 将默认方法声明为 static。 别担心，您仍然可以在继承自它的 class 中覆盖它。
2. 在调用 class 的 static 方法时，使用相同类型的语法调用默认方法，只需将接口名称替换为 class 名称即可。

此代码适用于 C#8 或更高版本，如果针对 .NET 框架构建，它将不起作用。 我用 C#9 在 Windows 10 上运行它，在 .NET 6 上运行，预览 5。

例子：

public interface IGreeter
{
   private static int DisplayCount = 0;
   public static void SayHello(string name)
   {
      DisplayCount++;
      Console.WriteLine($"Hello {name}! This method has been called {DisplayCount} times.");
   }
}

public class HappyGreeter : IGreeter
{
   public void SayHello(string name)
   {
      // what can I put here to call the default method?
      IGreeter.SayHello(name);
      Console.WriteLine("I hope you're doing great!!");
   }
}

public class CS8NewFeatures
{
   // This class holds the code for the new C# 8 features.
   //
   public void RunTests()
   {
      TestGreeting();
   }

   private void TestGreeting()
   {
      // Tests if a default method may be called after a class has implemented it.
      //
      var hg = new HappyGreeter();
      hg.SayHello("Pippy");
      hg.SayHello("Bob");
      hg.SayHello("Becky");
   }
}

示例 Output：

Hello Pippy! This method has been called 1 times.
I hope you're doing great!!
Hello Bob! This method has been called 2 times.
I hope you're doing great!!
Hello Becky! This method has been called 3 times.
I hope you're doing great!!

Static 字段现在也允许在接口中，如本示例所示。

如果由于某种原因您不能使用 static 接口方法，那么您始终可以依赖以下技术：

1. 将默认方法实现代码放入单独的 static 方法中。
2. 从默认实现方法中调用此 static 方法。
3. 在接口方法的 class 实现的顶部调用此 static 方法。

例子：

public class DefaultMethods
{
   // This class is used to show that a static method may be called by a default interface method.
   //
   public static void SayHello(string name) => Console.WriteLine($"Hello {name}!");
}

public interface IGreeter
{
   void SayHello(string name) => DefaultMethods.SayHello(name);
}

public class HappyGreeter : IGreeter
{
   public void SayHello(string name)
   {
      // what can I put here to call the default method?
      DefaultMethods.SayHello(name);
      Console.WriteLine("I hope you're doing great!!");
   }
}

public class CS8NewFeatures
{
   // This class holds the code for the new C# 8 features.
   //
   public void RunTests()
   {
      TestGreeting();
   }

   private void TestGreeting()
   {
      // Tests if a default method may be called after a class has implemented it.
      //
      var hg = new HappyGreeter();
      hg.SayHello("Bob");
   }
}

样品 Output：

Hello Bob!
I hope you're doing great!!