C#如何搜索最佳匹配重载方法

Question

我收到了几个关于在C#中调用重载（或者应该调用隐藏）方法的问题。假设我有如下类：

class ParaA {}
class ParaB : ParaA {}
class ParaC : ParaB {}
class TheBaseClass 
{
   public void DoJob (ParaA a){Console.WriteLine ("DoJob in TheBaseClass is being invoked");}
}
class TheDerivedClass : TheBaseClass 
{
  public void DoJob (ParaB b){Console.WriteLine ("DoJob in TheDerivedClass is being invoked");}
}
class Test
{
  //Case 1: which version of DoJob() is being called?
  TheDerivedClass aInstance= new TheDerivedClass ();
  aInstance.DoJob(new ParaA ());
  //Case 2: which version of DoJob() is being called?
  TheBaseClass aInstance= new TheDerivedClass ();
  aInstance.DoJob(new ParaA ());
  //Case 3: which version of DoJob() is being called?
  TheBaseClass aInstance= new TheDerivedClass ();
  aInstance.DoJob(new ParaB ());
  //Case 4: which version of DoJob() is being called?
  TheBaseClass aInstance= new TheDerivedClass ();
  aInstance.DoJob(new ParaC ());
}

我希望我已经明确了我要做的事情。我想知道当调用程序提供的参数与任何签名都不完全匹配，但与某些签名兼容时，C#将如何搜索要调用的方法的"匹配"版本。当方法不仅在类中重载，而且被派生类隐藏、重写或重载时，这让我更加困惑。上面的例子并没有涵盖所有可能的情况。这有什么术语吗？

提前谢谢大家！

Matthew

Answer 1

我添加了几行代码来编译：

void Main()
{
    var t = new Test();
    t.Run();
}
class ParaA {}
class ParaB : ParaA {}
class ParaC : ParaB {}
class TheBaseClass 
{
   public void DoJob (ParaA a){Console.WriteLine ("DoJob in TheBaseClass is being invoked");}
}
class TheDerivedClass : TheBaseClass 
{
  public  void DoJob (ParaB b){Console.WriteLine ("DoJob in TheDerivedClass is being invoked");}
}
public class Test
{
    public void Run()
    {
        //Case 1: which version of DoJob() is being called?
        TheDerivedClass aInstance= new TheDerivedClass ();
        aInstance.DoJob(new ParaA ());
        //Case 2: which version of DoJob() is being called?
        TheBaseClass aInstance2= new TheDerivedClass ();
        aInstance2.DoJob(new ParaA ());
        //Case 3: which version of DoJob() is being called?
        TheBaseClass aInstance3= new TheDerivedClass ();
        aInstance3.DoJob(new ParaB ());
        //Case 4: which version of DoJob() is being called?
        TheBaseClass aInstance4= new TheDerivedClass ();
        aInstance4.DoJob(new ParaC ());
    }
}

产生输出：

DoJob in TheBaseClass is being invoked
DoJob in TheBaseClass is being invoked
DoJob in TheBaseClass is being invoked
DoJob in TheBaseClass is being invoked

即每次都调用基类方法。

在情况1中，之所以调用它，是因为自变量是ParaA，而ParaA不是ParaB。在其他情况下，之所以调用它，是因为对象实例的类型为"TheBaseClass"。

以下是为说明方法重载而修改的相同代码：

void Main()
{
    var t = new Test();
    t.Run();
}
class ParaA {}
class ParaB : ParaA {}
class ParaC : ParaB {}
class TheBaseClass 
{
   public virtual void DoJob (ParaA a){Console.WriteLine ("DoJob in TheBaseClass is being invoked");}
}
class TheDerivedClass : TheBaseClass 
{
  public override void DoJob (ParaA b){Console.WriteLine ("DoJob in TheDerivedClass is being invoked");}
}
public class Test
{
    public void Run()
    {
        //Case 1: which version of DoJob() is being called?
        TheDerivedClass aInstance= new TheDerivedClass ();
        aInstance.DoJob(new ParaA ());
        //Case 2: which version of DoJob() is being called?
        TheBaseClass aInstance2= new TheDerivedClass ();
        aInstance2.DoJob(new ParaA ());
        //Case 3: which version of DoJob() is being called?
        TheBaseClass aInstance3= new TheDerivedClass ();
        aInstance3.DoJob(new ParaB ());
        //Case 4: which version of DoJob() is being called?
        TheBaseClass aInstance4= new TheDerivedClass ();
        aInstance4.DoJob(new ParaC ());
    }
}

现在的输出是：

DoJob in TheDerivedClass is being invoked
DoJob in TheDerivedClass is being invoked
DoJob in TheDerivedClass is being invoked
DoJob in TheDerivedClass is being invoked

每次都调用DerivedClass方法，因为对象的类型为"TheDerivedCClass"。

Answer 2

Case2-4调用TheBaseClass只是因为DoJob不是虚拟方法，并且aInstance的类型是TheBaseClass。

Case1调用TheBaseClass，因为它是一个直接匹配。