c# - 接口(interface)、继承和 'new' 关键字

Question

我想知道是否有人可以向我解释一下:

class Program
{
    static void Main()
    {
        AnotherDerivedClass d = new AnotherDerivedClass();
        Console.WriteLine(d.PrintMessage());

        IMsg m = d as IMsg;
        //Why this prints BaseClass.
        //How does it know that IMsg is implemented in the BaseClass.
        Console.WriteLine(m.PrintMessage());

        IMsg n = d as DerivedClass;
        //Why this prints BaseClass and not DerivedClass
        Console.WriteLine(n.PrintMessage());

        Console.Read();
    }
}

public interface IMsg
{
    string PrintMessage();
}

public class BaseClass : IMsg
{
    public string PrintMessage()
    {
        return "BaseClass";
    }
}

public class DerivedClass : BaseClass
{
    public new string PrintMessage()
    {
        return "DerivedClass";
    }
}

public class AnotherDerivedClass : DerivedClass
{
    public new string PrintMessage()
    {
        return "AnotherDerivedClass";
    }
}

Answer 1

您已经替换了派生类中的实现，而不是覆盖它们。如果您使用 BaseClass，将使用原始实现。

您需要将基类中的方法设为虚拟:

public class BaseClass : IMsg
{

    public BaseClass()
    {

    }

    public virtual string PrintMessage()
    {
        return "BaseClass";
    }
}

并在派生类中覆盖:

public class DerivedClass : BaseClass
{
    public DerivedClass()
    {

    }

    public override string PrintMessage()
    {
        return "DerivedClass";
    }
}

获取您指定的行为。