如何在C#中重写子类的重写方法中的子类型？

public class BaseClass
{
    public virtual void MyMethod()
    {
        Console.WriteLine("Base class method");
    }
}

public class SubClass : BaseClass
{
    public override void MyMethod()
    {
        Console.WriteLine("Sub class method");
    }
}

public class SubSubClass : SubClass
{
    public override void MyMethod()
    {
        Console.WriteLine("Sub-sub class method");
    }
}

public static void Main(string[] args)
{
    BaseClass baseObj = new BaseClass();
    SubClass subObj = new SubClass();
    SubSubClass subSubObj = new SubSubClass();

    baseObj.MyMethod();
    subObj.MyMethod();
    subSubObj.MyMethod();
}

Base class method
Sub class method
Sub-sub class method