在C#中，如果未将类或方法标记为密封的或虚的，那是什么? [英] In C#, if a class or a method is not marked as sealed or virtual, what is it?

问题描述

换句话说，默认值是什么(如果未指定任何内容)?我猜默认值是虚拟的，因为即使没有指定虚拟方法，也可以使用"new"关键字来覆盖基本方法.如果是这样，那为什么我们甚至需要虚拟选项?当我们需要阻止进一步的继承时，我们是否只能使用Sealed?

In other words, what's the default (if nothing is specified)? I'm guessing the default is virtual, because you can use the "new" keyword to override a base method even when the base method has no virtual specified. If that's the case then why do we even need a Virtual option? Couldn't we just use Sealed when we do need to prevent further inheritance?

推荐答案

默认情况下，C#方法是密封的-如果没有 virtual 关键字，则无法覆盖它们.

C# methods are sealed by default -- you cannot override them without the virtual keyword.

new 关键字隐藏基类中的方法.

The new keyword hides the method in the base class.

这就是我隐藏的意思:

public class HidingA
{
    public string Get()
    {
        return "A";
    }
}

public class HidingB : HidingA
{
    public new string Get()
    {
        return "B";
    }
}

HidingA a = new HidingA();
HidingB b = new HidingB();

Console.WriteLine(a.Get()); // "A"
Console.WriteLine(b.Get()); // "B"

HidingA c = new HidingB();
Console.WriteLine(c.Get()); // "A". Since we're calling this instance of "B" an "A",    
                            //we're using the "A" implementation.

现在，虚拟版本！

public class VirtualA
{
    public virtual string Get()
    {
        return "A";
    }
}

public class VirtualB : VirtualA
{
    public override string Get()
    {
        return "B";
    }
}
VirtualA a = new VirtualA();
VirtualB b = new VirtualB();

Console.WriteLine(a.Get()); // "A"
Console.WriteLine(b.Get()); // "B"

VirtualA c = new VirtualB();
Console.WriteLine(c.Get()); // "B". We overrode VirtualB.Get, so it's using the 
                            // "B" implementation of the method

因此，如果我们采用以 HidingA 作为参数并将其传递为 HidingB 的实例的方法，我们将获得 HidingA Get 方法的实现.

So if we make a method that takes HidingA as a parameter and pass it an instance of a HidingB, we're going to get the HidingA implementation of the Get method.

MSDN: http://msdn.microsoft.com/zh-cn/library/6fawty39.aspx

除非指定 sealed 关键字，否则打开类进行继承.

Classes are open for inheritance unless the sealed keyword is specified.

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