当应一类成员声明为虚函数（C＃）/可重写（VB.NET）？ [英] When should a class member be declared virtual (C#)/Overridable (VB.NET)?

问题描述

为什么不我选择抽象？什么是限制声明一个类成员的虚拟？只能方法声明为虚函数？

Why wouldn't I choose abstract? What are the limitations to declaring a class member virtual? Can only methods be declared virtual?

推荐答案

这是抽象方法或属性（都可以是虚拟的或抽象的）只能在抽象类中声明的，不能有身体，也就是你无法实现它在你的抽象类。

An abstract method or property (both can be virtual or abstract) can only be declared in an abstract class and cannot have a body, i.e. you can't implement it in your abstract class.

一个虚拟的方法或属性必须有一个机构，即你必须提供一个实现（即使身体是空的）。

A virtual method or property must have a body, i.e. you must provide an implementation (even if the body is empty).

如果有人想用你的抽象类，他将要实现一个类继承自它并明确实现抽象方法和属性，但可以选择不重写虚方法和属性。

If someone want to use your abstract class, he will have to implement a class that inherits from it and explicitly implement the abstract methods and properties but can chose to not override the virtual methods and properties.

例：

using System;
using C=System.Console;

namespace Foo
{
    public class Bar
    {
    	public static void Main(string[] args)
    	{
    		myImplementationOfTest miot = new myImplementationOfTest();
    		miot.myVirtualMethod();
    		miot.myOtherVirtualMethod();
    		miot.myProperty = 42;
    		miot.myAbstractMethod();
    	}
    }

    public abstract class test
    {
    	public abstract int myProperty
    	{
    		get;
    		set;
    	}

    	public abstract void myAbstractMethod();

    	public virtual void myVirtualMethod()
    	{
    		C.WriteLine("foo");
    	}

    	public virtual void myOtherVirtualMethod()
    	{
    	}
    }

    public class myImplementationOfTest : test
    {
    	private int _foo;
    	public override int myProperty
    	{
    		get { return _foo; }
    		set { _foo = value; }
    	}

    	public override void myAbstractMethod()
    	{
    		C.WriteLine(myProperty);
    	}

    	public override void myOtherVirtualMethod()
    	{
    		C.WriteLine("bar");
    	}
    }
}

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