类内的代理的默认值 [英] Default value of delegates inside classes

问题描述

using System;

public delegate void Printer(string s);

class Program
{
    public static void Main(string[] args)
    {
        Printer p = new Printer(delegate {});
        p+= myPrint;
        p("Hello");
        Console.ReadKey(true);
    }

    public static void myPrint(string s)
    {
        System.Console.WriteLine(s);
    }
}

好像我必须初始化一个委托一个空的匿名函数，以便能够使用 + = 。当我省略新的子句时， p 将被 null 和 + = 不起作用，这是有道理的。

It seems as if I have to initialize a delegate with an empty anonymous function to be able to use += later on. When I omit the new clause, p gets to be null and += doesn't work, which makes sense.

现在，当我有一个类代理实例，我可以执行以下操作：

Now, when I have a class with a delegate instance, I can do the following:

using System;

public delegate void Printer(string s);

class Program
{
    public static void Main(string[] args)
    {
        A a = new A();
        a.p += myPrint;
        a.p("Hello");
        Console.ReadKey(true);
    }

    public static void myPrint(string s)
    {
        System.Console.WriteLine(s);
    }
}


class A {
    public Printer p;
}

为什么允许这样做？代理实例有一个默认值 p ？它不能是 null ，因为我将无法使用 + = 为它分配一个新的回调。我试图通过关键字代理的默认值搜索这个问题，没有发现任何内容。另外，如果问题是否太基础，对不起。

Why is this allowed? Is there a default value for the delegate instance p? It can't be null because then I would not be able to assign it a new callback using +=. I have tried to search for this problem with the keywords "default value for delegates" and found nothing. Also, sorry if the question if too basic.

感谢您的帮助！

推荐答案

代理是引用类型，因此默认值为 null 。

但是，变量字段）不会默认初始化：

However, variables (unlike fields) are not initialized by default:

Printer p;
p += myPrint; // doesn't work: uninitialized variable

您需要初始化变量才能使用它：

You need to initialize the variable before you can use it:

Printer p = null;
p += myPrint;

或

Printer p;
p = null;
p += myPrint;

---

请注意，对于代表（但不是事件！）

---

Note that for delegates (but not events!)

p += myPrint;

是

p = (Printer)Delegate.Combine(p, new Printer(myPrint));

这篇关于类内的代理的默认值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！