为什么一定要在新的（）约束要求的公共构造？ [英] Why must the new() constraint require a public constructor?

问题描述

新的约束规定，任何   在泛型类的类型参数   声明必须有一个公共的   参数的构造函数。

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor.

来源： http://msdn.microsoft .COM / EN-US /库/ sd2w2ew5（V = VS.80）的.aspx

如果我想我的泛型类有一个保护参数的构造函数呢？举例来说，如果我想编写一个辛格尔顿类我附加其他类，使它们辛格尔顿取值，我不希望派生类是实例化 - 一切都应该通过 .Instance 属性

What if I wanted my generic class to have a protected parameterless constructor instead? For instance, if I want to write a Singleton class which I "attach" to other classes to make them Singletons, I don't want the derived classes to be instantiable - everything should go through the .Instance property.

internal class Singleton<T> where T : new()
{
    public static T Instance { get; private set; }

    static Singleton()
    {
        Singleton<T>.Instance = new T();
    }
}

internal class OnlyOneOfMe : Singleton<OnlyOneOfMe>
{
    protected OnlyOneOfMe()
    {
    }
}

这样，辛格尔顿＆LT; T＆GT; 能够创建 OnlyOneOfMe 类的唯一实例，但没有别人能（除非它是一个子类）。

This way, Singleton<T> is able to create the only instance of the OnlyOneOfMe class, but nothing else can (unless it is a subclass).

如果一个通用的父类可以访问泛型类型的保护成员？

"What if a generic parent class could access the generic type's protected members?"

推荐答案

由于是约束的定义。这有点像问为什么 T：类要求 T 是引用类型。这是真正的定义。

Because that is the definition of the constraint. It's a bit like asking why does T : class require that T be a reference type. It's true by definition.

此外，如果它不是一个公共构造函数，这将是约束的地步？该类接收类型参数 T 将无法调用构造函数，如果不是公开的。

Additionally, if it weren't a public constructor, what would be the point of the constraint? The class receiving the type parameter T wouldn't be able to call the constructor if it weren't public.

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