在对通用类施加约束时使用new() [英] Use of new() while applying a constraint on generic class

问题描述

我已经使用泛型已有一段时间了，并且已经广泛使用它们.最近，当我在一个涉及泛型的项目中工作时，有些事情让我震惊，我希望有人可以澄清一下.假设我声明一个类如下:

Hi,
I have been using generics for quite a while and have already used them extensively.Recently when i was working on a project involving generics some thing struck me which i want somebody to please clarify. Suppose i declare a class as following:

public class Stack<t> where T : IComparable,new()
{
....
}

在这里，处于T位置的类应实现IComparable接口，并且还应实现一个默认构造函数.但是，我们也知道，如果不提供默认构造函数，则编译器会自动插入默认构造函数.那么在通用类上编写"new()"约束有什么用呢?

Here, the class which takes the position of T should implement IComparable interface and should also implement a default constructor. However we also know the fact that if we don`t provide a default constructor, the compiler automatically inserts a default constructor. So whats the use of writing a "new()" constraint on the generic class ?

推荐答案

这意味着phe无参数构造函数必须以一种或另一种方式存在. br/>

请考虑以下内容:

That means, phe parameterless constructor must exist, one way or another.


Consider the following:

public class Test1 {
   public Test1(string somestring) { }
}

public class Test2<T1> where T1 : new() {
}

现在，如果我尝试定义实例

Now if I try to define an instance

Test2<Test1> testInstance = new Test2<Test1>();

那会失败，

That would fail with

''App.Test1'' must be a non-abstract type with a public parameterless constructor in order to use it as parameter ''T1'' in the generic type or method ''App.Test2<T1>''  

这篇关于在对通用类施加约束时使用new()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！