为什么一个集合初始化表达式需要的IEnumerable中实现？ [英] Why does a collection initializer expression require IEnumerable to be implemented?

问题描述

为什么这生成编译器错误：

Why does this generate a compiler error:

class X { public void Add(string str) { Console.WriteLine(str); } }

static class Program
{
    static void Main()
    {
        // error CS1922: Cannot initialize type 'X' with a collection initializer
        // because it does not implement 'System.Collections.IEnumerable'
        var x = new X { "string" };
    }
}

但这并不：

but this doesn’t:

class X : IEnumerable
{
    public void Add(string str) { Console.WriteLine(str); }
    IEnumerator IEnumerable.GetEnumerator()
    {
        // Try to blow up horribly!
        throw new NotImplementedException();
    }
}

static class Program
{
    static void Main()
    {
        // prints "string" and doesn’t throw
        var x = new X { "string" };
    }
}

什么是限制集合初始化的原因 - 这是语法糖到添加方法的调用 - 来实现它没有一个接口的类的有无的一个添加方法和未使用的？

What is the reason for restricting collection initializers — which are syntactic sugar for a call to an Add method — to classes that implement an interface which doesn’t have an Add method and which isn’t used?

推荐答案

这是对象的初始化没有按'吨;一个的集的初始化一样。它是如此，它的应用类这实际上代表的集合，而不仅仅是那些随意其中有一个添加方法。我不得不承认，每隔一段时间我实现的IEnumerable 明确，只允许集合初始化 - 但抛出 NotImplementedException 从的GetEnumerator（）。

An object initializer doesn't; a collection initializer does. It's so that it's applied to classes which really represent collections, rather than just arbitrary ones which have an Add method. I have to admit that every so often I've "implemented" IEnumerable explicitly, just to allow collection initializers - but thrown a NotImplementedException from GetEnumerator().

请注意在C＃3的发展早期，集合初始化不得不实施的ICollection< T> ，但被认为是过于严格。 MADS托格森博客上讲述这个变化和背后需要的IEnumerable ，早在2006年。

Note that early in C# 3's development, collection initializers had to implement ICollection<T>, but that was found to be too restrictive. Mads Torgersen blogged about this change, and the reason behind requiring IEnumerable, back in 2006.

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