如何将项目添加到集合，而消费它？ [英] How to add items to a collection while consuming it?

问题描述

下面的例子引发InvalidOperationException集合已修改;枚举操作可能无法执行。执行code时。

The example below throws an InvalidOperationException, "Collection was modified; enumeration operation may not execute." when executing the code.

var urls = new List<string>();
urls.Add("http://www.google.com");

foreach (string url in urls)
{
    // Get all links from the url
    List<string> newUrls = GetLinks(url);

    urls.AddRange(newUrls); // <-- This is really the problematic row, adding values to the collection I'm looping
}

我如何以更好的方式重写这个？我猜一个递归的解决方案吗？

How can I rewrite this in a better way? I'm guessing a recursive solution?

推荐答案

您不能，基本上是这样。你真的想在这里什么是队列：

You can't, basically. What you really want here is a queue:

var urls = new Queue<string>();
urls.Enqueue("http://www.google.com");

while(urls.Count != 0)
{
    String url = url.Dequeue();
    // Get all links from the url
    List<string> newUrls = GetLinks(url);
    foreach (string newUrl in newUrls)
    {
        queue.Enqueue(newUrl);
    }
}

这是稍微难看，由于有不是在队列℃的的AddRange 方法; T＆GT; ，但我认为它基本上你想要什么。

It's slightly ugly due to there not being an AddRange method in Queue<T> but I think it's basically what you want.

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