Question

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 } 

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

Answer 1

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);     } } 

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