How to add items to a collection while consuming it?

Question

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

var urls         


        

Answer 1

Jon's approach is right; a queue's the right data structure for this kind of application.

Assuming that you'd eventually like your program to terminate, I'd suggest two other things:

• don't use string for your URLs, use System.Web.Uri: it provides a canonical string representation of the URL. This will be useful for the second suggestion, which is...
• put the canonical string representation of each URL you process in a Dictionary. Before you enqueue a URL, check to see if it's in the Dictionary first.