How to remove elements from a generic list while iterating over it?

Question

I am looking for a better pattern for working with a list of elements which each need processed and then depending on the outcome are removed from the list.

You can't use .Remove(element) inside a foreach (var element in X) (because it results in Collection was modified; enumeration operation may not execute. exception)... you also can't use for (int i = 0; i < elements.Count(); i++) and .RemoveAt(i) because it disrupts your current position in the collection relative to i.

Is there an elegant way to do this?

Answer 1

Iterate your list in reverse with a for loop:

for (int i = safePendingList.Count - 1; i >= 0; i--) {     // some code     // safePendingList.RemoveAt(i); } 

Example:

var list = new List<int>(Enumerable.Range(1, 10)); for (int i = list.Count - 1; i >= 0; i--) {     if (list[i] > 5)         list.RemoveAt(i); } list.ForEach(i => Console.WriteLine(i)); 

Alternately, you can use the RemoveAll method with a predicate to test against:

safePendingList.RemoveAll(item => item.Value == someValue); 

Here's a simplified example to demonstrate:

var list = new List<int>(Enumerable.Range(1, 10)); Console.WriteLine("Before:"); list.ForEach(i => Console.WriteLine(i)); list.RemoveAll(i => i > 5); Console.WriteLine("After:"); list.ForEach(i => Console.WriteLine(i));