InvertedAcceleration InvertedAcceleration - 1 month ago 7
C# Question

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

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

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));
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