Question

I have a collection lets say list of integers and looping through it using foreach, but if I have to insert data into the list during the loop, how do I do that ? I’m getting

Unhandled Exception: System.InvalidOperationException: Collection was
modified enumeration operation may not execute.

This is the sample code below. I thought AsReadonly() would return a collection that was prior to modified state. That is not the case here.

ints = new List<int>(30);
ints.AddRange(Enumerable.Range(1, 10));
int y = 11;

foreach(int x in ints.AsReadOnly())
{
    ints.Add(y++);
    Console.WriteLine(x);
}

Answer 1

As others mentioned AsReadonly() only produces a wrapper around the original list.

Something like this would work. But I am not sure what you are trying to do so it might not be helpful.

var ints = new List<int>();
ints.AddRange(Enumerable.Range(1, 10));
int y = 11;

var moreInts = new List<int>();

foreach(int x in ints.AsReadOnly())
{
    moreInts.Add(y++);
    Console.WriteLine(x);
}

ints.AddRange(moreInts);