Collection was modified after the enumerator was instantiated.

IF you are using Foreach loop for modifying collection then you will get this error as below.

List<string> li = new List<string>();

            li.Add("bhanu");

            li.Add("test");

            foreach (string s in li)

            {

                li.Remove(s);

            }

Solution - use For Loop as below.

            for (int i = 0; i < li.Count; i++)

            {

                li.RemoveAt(i);

                i--;

            }