Maybe you are trying to do
string combinedString = string.Join( ",", myList.ToArray() );
You can replace “,” with what you want to split the elements in the list by.
Edit: As mentioned in the comments you could also do
string combinedString = string.Join( ",", myList);
Reference:
Join<T>(String, IEnumerable<T>)
Concatenates the members of a collection, using the specified separator between each member.