List<string> source = new List<string> {"A", "B", "C"};

List<string> items = new List<string>(source);
//                   ^^^ IDE0028: Collection can be simplified
items[^1] = "X";

List<string> items = new List<string>(source)
{
    [^1] = "X" // CS1913: Member '[^1]' cannot be initialized. It is not a field or property.
};