char[] char_list = new char[] { 'a','b','c'};
                var temp_list = char_list.ToList();
                temp_list.Insert(1, 'd');
                string result = new string(temp_list.ToArray());//adbc

------解决方案--------------------
先装换成List然后再转成数组
char[] char_list = new char[] { 'a','b','c'};
List<char> listchar=new List<char>();
foreach(char v in char_list)
{
 listchar.add(v);
}
char[] char_newlist=(char[])listchar.ToArry(typeof(char));