List<string> list = new List<string> { "1", "3", "2", "3", "4" };
string strs = "";
foreach (var item in list)
{
     strs += item;
}

List<string> list = new List<string> { "1", "3", "2", "3", "4" };
string strs = (from s in list
               select s).Aggregate((s, next) => s + next);

但是使用 + 来连接字符串，性能低下，极大浪费