C# 字符串的累加优化

//未优化前 运行结果为ABC
  var strList = new List<string>() { "A", "B", "C" };
            var str1 = "";
            foreach (var str in strList)
            {
                str1 += str;
            }

//优化后 Append运行结果是 ABC  ；AppendLine运行结果是A B C 
            var str2 = new StringBuilder();
            foreach (var str in strList)
            {
                //Append 是累加 ；AppendLine是累加并且加一个空格
                str2.Append(str);
                str2.AppendLine(str);
            }            

//最简介代码的优化 运行结果为ABC
var str5 = strList.Aggregate((a, b) => a + b);