string s1, s2, s3;
int p1;
s1 = "Hello world! ";
s2 = "Lucy";
p1 = s1.Length;
// 首先 Insert 返回在指定索引处插入指定字符串的新字符串
// p1 是s1字符串的长度
// p1 - 6的下标插入s2的字符串
// "Hello " + "wLucy" + "orld! "
// Hello wLucyorld!
s3 = s1.Insert(p1 - 6, s2);
Console.WriteLine(s3);
Hello wLucyorld!