关于AppendFormat("{0:X2}")

StringBuilder sb = new StringBuilder();
sb.AppendFormat("{0:X2}", 10);
Response.Write(sb);

输出A

X2表示十六进制格式（大写），域宽2位，不足的左边填0。

int[] recvLen = { 10, 11, 12, 13, 14, 15, 16, 17, 18 };
StringBuilder uid = new StringBuilder();
for (int i = 0; i <= recvLen.Length - 1; i++)
    uid.AppendFormat("{0:X2}\r\n", recvLen[i]);

Console.WriteLine(uid.ToString());

输出：
0A
0B
0C
0D
0E
0F
10
11
12