hCom1=CreateFile("COM1", //串口號
GENERIC_READ|GENERIC_WRITE, //允許讀寫
0, // 通訊設備必須以獨佔方式打開
NULL, //無安全屬性
OPEN_EXISTING, //通訊設備已存在
0,0);

我是以此方式連接上 COM1 的設備。
但是如果設備是在com10時，也就是"COM1"變成"COM10"時

(hCom1 == INVALID_HANDLE_VALUE) 就會成立，連接不上COM10了

經過測試COM1 ~ COM9均可以正常連線，不知為什麼變成2位數以上的com port時就會結接失敗了。

煩請各位大大給予解答指教。

我找到解答了!!

CreateFile() can be used to get a handle to a serial port. The "Win32 Programmer's Reference" entry for "CreateFile()" mentions that the share mode must be 0, the create parameter must be OPEN_EXISTING, and the template must be NULL.

CreateFile() is successful when you use "COM1" through "COM9" for the name of the file; however, the message "INVALID_HANDLE_VALUE" is returned if you use "COM10" or greater.

If the name of the port is \\.\COM10, the correct way to specify the serial port in a call to CreateFile() is as follows:

   CreateFile(
      "\\\\.\\COM10",     // address of name of the communications device
      fdwAccess,          // access (read-write) mode
      0,                  // share mode
      NULL,               // address of security descriptor
      OPEN_EXISTING,      // how to create
      0,                  // file attributes
      NULL                // handle of file with attributes to copy
   );

NOTES: This syntax also works for ports COM1 through COM9. Certain boards will let you choose the port names yourself. This syntax works for those names as well.