在開發一個Modbus的串列埠監測工具軟體的時候，啟動工具軟體，希望一開始就能在下拉框檢測到當前有效的USB串列埠。

剛開始做的時候是用的窮舉法，就是“COM0”~“COM15”一個一個嘗試開啟。但是，這樣做一個是效率低下，另一個是換了新的USB-串列埠介面卡，有可能虛擬串列埠埠號超過了COM15，這樣就找不到可用串列埠了。網上查找了資料，通過讀取windows登錄檔的方式，很好解決了這個問題。

void CSerialCtrlDemoDlg::GetAllCom(void)
{
	long   lReg; 
	HKEY   hKey;
	DWORD  MaxValueLength; 
	DWORD  dwValueNumber; 

	lReg=RegOpenKeyExA(HKEY_LOCAL_MACHINE, "HARDWARE\\DEVICEMAP\\SERIALCOMM", 0, KEY_QUERY_VALUE, &hKey); 
	if(lReg!=ERROR_SUCCESS) 
	{ 
	  AfxMessageBox("Please check comport!\n"); 
	  return; 
	} 

	lReg=RegQueryInfoKeyA(  hKey, 
							NULL, 
							NULL, 
							NULL, 
							NULL, 
							NULL, 
							NULL, 
							&dwValueNumber, //返回和hKey關聯的值 
							&MaxValueLength, 
							NULL, 
							NULL, 
							NULL); 
	if( lReg != ERROR_SUCCESS )  
	{	//沒有成功
		AfxMessageBox("Getting Info Error!\n"); 
		return; 
	} 

	LPSTR  pValueName,	pCOMNumber; 
	DWORD  cchValueName,dwValueSize=6; 

	for(DWORD  i=0; i < dwValueNumber; i++) 
	{ 
		cchValueName = MaxValueLength + 1; 
		dwValueSize  = 6; 
		pValueName=(LPSTR)VirtualAlloc(NULL, cchValueName, MEM_COMMIT, PAGE_READWRITE); 
		
		lReg=RegEnumValueA( hKey, 
							i, 
							pValueName, 
							&cchValueName, 
							NULL, 
							NULL, 
							NULL, 
							NULL); 
  
		if( (lReg != ERROR_SUCCESS) && (lReg != ERROR_NO_MORE_ITEMS) ) 
		{ 
			AfxMessageBox("Enum  Registry  Error or No More Items!\n"); 
			continue; 
		} 
  
		pCOMNumber = (LPSTR)VirtualAlloc(NULL, 6, MEM_COMMIT, PAGE_READWRITE); 
		lReg=RegQueryValueExA(  hKey, 
								pValueName, 
								NULL, 
								NULL, 
								(LPBYTE)pCOMNumber, 
								&dwValueSize); 
  
		if(lReg != ERROR_SUCCESS) 
		{ 
			AfxMessageBox("Can not get the name of the port"); 
			continue; 
		} 

		CString strCommList;
		
		//CharToUnicode(pCOMNumber,&strCommList);
		strCommList.Format("%s", pCOMNumber);

		BOOL m_bInsert=0;
  
		if( ((CComboBox*)GetDlgItem(IDC_COMBO_SN))->GetCount() == 0 )
			((CComboBox*)GetDlgItem(IDC_COMBO_SN))->AddString(strCommList);
		else
		{
			CString strTemp=strCommList;
			strCommList.TrimLeft("COM");

			int icurrent;

			for( icurrent=0; icurrent<((CComboBox*)GetDlgItem(IDC_COMBO_SN))->GetCount(); icurrent++ )
			{
				CString strCurrent;

				((CComboBox*)GetDlgItem(IDC_COMBO_SN))->GetLBText(icurrent,strCurrent);

				strCurrent.TrimLeft("COM");
				if( _ttol(strCurrent) > _ttol(strCommList) )
				{
					((CComboBox*)GetDlgItem(IDC_COMBO_SN))->InsertString(icurrent,strTemp);
					m_bInsert = 1;

					break;
				}
			}

			if(!m_bInsert)
				((CComboBox*)GetDlgItem(IDC_COMBO_SN))->InsertString(icurrent,strTemp);
		}

		 VirtualFree(pValueName,0,MEM_RELEASE); 
		 VirtualFree(pCOMNumber,0,MEM_RELEASE); 
	} 
}