使用程式讀取 Windows 無線網路訊號品質

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요약

昨天從 RDP 連線瞬斷問題追出是無線路由器的 Smart Connect 機制觸發網卡自動切換 2.4G/5G 頻段造成，而 Smart Connect 也在留言區被刷了不少負評，我目前的結論是「除非是走 WiFi 7 有 MLO，不然還是 2.4G 跟 5G 切成不同 SSID 較好」。 不過會... Read More

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昨天從 RDP 連線瞬斷問題追出是無線路由器的 Smart Connect 機制觸發網卡自動切換 2.4G/5G 頻段造成，而 Smart Connect 在臉書貼文留言被刷了不少負評，我目前的結論是「除非是走 WiFi 7 有 MLO，不然還是 2.4G 跟 5G 切成不同 SSID 較好」。

不過會自動切換頻段應也源於訊號品質不佳，讓我開始關注工作機的 WiFi 訊號品質是否太差？

我的無線路由器位置位於房子中央，工作機則在房間邊角中間還隔了牆，訊號確實可能會差一些，我想寫個程式長期觀測 WiFi 品質確認這點，看看白天晚上、不同時段是否有差別。

研究了一下，查看 WiFi 品質數字最簡單的做法是用 CLI netsh wlan show intefaces，顯示結果中的 Signal (訊號) 百分比即為訊號品質：

PS D:\> netsh wlan show interfaces

There is 1 interface on the system: 

 Name : Wi-Fi AX210
 Description : Intel(R) Wi-Fi 6E AX210 160MHz
 GUID : 4dba6ce4-dc0f-49dd-5a4e-66626be68720
 Physical address : f4:7b:09:ab:cd:ef
 Interface type : Primary
 State : connected
 SSID : M-WiFi-SSID
 AP BSSID : 12:34:56:78:90:ab
 Band : 5 GHz
 Channel : 157
 Connected Akm-cipher : [ akm = 00-0f-ac:08, cipher = 00-0f-ac:04 ]
 Network type : Infrastructure
 Radio type : 802.11ax
 Authentication : WPA3-Personal (H2E)
 Cipher : CCMP
 Connection mode : Auto Connect
 Receive rate (Mbps) : 216
 Transmit rate (Mbps) : 144
 Signal : 60% 
 Rssi : -72
 Profile : My-WiFi-SSID
 QoS MSCS Configured : 0
 QoS Map Configured : 0
 QoS Map Allowed by Policy : 0

程式要取得品質數據，無腦寫法是呼叫 netsh 並解析輸出結果，但呼叫外部程序偏笨重，解析文字則要考慮語系，我覺得不甚理想。netsh 可以查到資料代表背後有對映的 Windows API，我很快找到 Native Wifi API。

描述無線網卡介面的資料結構 WLAN_ASSOCIATION_ATTRIBUTES 有個 WLAN_SIGNAL_QUALITY wlanSignalQuality 表示網路訊號品質百分比值，其值介於 0 到 100 之間。0 表示實際 RSSI 訊號強度 -100 dbm、100 表示實際 RSSI 訊號強度 -50 dbm。用線性插補可計算 wlanSignalQuality 值介於 1 到 99 之間的 RSSI 訊號強度值。

找到方法，就可以寫個共用函式。完整程序是先 WlanOpenHandle() 與無線自動設定服務(WLAN AutoConfig Service）建立連線，WlanEnumInterfaces() 列舉使用中的無線網卡，從清單取得第一個介面的識別碼 Guid，接著呼叫 WlanQueryInterface() 取得該介面包含 WiFi 訊號品質的即時資料。考量 WlanOpenHandle() 建立本機 RPC 連線會耗用較多資源，而網卡不會三不五時異動，故每次重複列舉介面純屬多餘，故我決定重複使用連線及記憶介面識別 ID，定期查詢直接呼叫 WlanQueryInterface()，如此程式較省資源、執行效率更好。

using System.Runtime.InteropServices;

static class PInvokeHelper
{
 static IntPtr _hClient = IntPtr.Zero;
 static Guid _ifGuid;
 static string _ssid = string.Empty;
 const uint ERROR_SUCCESS = 0;
 const uint WLAN_INTF_OPCODE_CURRENT_CONNECTION = 7;

 [DllImport("wlanapi.dll")]
 static extern uint WlanOpenHandle(uint dwClientVersion, IntPtr pReserved,
 out uint pdwNegotiatedVersion, out IntPtr phClientHandle);

 [DllImport("wlanapi.dll")]
 static extern uint WlanEnumInterfaces(IntPtr hClientHandle, IntPtr pReserved,
 out IntPtr ppInterfaceList);

 [DllImport("wlanapi.dll")]
 static extern uint WlanQueryInterface(IntPtr hClientHandle, ref Guid pInterfaceGuid,
 uint OpCode, IntPtr pReserved, out uint pdwDataSize, out IntPtr ppData,
 out uint pWlanOpcodeValueType);

 [DllImport("wlanapi.dll")]
 static extern void WlanFreeMemory(IntPtr pMemory);

 [DllImport("wlanapi.dll")]
 static extern uint WlanCloseHandle(IntPtr hClientHandle, IntPtr pReserved);

 [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
 struct WLAN_INTERFACE_INFO
 {
 public Guid InterfaceGuid;
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
 public string strInterfaceDescription;
 public uint isState;
 }

 [StructLayout(LayoutKind.Sequential)]
 struct DOT11_SSID
 {
 public uint uSSIDLength;
 [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
 public byte[] ucSSID;
 }

 [StructLayout(LayoutKind.Sequential)]
 struct WLAN_ASSOCIATION_ATTRIBUTES
 {
 public DOT11_SSID dot11Ssid;
 public uint dot11BssType;
 [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)]
 public byte[] dot11Bssid;
 public uint dot11PhyType;
 public uint uDot11PhyIndex;
 public uint wlanSignalQuality; // 0–100; RSSI (dBm) = (quality / 2) - 100
 public uint ulRxRate;
 public uint ulTxRate;
 }

 [StructLayout(LayoutKind.Sequential)]
 struct WLAN_SECURITY_ATTRIBUTES
 {
 public int bSecurityEnabled;
 public int bOneXEnabled;
 public uint dot11AuthAlgorithm;
 public uint dot11CipherAlgorithm;
 }

 [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
 struct WLAN_CONNECTION_ATTRIBUTES
 {
 public uint isState;
 public uint wlanConnectionMode;
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
 public string strProfileName;
 public WLAN_ASSOCIATION_ATTRIBUTES wlanAssociationAttributes;
 public WLAN_SECURITY_ATTRIBUTES wlanSecurityAttributes;
 }

 static void EnsureInitialized()
 {
 if (_hClient != IntPtr.Zero) return;

 uint err = WlanOpenHandle(2, IntPtr.Zero, out _, out _hClient);
 if (err != ERROR_SUCCESS)
 {
 _hClient = IntPtr.Zero;
 throw new InvalidOperationException($"WlanOpenHandle failed: {err}");
 }

 err = WlanEnumInterfaces(_hClient, IntPtr.Zero, out IntPtr pIfList);
 if (err != ERROR_SUCCESS)
 {
 Reset();
 throw new InvalidOperationException($"WlanEnumInterfaces failed: {err}");
 }

 uint count = (uint)Marshal.ReadInt32(pIfList);
 if (count == 0)
 {
 WlanFreeMemory(pIfList);
 Reset();
 throw new InvalidOperationException("No WLAN interfaces found.");
 }

 // dwNumberOfItems, dwIndex 各 4 bytes，+8 取得第一個 WLAN_INTERFACE_INFO 結構
 

搞定上述關鍵，寫幾行 C# 就能做到定期監測 WiFi 訊號品質囉~

Console.WriteLine("Press Ctrl+C to stop.");

using 

補充，跟 AI 學到 async 時代的非同步定期輪詢寫法，建立 PeriodicTimer，用 WaitForNextTickAsync() 等待下次觸發，如此可透過 CancellationTokenSource 即時中止程序，比傳統 while 迴圈加 Thread.Spleep() 寫法更符合非同步精神。

Build a C# WiFi signal monitor using Windows Native Wifi API instead of parsing netsh, retrieving quality/RSSI efficiently via cached WLAN handles and interface IDs, with async periodic polling using PeriodicTimer.