经过几天的时间，终于找到了为什么我没有得到SSID在onCapabilitiesChanged。
在创建对象时，我们需要在ConnectivityManager.NetworkCallback构造函数中传递FLAG_INCLUDE_LOCATION_INFO。
在默认构造函数中，位置敏感信息被隐藏，因为我们无法在onCapabilitiesChanged中获取SSID。一旦我们创建了带有标志的NetworkCallback对象，我们将获得SSID。
链接

获取SSID和BSSID API31 Xamarin C#示例

所需权限：更改网络状态、访问精细位置
如果API〈31TransportInfo将返回空值

using Android.Content;
using Android.Net;
using Android.Net.Wifi;

protected override void OnStart()
    {
        base.OnStart();

        NetworkRequest request = new NetworkRequest.Builder().AddTransportType(transportType: TransportType.Wifi).Build();
        ConnectivityManager connectivityManager = Android.App.Application.Context.GetSystemService(Context.ConnectivityService) as ConnectivityManager;

        NetworkCallbackFlags flagIncludeLocationInfo = NetworkCallbackFlags.IncludeLocationInfo;
        NetworkCallback networkCallback = new NetworkCallback((int)flagIncludeLocationInfo);
        connectivityManager.RequestNetwork(request, networkCallback);
    }

    private class NetworkCallback : ConnectivityManager.NetworkCallback
    {
        public NetworkCallback(int flags) : base(flags)
        {
        }

        public override void OnCapabilitiesChanged(Network network, NetworkCapabilities networkCapabilities)
        {
            base.OnCapabilitiesChanged(network, networkCapabilities);
            WifiInfo wifiInfo = (WifiInfo)networkCapabilities.TransportInfo;

            if (wifiInfo != null)
            {
                string ssid = wifiInfo.SSID.Trim(new char[] {'"', '\"' });
                string bssid = wifiInfo.BSSID;
            }
        }
    }

点击Android API参考.连接管理器.网络回调（int）!

将new NetworkCallback()替换为new NetworkCallback(ConnectivityManager.NetworkCallback.FLAG_INCLUDE_LOCATION_INFO)。

示例代码：

final NetworkRequest request =
      new NetworkRequest.Builder()
      .addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
      .build();
 final ConnectivityManager connectivityManager =
      context.getSystemService(ConnectivityManager.class);
 final NetworkCallback networkCallback = new NetworkCallback(ConnectivityManager.NetworkCallback.FLAG_INCLUDE_LOCATION_INFO) {
      ...
      @Override
      void onAvailable(Network network) {}

      @Override
      void onCapabilitiesChanged(Network network, NetworkCapabilities networkCapabilities) {
          WifiInfo wifiInfo = (WifiInfo) networkCapabilities.getTransportInfo();
      }
      // etc.
 };
 connectivityManager.requestNetwork(request, networkCallback); // For request
 connectivityManager.registerNetworkCallback(request, networkCallback); // For listen

参考链接：

网络回调#网络回调（整数）

此方法适用于所有Android版本，但适用于Android 8. 0（Oreo）和更高版本，**您需要有定位服务（GPS）以访问Wi-Fi SSID。**此外，您需要在运行时请求ACCESS_COARSE_LOCATION和ACCESS_FINE_LOCATION权限，并在清单文件中声明它们。如果您希望在后台访问Wi-Fi SSID，你可以写一个前台服务来处理这个问题。

private fun checkWIFIName(context: Context) : Boolean {
    val wifiManager = context.applicationContext.getSystemService(WIFI_SERVICE) as WifiManager
    val wifiInfo = wifiManager.connectionInfo
    println("SSID : ${wifiInfo.ssid}")
    return wifiInfo.ssid.trim().equals("\"$SSID\"", true)

}