经过多次尝试和失败，我能够实现此处提供的方法: ***链接

After lot of tries and failures, I was able to implement the approach provided here: *** Link

我将此代码从Java更改为Xamarin C#，并能够以编程方式强制Cellular或Wifi作为首选网络.

I changed this code from Java to Xamarin C# and was able to force Cellular or Wifi as the preferred network programmatically.

我的实现:

using Android.Net;
public SomeClass{
    public static Context _context = Android.App.Application.Context;

    ....

    /// <summary>
    /// Forces the wifi over cellular.
    /// </summary>
    public static void ForceWifiOverCellular()
    {
        ConnectivityManager connection_manager = (ConnectivityManager)_context.GetSystemService(Context.ConnectivityService);

        NetworkRequest.Builder request = new NetworkRequest.Builder();
        request.AddTransportType(TransportType.Wifi);

        var callback = new ConnectivityManager.NetworkCallback();
        connection_manager.RegisterNetworkCallback(request.Build(), new CustomNetworkAvailableCallBack());

    }

    /// <summary>
    /// Forces the cellular over wifi.
    /// </summary>
    public static void ForceCellularOverWifi()
    {
        ConnectivityManager connection_manager = (ConnectivityManager)_context.GetSystemService(Context.ConnectivityService);

        NetworkRequest.Builder request = new NetworkRequest.Builder();
        request.AddTransportType(TransportType.Cellular);

        connection_manager.RegisterNetworkCallback(request.Build(), new CustomNetworkAvailableCallBack());
    }
}


/// <summary>
/// Custom network available call back.
/// </summary>
public class CustomNetworkAvailableCallBack : ConnectivityManager.NetworkCallback
{
    public static Context _context = Android.App.Application.Context;

    ConnectivityManager connection_manager = (ConnectivityManager)_context.GetSystemService(Context.ConnectivityService);

    public override void OnAvailable(Network network)
    {
        //ConnectivityManager.SetProcessDefaultNetwork(network);    //deprecated (but works even in Android P)
        connection_manager.BindProcessToNetwork(network);           //this works in Android P
    }
}

用法:

1. 我需要强制Cellular的地方，只需致电:

SomeClass.ForceCellularOverWifi();

SomeClass.ForceCellularOverWifi();

1. 需要强制Wifi的地方，只需致电:

SomeClass.ForceWifiOverCellular();

SomeClass.ForceWifiOverCellular();

希望这对其他人有帮助.

Hope this helps others.