在 .NET MAUI 应用中使用依赖项注入的 NullReferenceException

问：

在我的 .NET MAUI 7 应用程序中使用依赖项注入时，我遇到了一个奇怪的错误。当我为请求实例化一个新的请求时，对我的 API/令牌登录 URL 的 API 调用可以完美运行，但是当使用服务类、singletonService 在我的构建器中引用该类和视图模型以在 maui 中传递无参数构造函数要求时，我收到此错误。我也尝试过其他方法，例如服务定位器方法而不是视图模型方法，但这没有区别。我开始怀疑 .NET MAUI DI 系统是否存在问题。NullReferenceExceptionHttpClient

这是我针对此问题的所有代码：

毛伊岛程序.cs

public static class MauiProgram
{
    public static MauiApp CreateMauiApp()
    {
        var builder = MauiApp.CreateBuilder();

        builder.Services.AddSingleton<IHttpService, HttpService>();
    
        builder
            .UseMauiApp<App>()
            .UseMauiCommunityToolkit()
            .ConfigureFonts(fonts =>
            {
                fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
                fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
            });
            
#if DEBUG
        builder.Logging.AddDebug();
#endif

        return builder.Build();
    }
}

HttpService.cs：

public interface IHttpService
{
    Task<HttpResponseMessage> SendAsync(HttpRequestMessage request);
    // Add other HTTP methods as needed
}

public class HttpService : IHttpService
{
    private readonly HttpClient _httpClient;

    public HttpService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request)
    {
        return await _httpClient.SendAsync(request);
    }
}

登录页面视图模型.cs

public class LoginPageViewModel : BaseViewModel
{
    private readonly IHttpService _httpService;

    public LoginPageViewModel(IHttpService httpService)
    {
        _httpService = httpService;
    }

    // You can add more properties and methods specific to the view model
    public IHttpService HttpService => _httpService;
}

LoginPage.xaml.cs（这是我收到错误的页面）

public partial class LoginPage : ContentPage
{
    private string uText, pText;

    public LoginPage()
    { 
        InitializeComponent();
        var httpService = DependencyService.Get<IHttpService>();
        var viewModel = new LoginPageViewModel(httpService);
        BindingContext = viewModel;
    }

    void OnEntryUsernameTextChanged(object sender, TextChangedEventArgs e)
    {
        uText = userNameEntry.Text;
    }

    void OnEntryUsernameTextCompleted(object sender, EventArgs e)
    {
        uText = ((Entry)sender).Text;
    }

    void OnEntryPasswordTextChanged(object sender, TextChangedEventArgs e)
    {
        pText = PasswordEntry.Text;
    }

    void OnEntryPasswordTextCompleted(object sender, EventArgs e)
    {
        pText = ((Entry)sender).Text;
    }

    protected override bool OnBackButtonPressed()
    {
        Application.Current.Quit();
        return true;
    }

    private async void OnLoginClicked(object sender, EventArgs e)
    {
        var viewModel = (LoginPageViewModel)BindingContext;
        string loginUsername = uText.Trim();
        string loginPassword = pText.Trim();

        //Make API call to login to the server
        //This will be a call to recieve a token which is then attached to the rest
        //of the API calls, as well as to check whether the user exists in the
        //database and has entered correct login details
        
        //HttpClient client = new HttpClient();
        var request = new HttpRequestMessage(
            HttpMethod.Get, "https://localhost:44386/token");
        var collection = new List<KeyValuePair<string, string>>();
        collection.Add(new("grant_type", "password"));
        collection.Add(new("username", loginUsername));
        collection.Add(new("password", loginPassword));
        var content = new FormUrlEncodedContent(collection);
        request.Content = content;
        var response =
--->        await viewModel.HttpService.SendAsync(request); <--- Null reference ex
        await Task.Delay(3000);
        if (response.IsSuccessStatusCode)
        {
            var token = await response.Content.ReadFromJsonAsync<Token>();
    
            await SecureStorage.Default.SetAsync(
                "accessTokenKey", token.access_token);
       
            await Shell.Current.GoToAsync("///home");
        }
        else
        {
            await DisplayAlert("Error", response.StatusCode.ToString(), "Retry");
        }
    }

    private async void OnRegisterPageClicked(object sender, EventArgs e)
    {
        await Shell.Current.GoToAsync("///register");
    }
}

我尝试了许多不同的方法，甚至咨询了 Chat GPT 以尝试获得一些建议，但即使是 Chat GPT 也被这个难住了。我已经对 MVC 应用程序进行了依赖注入，并且从未遇到过这样的问题。