當私有建構式有引數時創建單例

我想在 StudentProvider 中實作單例模式，然后通過介面訪問方法。StudentProvider 建構式接受很少的引數。這是沒有單例的示例作業代碼。

public interface IStudentProvider
{
    Task<StudentViewModel> GetStudentAsync();
}

public class StudentProvider : IStudentProvider
{
    private readonly HttpContext httpContext;
    private readonly IActionContextAccessor actionContextAccessor;
    private readonly IConfiguration configuration;
    private readonly IUnitOfWork unitOfWork;
    private readonly string host;

    public StudentProvider(IHttpContextAccessor _httpContextAccessor, IActionContextAccessor _actionContextAccessor, IConfiguration _configuration, IUnitOfWork _unitOfWork)
    {
        httpContext = _httpContextAccessor.HttpContext;
        
        actionContextAccessor = _actionContextAccessor;
        configuration = _configuration;
        unitOfWork = _unitOfWork;
        host = _httpContextAccessor.HttpContext.Request.Host.Host;
    }

    public async Task<StudentViewModel> GetStudentAsync()
    {
        var std = new StudentViewModel();

        // httpContext, actionContextAccessor, configuration, unitOfWork and host uses here

        return std;
    }
}

現在我將其轉換為單個，這是代碼：

public interface IStudentProvider
{
    Task<StudentViewModel> GetStudentAsync();
}

public sealed class StudentProvider : IStudentProvider
{
    private readonly HttpContext httpContext;
    private readonly IActionContextAccessor actionContextAccessor;
    private readonly IConfiguration configuration;
    private readonly IUnitOfWork unitOfWork;
    private readonly string host;

    private static StudentProvider instance = null;

    public static StudentProvider GetInstance
    {
        get
        {
            if (instance == null)
            {
                instance = new StudentProvider();
            }

            return instance;
        }
    }

    private StudentProvider(IHttpContextAccessor _httpContextAccessor, IActionContextAccessor _actionContextAccessor, IConfiguration _configuration, IUnitOfWork _unitOfWork)
    {
        httpContext = _httpContextAccessor.HttpContext;

        actionContextAccessor = _actionContextAccessor;
        configuration = _configuration;
        unitOfWork = _unitOfWork;
        host = _httpContextAccessor.HttpContext.Request.Host.Host;
    }

    public async Task<StudentViewModel> GetStudentAsync()
    {
        var std = new StudentViewModel();

        // httpContext, actionContextAccessor, configuration, unitOfWork and host uses here

        return std;
    }
}

上述單例代碼的問題instance = new StudentProvider();是期待我無法傳遞的引數。

如何將引數從單例實體傳遞給建構式？

uj5u.com熱心網友回復：

看來您正在使用 ASP.NET 并且它是依賴項注入。如果是這樣，您可以使用AddSingleton注冊您的提供程式而不是實作您自己的單例模式。辛格爾頓。

順便說一句，您的提供者依賴于 HttpContext 這意味著您需要為不同的請求創建不同的實體。

uj5u.com熱心網友回復：

正如@Jon Skeet 所建議的，最好使用依賴注入。

我也會向@Xiaofeng Zheng 推薦使用工廠模式的單例依賴注入的解決方案。

如果所有這些都不滿足，您可以使用以下解決方案。

您需要將 IServiceProvider 的參考保留為可全域訪問的啟動檔案中的單例。

public class Startup
{
  public static IServiceProvider ServiceProvider { get; private set; }

  public void Configure(IApplicationBuilder app, IWebHostEnvironment env, IServiceProvider serviceProvider) {
    ...

    ServiceProvider = serviceProvider;
  }
}

然后，您可以訪問 StudentProvider 中的 Startup.ServiceProvider 以創建其他依賴項的實體。

using Microsoft.Extensions.DependencyInjection;

public sealed class StudentProvider : IStudentProvider
{
    private readonly HttpContext httpContext;
    private readonly IActionContextAccessor actionContextAccessor;
    private readonly IConfiguration configuration;
    private readonly IUnitOfWork unitOfWork;
    private readonly string host;

    private static StudentProvider instance = null;

    public static StudentProvider GetInstance
    {
        get
        {
            if (instance == null)
            {
                instance = new StudentProvider(
                    Startup.ServiceProvider.GetService<IHttpContextAccessor>(), 
                    Startup.ServiceProvider.GetService<IActionContextAccessor>(), 
                    Startup.ServiceProvider.GetService<IConfiguration>(), 
                    Startup.ServiceProvider.GetService<IUnitOfWork>()
                );
            }

            return instance;
        }
    }

    private StudentProvider(IHttpContextAccessor _httpContextAccessor, IActionContextAccessor _actionContextAccessor, IConfiguration _configuration, IUnitOfWork _unitOfWork)
    
}

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