c# - IHttpContextAccessor 在类中为 null

Question

当我尝试从 IHttpContextAccessor 获取 httpcontext 时遇到问题，类中字段始终为 null。

这是我的startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddDistributedMemoryCache();
    services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    services.AddTransient<IUserContextServices, UserContextService>();
    services.AddSession(options =>
    {
        options.Cookie.Name = ".AdventureWorks.Session";
        options.IdleTimeout = TimeSpan.FromSeconds(10);
    });
}

这是我的 UserContext 类，它实现了 IUserContext 接口(interface)

public class UserContextService : IUserContextServices
{
    private readonly IHttpContextAccessor contextAccessor;
    PQADBContext _context = new PQADBContext();

    public UserContextService(IHttpContextAccessor accessor)
    {
        contextAccessor = accessor;
    }

   public UserContextService()
    {

    }

    public HttpContext Context
    {
        get
        {
            return contextAccessor.HttpContext;
        }
    }
   public int UserID()
    {
       return Context.Session.GetID("UserID").ConvertToInt();
    }
    public bool isLogin()
    {
        return Context.Session.GetBoolean("isLogin").ConvertToBool();
    }
      public UserAccount CreateSession(LoginViewModel logindata, bool EncryptPwd = true)
    {
        string error;
        error = "";
        try
        {
            string EncPwd = EncryptPwd ? EncryptDecryptHelper.Encrypt(logindata.Password) : logindata.Password;
            var UserDetail =_context.UserAccount.Where(e => e.P_No == logindata.PNo && e.Password == EncPwd).SingleOrDefault();


            if (UserDetail != null)
            {

                //HttpContext.Session.SetInt32()

                // ///put all the properties in session variables
                 Context.Session.SetBoolean("isLogin", true);
                Context.Session.SetID("UserID",UserDetail.AccountId);
                Context.Session.SetID("P_No",Convert.ToInt32(UserDetail.P_No));
                Context.Session.SetBoolean("isActive", true);
                Context.Session.SetBoolean("Email", true);
                Context.Session.SetID("RoleId", 1);
                Context.Session.SetString("userName", "admin"); 
   }

httpContext 在上面的类中可用，并且还设置了 session 值，但是当我尝试访问此类中的 httpcontext 时，它给了我空对象引用

public class UserService
{
   private readonly IHttpContextAccessor contextAccessor;
    public IUserContextServices _userContext = new UserContextService();
   public UserService()
    {  
    }
 public bool CreateEmployee(AppEmployees appemployee, int RoleId, bool isEmailSend, out string error)
    {
appemployee.CreatedBy = _userContext.UserID(); //this line shows null reference exception
        appemployee.CreatedDate = DateTime.Now;
}

Answer 1

您正在使用无参数构造函数新建UserContextService

public IUserContextServices _userContext = new UserContextService();

而不是依赖依赖注入(inject)。

您需要配置您的 UserService 以便与您的 DI 容器一起使用 - 通过构造函数注入(inject)

public class UserService
{
    private readonly IUserServiceContext _userServiceContext;

    public UserService(IUserServiceContext userServiceContext)
    {
        _userServiceContext = userServiceContext;
    }
}

您还需要修改 Startup.cs 以注册 UserService 并且您可能希望它也实现一个接口(interface)