应用程序上的异步调用开始返回Null

时间：2019-05-06 23:29:58

标签： asp.net-core razor entity-framework-core

当您导航到主页时，我有一个电话检索数据库中的所有类别。这个想法是索引页面动态显示类别列表。类别是通过使用EF .Net Core来放置将类别插入数据库的代码来放置的-这就是数据库的外观。

https://gyazo.com/c407f03ae60a82c6fa6f9ee12105089d

编辑：获取类别：

[AllowAnonymous]
        public async Task<List<Category>> GetCategoriesFromDb()
        {
            return await _context.Categories.Where(x => x.ParentId == 0).ToListAsync();
        }

它抛出此错误：

https://gyazo.com/7e27dda9ff57b6e28b58b563abd05767 

ArgumentNullException: Value cannot be null.
Parameter name: source
System.Linq.Enumerable.Count<TSource>(IEnumerable<TSource> source)
AspNetCore.Views_Home__categories.ExecuteAsync() in _categories.cshtml
+
    @if (Model.Count() > 0)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage 
page, ViewContext context)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, 
ViewContext context, bool invokeViewStarts)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context) = 
AspNetCore.Views_Home_Index.ExecuteAsync() in Index.cshtml
+
<partial name="_categories" , for="Categories" />

似乎在随后导航到页面（字面上刷新一次）后，错误消失了。

这是发生故障的控制器：

https://gyazo.com/ee6db603cde49a7b704b200e7e5fa2dc 

// Model stores an IEnumerable of Products and Categories to be used with the two partial views.
            Index myModel = new Index();
            myModel.Categories = new List<Category>();
            myModel.Products = new List<Product>();

            myModel.Categories = await GetCategories();

这是代码和有疑问的视图：

https://gyazo.com/634b6867601fc582836e284cdacb0ca3 

@{
@model List<Category>
}


<div>

    @if (Model.Count() > 0)
    {
        <a class="btn btn-secondary" href="~/Home/Index/" style="margin-bottom:10px;">All</a>
        @foreach (var item in Model)
        {

            <a class="btn btn-secondary" href="~/Home/Index?Category=@item.Name&Zip=@ViewBag.Zip" style="margin-bottom:10px;">@item.Name</a>
        }


    }
    else
    {
        <a class="btn btn-secondary" href="~/Home/Index/" style="margin-bottom:10px;">All</a>
    }
</div>

这是视图模型：

https://gyazo.com/e5053f6827d2198742279eb37071c733 

namespace Market.Views.Home
{
public class Index
{

    public List<Product> Products { get; set; }
    public List<Category> Categories { get; set; }

}

如果我再次导航至页面，该错误将消失。 恐怕这是我的应用程序返回500错误的0.4％的原因。

我尝试将其从IEnumerable更改为List-但我实际上不知道如何解决该问题。似乎我需要处理类别列表为null的情况，但是我希望视图根本不崩溃，而要等到检索到所有类别。

我可以说是因为当Razor视图点击if语句检查是否有类别时，类别列表为null。我认为应该简单地重试，直到有类别为止，但是我不确定。

编辑： 经过进一步检查，根据Azure，我的SQL数据库服务器上发生了错误。它带有此异常，它似乎与我在本地遇到的问题有关。

System.NullReferenceException: Object reference not set to an instance of an object.
   at Market.Controllers.HomeController.GetGeoProducts(Nullable`1 Distance, String productSearch, String Zip, String Category) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 162
   at Market.Controllers.HomeController.Index(String Category, Double Distance, String productSearch, String Zip, String sortOrder, String currentFilter, Nullable`1 pageNumber) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 80
   at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.MigrationsEndPointMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

1 个答案:

答案 0 :(得分：0)

感谢帮助人员-这最终成为日志记录中的重要课程。

我更改了代码以获取类别，以仅创建预填充的SelectList，而不是从数据库中检索它。

@{
var Categories = new List<Category>(){
            //new Category
            //{
            //    CategoryId = 1,
            //    ParentId = 0,
            //    Name = "All",
            //    ParentCategory = null
            //},
            new Category
            {
                CategoryId = 2,
                ParentId = 0,
                Name = "Electronics",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 3,
                ParentId = 0,
                Name = "Furniture",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 4,
                ParentId = 0,
                Name = "Clothing",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 5,
                ParentId = 0,
                Name = "Shoes",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 6,
                ParentId = 0,
                Name = "Video Games",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 7,
                ParentId = 0,
                Name = "Books",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 8,
                ParentId = 0,
                Name = "Tools",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 9,
                ParentId = 0,
                Name = "Vehicles",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 10,
                ParentId = 0,
                Name = "Free",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 11,
                ParentId = 0,
                Name = "Jewelry",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 12,
                ParentId = 0,
                Name = "Crafts",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 13,
                ParentId = 0,
                Name = "Crafts",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 14,
                ParentId = 0,
                Name = "Appliances",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 15,
                ParentId = 0,
                Name = "Health",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 16,
                ParentId = 0,
                Name = "Tickets",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 17,
                ParentId = 0,
                Name = "Services",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 18,
                ParentId = 0,
                Name = "Home & Garden",
                ParentCategory = null
            },
            new Category
            {
                CategoryId = 19,
                ParentId = 0,
                Name = "Collectibles",
                ParentCategory = null
            }
};

但是错误仍然被抛出-所以我在添加NLog之后检查了日志：

    2019-05-06 12:09:56.6564|ERROR|Market.Controllers.HomeController|MaxMind.GeoIP2.Exceptions.AddressNotFoundException: The address ::1 is not in the database.
   at MaxMind.GeoIP2.DatabaseReader.Execute[T](String ipStr, IPAddress ipAddress, String type, Boolean throwOnNullResponse)
   at MaxMind.GeoIP2.DatabaseReader.City(IPAddress ipAddress)
   at Market.Controllers.HomeController.GetGeoProducts(Nullable`1 Distance, String productSearch, String Zip, String Category) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 169
   at Market.Controllers.HomeController.Index(String Category, Nullable`1 Distance, String productSearch, String Zip, String sortOrder, String currentFilter, Nullable`1 pageNumber) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 85
2019-05-06 12:12:07.8714|ERROR|Market.Controllers.HomeController|MaxMind.GeoIP2.Exceptions.AddressNotFoundException: The address ::1 is not in the database.
   at MaxMind.GeoIP2.DatabaseReader.Execute[T](String ipStr, IPAddress ipAddress, String type, Boolean throwOnNullResponse)
   at MaxMind.GeoIP2.DatabaseReader.City(IPAddress ipAddress)
   at Market.Controllers.HomeController.GetGeoProducts(Nullable`1 Distance, String productSearch, String Zip, String Category) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 169
   at Market.Controllers.HomeController.Index(String Category, Nullable`1 Distance, String productSearch, String Zip, String sortOrder, String currentFilter, Nullable`1 pageNumber) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 85
2019-05-06 12:21:50.0824|ERROR|Market.Controllers.HomeController|MaxMind.GeoIP2.Exceptions.AddressNotFoundException: The address ::1 is not in the database.
   at MaxMind.GeoIP2.DatabaseReader.Execute[T](String ipStr, IPAddress ipAddress, String type, Boolean throwOnNullResponse)
   at MaxMind.GeoIP2.DatabaseReader.City(IPAddress ipAddress)
   at Market.Controllers.HomeController.GetGeoProducts(Nullable`1 Distance, String productSearch, String Zip, String Category) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 169
   at Market.Controllers.HomeController.Index(String Category, Nullable`1 Distance, String productSearch, String Zip, String sortOrder, String currentFilter, Nullable`1 pageNumber) in C:\Users\nvn\source\repos\market\Market.Core\Market.Core\Controllers\HomeController.cs:line 85

由于某种原因，实际错误-在GeoIP DB中找不到IP地址-隐藏在产品中抛出的堆栈跟踪中。不确定为什么，但是要么使用了免费版本中没有的IP范围的人员，要么以在开发模式下进行调试的方式存在一些环回错误。

无论如何，这就是我的处理方式-

// Determine IP address of request.
                    var ipAddress = HttpContext.Connection.RemoteIpAddress;
                    try
                    {
                        // Grab the city if it's not null
                        var city = reader.City(ipAddress);
                        ViewBag.Zip = city.Postal.Code;
                        ViewBag.CurrentDistance = Distance;
                    }
                    catch(AddressNotFoundException e)
                    {

                        logger.Warn(e.ToString());
                        // Grab the city if it's not null
                        var city = reader.City("insert a random IP here");
                        ViewBag.Zip = city.Postal.Code;
                        ViewBag.CurrentDistance = Distance;
                    }

感谢大家的帮助。

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