razor - Nested foreach loop in ASP.NET MVC Core NullReferenceException -
i'm trying add navigation sidebar in simple web app has 2 primary models, , i'm running peculiar error when try use nested loop render data in sidebar.
my app has 2 models category , thing in example below:
public class category { public int id { get; set; } public string name { get; set; } public virtual icollection<thing> things { get; set; } } public class thing { public int id { get; set; } public string title { get; set; } public string description { get; set; } public int categoryid { get; set; } public virtual category category { get; set; } } i want add sidebar contains list of categories. created viewcomponent i'm referencing in _layout.cshtml so:
// sidebarviewcomponent.cs public class sidebarviewcomponent : viewcomponent { private readonly mycontext db; public sidebarviewcomponent(mycontext context) { db = context; } public async task<iviewcomponentresult> invokeasync() { var categories = await getcategoriesasync(); return view(categories); } private task<list<category>> getcategoriesasync() { return db.category.tolistasync(); } } // view file sidebarviewcomponent @model ienumerable<myapp.models.category> <div id="sidebar"> @foreach (var category in model) { <div class="header">@category.name</div> @foreach (var thing in @category.things) { <div>@thing.title</div> } </div> } </div> // _layout.cshtml <!doctype html> <html> <head> <meta charset="utf-8" /> <title>@viewdata["title"]</title> <!-- stylesheets --> <link rel="stylesheet" href="~/css/site.css"> </head> <body> @await component.invokeasync("sidebar") @renderbody() </body> </html> when access page that's rendered thingcontroller, works perfectly. on home page or page handled categorycontroller, error "nullreferenceexception: object reference not set instance of object." @ line @foreach (var thing in @category.things).
how can sidebar work on every page?
okay, able working changing viewcomponent's getcategoriesasync method use eager loading of related collection:
private task<list<category>> getcategoriesasync() { return db.category.include(c => c.things).tolistasync(); } this microsoft doc pointed me in right direction.
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