ArgumentNullException: Value cannot be null. (Parameter 'source') - sql-server

Question

I have parent and childs nested tables.
Here is my model:
public class Categories
{
[Key]
public int CategoriesId { get; set; }
public int Order { get; set; }
public string CategoryName { get; set; }
public List<News> News { get; set; }
}
public class News
{
[Key]
public int NewsId { get; set; }
public int CategoriesId { get; set; }
public string Content { get; set; }
public DateTime Date { get; set; }
...
public List<Comments> Comments { get; set; }
public Categories Categories { get; set; }
}
public class Comments
{
[Key]
public int CommentsId { get; set; }
public int NewsId { get; set; }
public string Comment { get; set; }
...
public News News { get; set; }
}
public class NewsImages
{
[Key]
public int ImageId { get; set; }
public int NewsId { get; set; }
public string ImageUrl { get; set; }
public bool Cover { get; set;}
...
public News News { get; set; }
}
I'm trying to send it from ViewComponent to View;
public async Task<IViewComponentResult> InvokeAsync()
{
var group = _dbContext.Categories.Where(k => k.Order != 0).OrderBy(h => h.Order)
.Select(c => new
{
C = c,
N = c.News.OrderByDescending(n => n.Date).Take(5)
.Select(r => new
{
Y = r.Comments,
R = r.NewsImages.Where(rs => rs.Cover == true).FirstOrDefault()
})
});
var model = group
.Select(m => m.C);
return View(await model.ToListAsync()) ;
}
I am sure there are enough News records for every Category, But I get error :
ArgumentNullException: Value cannot be null. (Parameter 'source')
AspNetCore.Views_Shared_Components_IndexKategori_Default.ExecuteAsync() in Default.cshtml
var bp = k.News.FirstOrDefault();
if I use that code works fine :
var model = _dbContext.Categories
.Include(h => h.News).ThenInclude(h => h.Comments)
.Include(h => h.News).ThenInclude(h => h.NewsImages)
.Where(h => h.Order != 0)
.OrderBy(h => h.Order)
But when I use the code above, a few records appear for some categories, and some categories react as if there are no records.
Where am I making mistakes?
Thank you in advance for those who helped ..

Answer 1

Whenever you have a big LINQ statement that throws an exception, and you can't find where the exception comes form, translate the LINQ into smaller steps, and ToList() every step.
public async Task<IViewComponentResult> InvokeAsync()
{
// Temp code: small steps, ToList after every step
var a = dbContext.Categories.Where(category => category.Order != 0).ToList();
var b = a.OrderBy(category => category.Order).ToList();
var c = b.Select(category => new
{
Category = category,
News = category.News.OrderByDescending(news => news.Date)
.Take(5)
.ToList();
})
.ToList();
var d = c.Select(item => new
{
Category = item.Category,
NewsItems = item.News.Select(news => new
{
Comments = news.Comments,
Images = news.NewsImages.Where(newsImage => newsImage.Cover).ToList(),
})
.ToList(),
})
.ToList();
var e = d.Select(item => new
{
Category = item.Category,
NewsItems = item.NewsItems.Select(newsItem => new
{
Comments = newsItem.Comments,
Images = images.FirstOrDefault();
})
.ToList(),
})
.ToList();
// original code:
var group = _dbContext.Categories.Where...
}
I'm sure that your debugger will tell you which step is incorrect.