Nullable reference type when using DTO classes-CodePudding

Product

        public int Id { get; set; }
        public string ProductName { get; set; }
        public string ProductDescription { get; set; }
        public Category Category {get; set;}

Category

        public int Id { get; set; }
        public string CategoryName { get; set; }
        public string CategoryDescription { get; set; }
        public Product Product{get; set;}

ProductViewModel

    public int Id { get; set; }
    public string ProductName { get; set; }
    public string ProductDescription { get; set; }
    public Category Category {get; set;}

Controller

     public IActionResult GetProducts()
     {
        using var dbContext = new DatabaseContext();
        var result = dbContext.Products.Where(i => i.Id == id)
                                 .ThenInclude(i => i.Category)
                                 .ToList();
        List<ProductViewModel> productList = new List<ProductViewModel>();
        foreach(var item in result)
        {
         var model = ProductViewModel()
         {
             Id = item.Id,
             ProductName = item.ProductName,
             ProductDescription = item.ProductDescription,
             Category = new Category()
             { 

             // Error: item.Category.CategoryName = 'item.Category.CategoryName' threw an 
                 exception of type 'System.NullReferenceException'

             CategoryName= item.Category.CategoryName, 
             CategoryDescription = item.Category.CategoryDescription, 
             Id = item.Category.Id 
             }
          productList.Add(model); 
         }
     }

How can I ignore null values in my DTO class.

I get my products from database. Since the category is null in some products, I get an error when mapping it to my DTO class.

CodePudding user response：

Use Select for projecting Model to DTO. It will create optimal SQL and should avoid loading unwanted data into the memory. Also EF should resolve nullability automatically.

public IActionResult GetProducts()
{
    using var dbContext = new DatabaseContext();

    var productList = dbContext.Products
        .Where(i => i.Id == id)
        .Select(item => new ProductViewModel
        {
            Id = item.Id,
            ProductName = item.ProductName,
            ProductDescription = item.ProductDescription,
            Category = new Category
            { 
                CategoryName = item.Category.CategoryName, 
                CategoryDescription = item.Category.CategoryDescription, 
                Id = item.Category.Id 
            }
        })
        .ToList();
    ...
}

CodePudding user response：

Answer of @Svyatoslav Danyliv is the clean solution to your problem! However below my guess to your exception in your current solution.

I think it is because you are using ThenInclude. ThenInclude is usually used when you need to go into deeper includes.

Example: human --> head --> eye

dbContext.Humans.Include(x => x.Head).ThenInclude(x => x.Eye);

Your adjusted code:

var result = dbContext.Products.Include(i => i.Category).Where(i => i.Id == id).ToList();

Change ThenInclude with Include and give it a try! :)