在 ASP.NET MVC 中使用连接表时出现 Null 异常

Question

I have product , Productsize and Size entities.

Productsize entity contains ProductID and SizeID which are foreign key of Product and Size entity. I am adding size filter in shop page. List of size ids are coming from view.I am checking if the size ids are there in Productsize entity and displaying that product only. In if condition it is showing null exception

List<int> sizeIDs = !string.IsNullOrEmpty(SizeIds) ? SizeIds.Split(',').Select(x => int.Parse(x)).ToList() : new List<int>();

var products = db.Products.ToList();

if (sizeIDs.Any())
{
    products = products.Where(x=>sizeIDs.Contains(x.ProductSizes.FirstOrDefault()
                                          .SizeID)).ToList();
}    

Answer 1

try:

List<int> sizeIDs = !string.IsNullOrEmpty(SizeIds) ? SizeIds.Split(',').Select(x => int.Parse(x)).ToList() : new List<int>();

var products = db.Products.ToList();

if (sizeIDs.Any())
{
    products = products.Where(x=>sizeIDs.Contains(x.ProductSizes.FirstOrDefault()?
                                          .SizeID)).ToList();
}    

.FirstOrDefault() ? .SizeID

or:

List<int> sizeIDs = !string.IsNullOrEmpty(SizeIds) ? SizeIds.Split(',').Select(x => int.Parse(x)).ToList() : new List<int>();

var products = db.Products.ToList();

if (sizeIDs.Any())
{
    products = products.Where(x=>x.ProductSizes.Any() && sizeIDs.Contains(x.ProductSizes.First().SizeID)).ToList();
} 

Update: lets try this query:

List<int> sizeIDs = !string.IsNullOrEmpty(SizeIds) ? SizeIds.Split(',').Select(x => int.Parse(x)).ToList() : new List<int>();

            var products = db.Products.ToList();

            if (sizeIDs.Any())
            {
                products = products.Where(x => x.ProductSizes.Any(s=> sizeIDs.Contains(s.SizeID)) ).ToList();
            }