System.InvalidOperationException issue in LINQ expression

Question

When I run the following Linq:

var selectedProduct = db.Products.FirstOrDefault(a => a.ProductNr == productNr)?.Id; model.PackTypes = db.Zones .Where(az => az.ProductId == selectedProduct && az.StoragePrio > 0) .ToList() .DistinctBy(p => p.PackType) .OrderBy(x => x.PackType) .Select(x => new DropdownItemViewModel<int> { Id = (int)x.PackType, Name = x.PackType.Translate() }); return true;

I get this error:

System.InvalidOperationException: 'Nullable object must have a value.' on this code Id = (int)x.PackType,

Now I know I must do a nullcheck so I have tried this:

if (x.PackType != null) return new DropdownItemViewModel<int> { Id = (int)x.PackType, Name = x.PackType.Translate() }; return null;

Still doesn't work, by that I mean I still have problem with NullCheck.

Can az.ProductId be null at some point? If not, why are you doing that query at all vs just exiting early if selectedProduct == null? — ProgrammingLlama
– ProgrammingLlama, Commented Aug 9, 2021 at 14:25
Why are you calling ToList in the middle of building the chained deferred methods query? What is PackType type? Does it can be converted to an integer using a simple cast? — user12031933
– user12031933, Commented Aug 9, 2021 at 14:26
It seems like you want .Where(az => az.ProductId == selectedProduct && az.StoragePrio > 0 && az.PackType != null) to filter out any entries where az.PackType == null. Unless there's something else that is null here? — ProgrammingLlama
– ProgrammingLlama, Commented Aug 9, 2021 at 14:29
If you want to force Linq-To-Objects after the Where don't use ToList but AsEnumerable. On that way you don't load all into memory at once but you can use deferred execution. — Tim Schmelter
– Tim Schmelter, Commented Aug 9, 2021 at 14:31

Svyatoslav Danyliv · Accepted Answer · 2021-08-09 15:02:07Z

This query more effective and should not have all mentioned errors:

var query = from p in db.Products where p.ProductNr == productNr join az in db.Zones on p.Id equals az.ProductId where az.StoragePrio > 0 && az.PackType != null select new { az.PackType }; model.PackTypes = query .Distinct() .OrderBy(x => x.PackType) .Select(x => new DropdownItemViewModel<int> { Id = (int)x.PackType, Name = x.PackType.Translate() }) .ToList();

Instead of two database requests this query sends only one. Also all operations are done on the server side.

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