尽管 SingleOrDefault() 会返回 null，但 EF Core LINQ 查询何时导致 null 合并？

Question

I have a query against the DbContext something like:

ctx.Items.SingleOrDefault(i => i.Id == *someQueryThatReturns0*).Code

The someQueryThatReturns0 is a query that looks up an Id, something like the following:

otherItems.Where(h => h.Id == xyz).Select(s => s.OtherId).SingleOrDefault()

When this inner query has no results, then the inner SingleOrDefault will return 0 because the Ids are int, and this means the outer Items.SingleOrDefault(..) should result in null.

But when referencing Items.SingleOrDefault(..).Code instead of getting NullRef exception, it will be just null.

This whole thing is part of a bigger query where it is used in to set values in a Select(), something like:

someComplexQuery.Select(x => new someDto(){
  ...
  someProp = x.Valid ? ctx.Items.SingleOrDefault(i => i.Id == *someQueryThatReturns0*).Code : null;
  ...
})

When executing the query I would have expected a NullRef exception, but instead someProp is simply coalesced to null basically. Is this the expected behavior simply because referencing .Code in the query like this will be translated into the SQL query somehow? Or am I missing something here?

Answer 1

  someProp = x.Valid ? ctx.Items.SingleOrDefault(i => i.Id == *someQueryThatReturns0*).Code : null;

This line should definitely return null at the SingleOrDefault, if Item object is a class, and therefore to throw NullReferenceException.

If it's a struct type then it would initialize the struct with all its values in turn default initialized (ie zero, null, etc.).

Assuming it's a class, the only way that someProp will be set to null eventually, is if Items collection has an item with Id equal to zero, and it's Code property is set to null , or if x.Valid is set to false .