返回一个空的可枚举而不是获取 nullReferenceException 仍然抛出异常

Question

I see the GetById function is returning null in this forEach which is throwing an exception, to prevent this I tried returning an empty Enumerable of that same type however I still get the System.NullReferenceException: Object reference not set to an instance of an object.

foreach (var categoryEntry in menus.GetById(options.OnlineOrdering.MenuId).Categories.OrderBy(i => i.Position) ?? Enumerable.Empty<MenuMenuCategory>())

Answer 1

You're accounting for the wrong null checking. If this is returning null :

menus.GetById(options.OnlineOrdering.MenuId)

Then this is throwing the error:

menus.GetById(options.OnlineOrdering.MenuId).Categories

Since you already have a null-coalescing check, you can use a null-conditional check on the expression that's failing:

menus.GetById(options.OnlineOrdering.MenuId)?.Categories

This would cause the entire expression to evaluate to null (never even trying to dereference the Categories property) is GetById() returns null . The result of that expression would then trigger the null-coalescing check you've added, and the loop should successfully iterate zero times as you intend.