Question

The following code works good

IEnumerable<GroupedSelectListItem> groupList = db.Categories.Select(p => new GroupedSelectListItem()
{
     GroupKey = p.ParentCategory.Name,
     GroupName = p.ParentCategory.Name,
     Text = p.Name,
     Value = p.Name
});

While the below code generates object reference not set to an instance error

List<Category> orderedList = new List<Category>();
var rootList = db.Categories.Where(c => c.ParentCategoryId == null).ToList();
    foreach (var item in rootList)
    {
        orderedList.Add(item);
        if (item.SubCategories.Count != 0)
        {
            foreach (var subcat in item.SubCategories)
            {
                orderedList.Add(subcat);
                if (subcat.SubCategories.Count != 0)
                {
                    foreach (var subsubcat in subcat.SubCategories)
                    {
                        orderedList.Add(subsubcat);
                    }
                }
            }
        }
    }
IEnumerable<GroupedSelectListItem> groupList = orderedList.Select(p => new GroupedSelectListItem()
{
     GroupKey = p.ParentCategory.Name,
     GroupName = p.ParentCategory.Name,
     Text = p.Name,
     Value = p.Name
});

Error Detail

Line 54: IEnumerable groupList = orderedList.Select(p => new GroupedSelectListItem()

NOTE: The table for Categories in database contains element with ParentCategoryId for some records equal to null

Answer 1

The first code snippet is using Linq-to-entities with automatic coalescence. It menas that if p doesn’t have ParentCategory it will work without problem on SQL level. The second example use Linq-to-objects where no such feature exists. You must manually check if ParentCategory property is filled prior to accessing its properites:

GroupKey = p.ParentCategory != null ? p.ParentCategory.Name : null