Question

下述法典行之有效。

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

下面的代码产生,但并未提及某一实例错误。

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
});

www.un.org/Depts/DGACM/index_spanish.htm Error Detail

第54条:数字群体List = 命令List.Select(p =>新集团SelectListItem()

<>光> 数据库的类别表包含一些与父母关系的内容,这些内容涉及某些记录,其价值等于零。

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

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