当我们在 Linq 上使用 .Select(x=>new object) 时，为什么我们不能在多方面选择任何属性

Question

i just wonder about something which has no sense to me. I'm using ASP.NET Core 5.0 and im using Linq.

I have a to class which has relationship One (MainCategory) - To - (Category) Many . I wanna write a linq which is gonna targeting to get MainCategoryName and CategoryID . Let me show to you my linq :

public IEnumerable<MainCategory> GetMainCategoryForFair()
{
    return DBContext.MainCategories.Where(x => x.MainCategoryIsFeatured == true
        && x.IsDeleted != true)
        .Select(x => new MainCategory
        {
            MainCategoryName = x.MainCategoryName,
            Categories = x.Categories
        }).ToList();
}

I know, that query gonna take all properties of Categories and this is the thing im trying to avoid. Is it possible ?

if it's possible,then how ?

if it's not, why ?

Answer 1

Sure. Just don't project all the properties. And normally if you are returning a different data shape, you would introduce a new class to return, instead of returning an Entity type with a bunch of null properties, eg

public IEnumerable<MainCategoryDTO> GetMainCategoryForFair()
{
    return DBContext.MainCategories.Where(x => x.MainCategoryIsFeatured == true
        && x.IsDeleted != true)
        .Select(x => new MainCategoryDTO()
        {
            MainCategoryName = x.MainCategoryName,
            Categories = x.Categories.Select( c => new CategoryDTO(){CategoryName = c.Name, ...}).ToList()
        }).ToList();
}