IQueryable vs ICollection (List)

Question

I have such a case study:

ToList() case:

    List<CategoryType> categories = (from c in categoryTypes where c.IsSysParam == isSysParamCategory select new CategoryType { Code = c.Code, CreateDate = c.CreateDate, EditDate = c.EditDate, IsProductCategory = c.IsProductCategory, IsSysParam = c.IsSysParam, Name = c.Name, TypeId = c.TypeId, ValueTypes = new List<ValueType>() }).ToList();

    List<ValueType> valueTypeList = new List<ValueType>();
    foreach (var c in categories.ToList())
    {
        valueTypeList = categoryTypes.Where(x => x.TypeId == c.TypeId).SelectMany(v => v.ValueTypes).Where(v => v.ParentValueId == null).ToList();
        c.ValueTypes = valueTypeList;
    }

IQueryable case:

When I change in first query - List<CategoryType> to IQueryable<CategoryType> and remove ToList() from the end of query then I dont have any result:

Question:

I am asking for an explanation, I do not understand why this is happening. I know that the IQueryable makes some part of the work on the database side.

Edit: The code is working, pictures shows the final effect.

I have:

  public IQueryable<CategoryType> CategoryTypePagination { get; set; }

and in the end of ToList() case:

this.CategoryTypePagination = categories.AsQueryable();

in IQueryable case just removed .AsQueryable()

Answer 1

您必须查看“延迟查询执行”和“立即查询执行”

Answer 2

Accrodingly to this , IQueryable uses something called lazy loading .

So the results of IQueryable aren't loaded until they are first used, for example in Sum , ToList or ToArray methods, while ToList requieres data to be loaded. Thus you see the difference after initailizing both objects.