集中或合并 LINQ select

Question

如何重构此代码以便集中投影？

public IEnumerable<ItemDto> GetItemsByType(int itemId, ItemType itemType)
{
    IEnumerable<ItemDto> items = null;
    try
    {
        var tempItems= _Items.Get(i => i.ItemId == itemId
            && o.Active == true);
        switch (itemType)
        {
            case ItemType.Normal:
                items = from item in tempItems
                        select new ItemDto
                        {
                            // many fields here
                        };
                break;
            case ItemType.Damaged:
                items = from item in tempItems
                        join itemDetail in _ItemDetails.Get() on item.ID equals itemDetail.ItemID
                        select new ItemDto
                        {
                            // many fields here
                        };
                break;
            case ItemType.Fixed:
                items = from item in tempItems
                        join itemDetail in _ItemDetails.Get() on item.ID equals itemDetail.ItemID
                        where item.Status.ToLower() == "fixed"
                        select new ItemDto
                        {
                            // many fields here
                        };
                break;

            // more case statements here...

            default:
                break;
        }
    }
    catch { ... }
}

基本上，我有很多案例陈述，每个案例陈述都有很长的投影。我担心一旦 DTO 需要更改，比如添加一个新字段，其他案例的预测可能彼此不一致（忘记或错过更新）。我怎样才能集中这个？

Answer 1

你可以这样做：

var baseQuery = from item in tempItems select item;
switch (itemType)
{
     case ItemType.Fixed:
         baseQuery = from item in baseQuery where item.ID equals itemID select item;
         break;
}

return (from item in baseQuery select new ItemDTO (...projection here... ));

Answer 2

你能这样处理吗？

var query = tempItems.AsQueryable();

switch(itemType)
{
   case ItemType.Damaged:
        query.Join(...);
        break;

   case ItemType.Fixed:
        query.Where(...);
}

query.Select(e => new ItemDto{//Lots of properties});

return query.ToList();

Answer 3

这种方法有帮助吗？

public IEnumerable<ItemDto> GetItemsByType2(int itemId, ItemType itemType)
{
    var cases = new Dictionary<ItemType, Func<IEnumerable<ItemDto>, IEnumerable<ItemDto>>>()
    {
        { ItemType.Normal, xs => xs },
        { ItemType.Damaged, xs =>
            from item in xs
            join itemDetail in _ItemDetails.Get() on item.ID equals itemDetail.ItemID
            select item },
        { ItemType.Fixed, xs =>
            from item in xs
            join itemDetail in _ItemDetails.Get() on item.ID equals itemDetail.ItemID
            where item.Status.ToLower() == "fixed"
            select item },
    };

    return cases[itemType](_Items.Get(i => i.ItemId == itemId && o.Active == true))
        .Select(x => new ItemDto { .... });
}

集中或合并 LINQ select

Centralizing or consolidating LINQ select

c#

linq