c# - LINQ 中的条件选择(如果为空，则改为选择)

Question

是否有一些“LINQ”方法可以有条件地选择数据，即如果第一个数据源为空，则从另一个数据源中选择？一个例子是，如果你有一个项目的树结构，你想从根中获取一些 Assets ，或者如果它是空的，从它的 child 中获取一些 Assets 。

我有以下示例:

IEnumerable<Item> items = ...;
// Item has a Assets property that returns IEnumerable<Asset>
// Item has a SubItems property that returns IEnumerable<Item>
//    i.e. other items with assets in them

// getting assets from a "main" item
var assets = item.Assets.Where(a => HasRelevantAsset(a));

// if there were no relevant assets in the "main" item
if (!assets.Any()) {
    // then reselect from "subitems" assets instead
    assets = item.SubItems.SelectMany(item => 
        item.Assets.Where(a => HasRelevantAsset(a)));
}

// HasRelevantAsset(Asset) is a static method that returns 
// true if it is the asset that is needed

Answer 1

我相信 LINQ 方式看起来会有点难看

var assets = item.Any(a=>HaRelevantAsset(a)) ? item.Where(a => HasRelevantAsset(a)) :
                     item.SubItems.SelectMany(item => 
                            item.Assets.Where(a => HasRelevantAsset(a)));

我会选择其他变体，扩展方法

public static IEnumerable<Asset> SelectRelevantAssets(this Item item)
{
     var assetsInItemFound = false;
     foreach (var asset in item.Assets)
     {
         if (HasRelevantAsset(asset))
         {
             assetsInItemFound = true;
             yield return asset;
         }
     }
     if (assetsInItemFound)
     {
         yield break;
     }
     else
     {
         foreach (var subItem in item.SubItems)         
             foreach (var asset in subItem.Assets)
                if (HasRelevantAsset(asset))
                    yield return asset;
     }
}

首先，我想尝试对 SelectRelevantAssets 进行递归调用，我想应该是

if (!assetsInItemFound)
         {
             yield break;
         }
         else
         {
             foreach (var subItem in item.SubItems)         
                 foreach (var asset in SelectRelevantAssets(subItem))
                        yield return asset;
         }

但这将包括在子项的 Assets 的 Items 集合中找到的 Assets