LINQ中的条件选择(如果为空，则选择)

本文关键字：选择 条件 LINQ 如果 | 更新日期: 2023-09-27 18:29:45

是否有某种"LINQ"方法可以对数据进行条件选择，即如果第一个源为空，则从另一个源进行选择？一个例子是，如果您有一个项的树结构，并且您想从根中获取一些资产，或者如果它是空的，则从它的子级中获取。

我有以下例子：

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

LINQ中的条件选择(如果为空，则选择)

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)));

我会选择另一种变体，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，我想会像

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

但这将包括在子项资产的Items集合中找到的资产

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