[英]Conditional selection in LINQ (select instead if empty)
是否有一些“LINQ”方式來條件選擇數據,即如果第一個數據為空則從另一個來源中選擇? 一個例子是,如果你有一個項目的樹結構,你想從一個根,或者如果它是空的,從它的孩子那里得到一些資產。
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
我相信LINQ方式看起來有點難看
var assets = item.Any(a=>HaRelevantAsset(a)) ? item.Where(a => HasRelevantAsset(a)) :
item.SubItems.SelectMany(item =>
item.Assets.Where(a => HasRelevantAsset(a)));
我會選擇其他變種,擴展方法
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,我想嘗試對SelectRelevantAssets進行遞歸調用,我想是這樣的
if (!assetsInItemFound)
{
yield break;
}
else
{
foreach (var subItem in item.SubItems)
foreach (var asset in SelectRelevantAssets(subItem))
yield return asset;
}
但這將包括在subItem的Items集合中找到的資產
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