基本解决方案:

allItems.ForEach(x => x.Combination.FirstOrDefault(ele => ele.Index == 0).ItemCount -= 5);

如注释中所述，如果只有1个元素的索引为1，那么您可以做的是:

As mentioned in comment if there is only 1 element with index 1 then you can do is :

   allItems.ForEach(x => x.Combination.First(ele => ele.Index == 0).ItemCount -= 5);

如果不确定给定索引是否存在于集合中，您可以做的是

If you are not sure that the given index exists in the collection you can do is

CombMin element = new CombMin();
int index = 25;//does not exists in the collection
element = allItems.FirstOrDefault(x => x.Combination.Any(ele => ele.Index == index));
if (element != null)
    element.Combination.FirstOrDefault(ele => ele.Index == index).ItemCount -= 5;