这个LINQ可以更高效吗？

Question

我有一个相当复杂的linq分组，有些重复让我烦恼，但我无法减少它。 有没有办法避免两次获得ID ==“XYZ”的项目列表？

var example = = new GdsObservableCollection<GroupedQueryResults>(
                items.Where(a => a.SubCategory3 != "{template}")
                     .GroupBy(item => item.SubCategory1)
                     .Select(g => new GroupedQueryResults
                                  {
                                     SubCategory = g.Key,
                                     SectionHeader = (g.Count(x => x.Id == "XYZ") > 0) ?
                                     "Category :" +  g.Where(x => x.Id == "XYZ")
                                                     .First().NewValue :
                                     "Item - " + itemNumber

...

Answer 1

我不会说效率更高，但可以更小一点，因为你可以在Any和First使用谓词：

var example = new GdsObservableCollection<GroupedQueryResults>(
            items.Where(a => a.SubCategory3 != "{template}")
                 .GroupBy(item => item.SubCategory1)
                 .Select(g => new GroupedQueryResults
                              {
                                 SubCategory = g.Key,
                                 SectionHeader = g.Any(x => x.Id == "XYZ") ?
                                 "Category :" +  g.First(x => x.Id == "XYZ").NewValue :
                                 "Item - " + itemNumber

Answer 2

通过使用Where + Select + FirstOrDefault链与C＃null-coalescing运算符结合，可以避免重复搜索x.Id == "XYZ" ：

SectionHeader = g
    .Where(x => x.Id == "XYZ")
    .Select(x => "Category :" +  x.NewValue)
    .FirstOrDefault() ?? "Item - " + itemNumber