c# - 如何使用 Group by 在 Linq 语句中引入 Let 关键字

Question

我有以下带有“Group by”子句的 Linq 语句，想知道如何引入 let 或任何其他语句以避免重复子查询，lifecycleEvents.Where(i => i.LifecycleEventId = = grouping.Key).First() 在下面的例子中

var completionTimeModels =
    from timeline in processTimelines 
    group timeline by timeline.LifecycleEventId into grouping
    select new CompletionTimeViewModel()
    {
        // How to avoid repeating the same query to find the life cycle event?
        Name = lifecycleEvents.Where(i => i.LifecycleEventId == grouping.Key).First().LifecycleEventName,
        DisplayName = lifecycleEvents.Where(i => i.LifecycleEventId == grouping.Key).First().LifecycleEventDisplayName
    };

Answer 1

var completionTimeModels =
from timeline in processTimelines

group timeline by timeline.LifecycleEventId into grouping
let foo = lifecycleEvents.First(i => i.LifecycleEventId == grouping.Key)
select new CompletionTimeViewModel()
{
    Name = foo.LifecycleEventName,
    DisplayName = foo.LifecycleEventDisplayName
};