如何介绍一下里面的Linq与集团发言let关键字

问题描述

我有集团通过条款如下LINQ的发言，并想知道如何引入一个让或任何其他语句，避免重复的子查询， lifecycleEvents.Where（I =＆GT; i.LifecycleEventId == grouping.Key）。首先（）在下面的例子

  VAR completionTimeModels =
    从时间轴上processTimelines
    组时间线timeline.LifecycleEventId到分组
    选择新CompletionTimeViewModel（）
    {
        //如何避免重复相同的查询，找到生命周期的事件吗？
        名称= lifecycleEvents.Where（I =＆GT; i.LifecycleEventId == grouping.Key）。首先（）LifecycleEventName，
        显示名称= lifecycleEvents.Where（I =＆GT; i.LifecycleEventId == grouping.Key）。首先（）LifecycleEventDisplayName。
    };
 

解决方案

  VAR completionTimeModels =
从时间轴上processTimelines

组时间线timeline.LifecycleEventId到分组
让富= lifecycleEvents.First（I =＆GT; i.LifecycleEventId == grouping.Key）
选择新CompletionTimeViewModel（）
{
    名称= foo.LifecycleEventName，
    显示名称= foo.LifecycleEventDisplayName
};
 

I have the following Linq statement with 'Group by' clause and would like to know how to introduce a let or any other statement to avoid repeating the sub query, lifecycleEvents.Where(i => i.LifecycleEventId == grouping.Key).First() in the following example

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
    };

解决方案

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
};

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