c#在LINQ中定义LET

本文关键字：定义 LET LINQ | 更新日期: 2023-09-27 18:04:32

我有多个使用相同LET变量的LINQ查询，我想以某种方式预定义这些。

IQueryable<RouteQueryModel> query =
    (from b in db.routes
     let avg_rating = b.ratings.Any() ? 
         b.ratings.Select(r => r.rating1).Average() : 
         0
     let distance_to_first_from_me = b.coordinates.
         Select(c => c.position).
         FirstOrDefault().
         Distance(DbGeography.FromText(currentLocation, 4326))
     let distance_to_last_from_me = b.coordinates.
         OrderByDescending(c => c.sequence).
         Select(d => d.position).
         FirstOrDefault().
         Distance(DbGeography.FromText(currentLocation, 4326))
     let distance_to_from_me = distance_to_first_from_me < distance_to_last_from_me ? 
         distance_to_first_from_me : 
         distance_to_last_from_me
     where b.endpoints.Any(e => values.Any(t => t == e.town.town_id))
     select new RouteQueryModel 
     { 
         b = b, 
         distance_to_from_me = distance_to_from_me.Value, 
         avg_rating = avg_rating
     }
 );

我在8个不同的查询中使用这三个distance_to let，有没有办法为那些我可以在查询中使用的模板?

c#在LINQ中定义LET

有一个简单的方法来预编译LINQ查询:

var distanceToFirstFromMe =
    CompiledQuery.Compile<Route, GeoCoordinates, Distance>((route, currentLocation) => {
        return route.coordinates
            .Select(c => c.position)
            .FirstOrDefault()
            .Distance(DbGeography.FromText(currentLocation, 4326));
    });

要在查询中使用它们，您可以简单地调用它们:

IQueryable<RouteQueryModel> query =
    (from b in db.routes
     let avg_rating = b.ratings.Any() ? 
         b.ratings.Select(r => r.rating1).Average() : 0
     let distance_to_first_from_me = distanceToFirstFromMe(b, currentLocation)
     // ...
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