如何“让”在lambda表达式？ [英] How to "let" in lambda expression?

问题描述

如何使用lambda表达式将此linq查询重写为Entity？

我要在我的lambda表达式中使用 let 关键字或等效项。

How can I rewrite this linq query to Entity on with lambda expression?
I want to use let keyword or an equivalent in my lambda expression.

var results = from store in Stores
              let AveragePrice =  store.Sales.Average(s => s.Price)
              where AveragePrice < 500 && AveragePrice > 250

对于某些类似问题，例如我在问题下的评论，建议

For some similar questions like what is commented under my question, it's suggested to

.Select(store=> new { AveragePrice = store.Sales.Average(s => s.Price), store})

这将计算每个项目的AveragePrice，而在我提到的查询风格中，表达式阻止计算平均次数。

which will calculate AveragePrice for each item, while in Query style I mentioned, let expression prevents to calculate average many times.

推荐答案

因此，可以使用扩展方法语法，比你目前正在使用。没有 let ，您只需使用多行lambda并声明一个变量：

So, you can use the extension method syntax, which would involve one lambda expression more than you are currently using. There is no let, you just use a multi-line lambda and declare a variable:

var results = Stores.Where(store => 
{
    var averagePrice = store.Sales.Average(s => s.Price);
    return averagePrice > 250 && averagePrice < 500;
});

请注意，我更改了平均价格比较，因为您的数据永远不会返回任何结果

Note that I changed the average price comparison, because yours would never return any results (more than 500 AND less that 250).

替代方法是

var results = Stores.Select(store => new { Store = store, AveragePrice = store.Sales.Average(s => s.Price})
    .Where(x => x.AveragePrice > 250 && x.AveragePrice < 500)
    .Select(x => x.Store);

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