如何“让"在 lambda 表达式中?

问题描述

如何使用 lambda 表达式将此 linq 查询重写为实体?
我想在我的 lambda 表达式中使用 let 关键字或等效关键字.

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

对于一些类似的问题，比如我的问题下的评论，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平均价格，而在我提到的查询样式中，let 表达式阻止多次计算平均值.

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，您可以使用扩展方法语法，这将比您当前使用的多一个 lambda 表达式.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，因为您的结果永远不会返回任何结果(大于 500 且小于 250).

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

替代方案是

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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