使用 Min 或 Max 时如何处理 LINQ 中的空值?

问题描述

我有以下 Linq 查询:

I have the following Linq query:

result.Partials.Where(o => o.IsPositive).Min(o => o.Result)

当 result.Partials.Where(o => o.IsPositive) 不包含元素时，我得到一个异常.除了将操作一分为二并检查是否为空之外，是否有一种优雅的方法来处理这个问题?我有一堂课充满了这样的操作.

I get an exception when result.Partials.Where(o => o.IsPositive) does not contains elements. Is there an elegant way to handle this other than splitting the operation in two and checking for null? I have a class full of operations like this one.

问题与 LINQ to Objects 相关.

The question is related with LINQ to Objects.

这是我得到的异常(翻译它说:序列为空):

This is the Exception I'm getting (translated it says: The sequence is empty):

推荐答案

Min

计算的简短总结

- 无中介(例外！)

   var min = result.Partials.Where(o => o.IsPositive).Min(o => o.Result);

这是您的情况:如果没有匹配的元素，则 Min 调用将引发异常 (InvalidOperationException).

This is your case: if there are no matching elements, then the Min call will raise an exception (InvalidOperationException).

 var min = result.Partials.Where(o => o.IsPositive)
                          .Select(o => o.Result)
                          .DefaultIfEmpty()
                          .Min();

DefaultIfEmpty 将在 0 元素上创建一个枚举，当列表中没有元素时.你怎么知道 0 是 Min 还是 0 代表没有元素的列表?

DefaultIfEmpty will create an enumeration over the 0 element, when there are no elements in the list. How do you know that 0 is the Min or if 0 stands for a list with no elements?

   var min = result.Partials.Where(o => o.IsPositive)
                            .Min(o => (decimal?)o.Result);

这里的 Min 要么是 null(因为它等于 default(decimal?))要么是找到的实际 Min.

Here Min is either null (because that's equal to default(decimal?)) or the actual Min found.

所以这个结果的消费者会知道:

So a consumer of this result will know that:

1. 当结果为 null 时，列表 没有元素
2. 当结果是十进制值时，列表有一些元素，这些元素的Min就是返回值.

1. When result is null then the list had no elements
2. When the result is a decimal value then the list had some elements and the Min of those elements is that returned value.

但是，如果这无关紧要，则可以调用 min.GetValueOrDefault(0).

However, when this doesn't matter, then min.GetValueOrDefault(0) can be called.

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