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，出现异常.除了将操作分为两部分并检查是否为null之外，是否有一种优雅的方式来处理此问题?我有一堂课，像这样的操作.

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有关.

The question is related with LINQ to Objects.

这是我得到的异常(翻译为:序列为空):

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

推荐答案

Min计算的简短摘要

A short summary of the calculation of a Min

   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();

当列表中没有元素时，

DefaultIfEmpty将在0元素上创建一个枚举.您怎么知道0是Min或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 0 stands for a list with no elements?

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

此处的min为空(defaul(decimal?))或找到的实际Min.因此，此结果的使用者将知道，如果结果为null，则列表中没有元素；当结果为十进制值时，则列表中包含一些元素，而这些元素的Min为该值.

Here min is either null (defaul(decimal?)) or the actual Min found. So a consumer of this result will know that if the result is null then the list had no elements and when the result is a value of decimal, then the list had some elements and the Min of those elements is that value.

但是，当这无关紧要时，可以调用min.GetValueOrDefault(0).

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

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