問題描述

我有以下 Linq 查詢:

I have the following Linq query:

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

當 result.Partials.Where(o => o.IsPositive) 不包含元素時，我得到一個異常.除了將操作一分為二并檢查是否為空之外，是否有一種優雅的方法來處理這個問題?我有一堂課充滿了這樣的操作.

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.

問題與 LINQ to Objects 相關.

The question is related with LINQ to Objects.

這是我得到的異常(翻譯它說:序列為空):

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

推薦答案

Min

- 無中介(例外！)

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

這是您的情況:如果沒有匹配的元素，則 Min 調用將引發異常 (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 將在 0 元素上創建一個枚舉，當列表中沒有元素時.你怎么知道 0 是 Min 還是 0 代表沒有元素的列表?

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);

這里的 Min 要么是 null(因為它等于 default(decimal?))要么是找到的實際 Min.

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

所以這個結果的消費者會知道:

So a consumer of this result will know that:

1. 當結果為 null 時，列表 沒有元素
2. 當結果是十進制值時，列表有一些元素，這些元素的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，如果這無關緊要，則可以調用 min.GetValueOrDefault(0).

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

這篇關于使用 Min 或 Max 時如何處理 LINQ 中的空值?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！