LINQ Max扩展方法在空集合上给出错误 [英] LINQ Max extension method gives an error on empty collections

问题描述

我有以下查询:

var maxNumber = dbContext.Where(a => a.Id == 9).Max(a => a.Sample_Num);

如果没有编号为9的ID，则会出现错误.如果没有ID为9，我想将结果默认为0.

If there is no Id of 9, I get an error. I like to default the result to 0 if there is no Id of 9.

我尝试过:

var maxNumber = dbContext.Where(a => a.Id == 9).Max(a => a.Sample_Num) ?? 0; 

以及其他变体，但无法使其正常工作

as well as other variations but was not able to get it to work

推荐答案

您可以使用Any检查是否存在匹配的元素:

You could use Any to check if there's a matching element:

int maxNumber = 0;
var id9 = dbContext.Where(a => a.Id == 9);
if(id9.Any())
{
    maxNumber = id9.Max(a => a.Sample_Num);
}

，或者您可以使用 DefaultIfEmpty(defaultValue) :

or you could use DefaultIfEmpty(defaultValue):

int maxNumber = dbContext
    .Where(a => a.Id == 9)
    .Select(a => a.Sample_Num)
    .DefaultIfEmpty(0)
    .Max();

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