c# - 正确处理 lambda 表达式中可能出现的 System.NullReferenceException

Question

这是有问题的查询

return _projectDetail.ExpenditureDetails
    .Where(detail => detail.ProgramFund == _programFund
         && detail.Expenditure.User == _creditCardHolder)
    .Sum(detail => detail.ExpenditureAmounts.FirstOrDefault(
        amount => amount.isCurrent && !amount.requiresAudit)
    .CommittedMonthlyRecord.ProjectedEac);

表结构
ProjectDetails (1 to Many) 支出明细
ExpenditureDetails(1 到多)ExpenditureAmounts
ExpenditureAmounts (1 to 1) CommittedMonthlyRecords

ProjectedEac 是 CommittedMonthlyRecords 上的小数字段。

我在单元测试中发现的问题(尽管不太可能发生)，即以下行可能为空:

detail.ExpenditureAmounts.FirstOrDefault(
    amount => amount.isCurrent && !amount.requiresAudit)

我的原始查询是一个嵌套循环，我会在其中多次访问数据库，我不想重复。我在这里查看了一些类似的问题，但是 the solution didn't seem to fit .

有什么想法吗？

Answer 1

为什么不检查 null？

return _projectDetail.ExpenditureDetails.Where(detail => 
        detail.ProgramFund == _programFund && 
        detail.Expenditure.User == _creditCardHolder
    ).Sum(detail => {
        var a = detail.ExpenditureAmounts.FirstOrDefault(
            amount => amount.isCurrent && !amount.requiresAudit
        );
        return a != null ? a.CommittedMonthlyRecord.ProjectedEac : 0.0m;
    });