转换为值类型“Double”失败，因为物化值为null [英] The cast to value type 'Double' failed because the materialized value is null

问题描述

代码

double cafeSales = db.InvoiceLines
    .Where(x =>
        x.UserId == user.UserId &&
        x.DateCharged >= dateStart &&
        x.DateCharged <= dateEnd)
    .Sum(x => x.Quantity * x.Price);

错误：

转换为值类型Double失败，因为物化值为null。结果类型的通用参数或查询必须使用可空类型。

The cast to value type 'Double' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

我已经看到已经存在：

转换为值类型Int32失败，因为物化值为空

转换为值类型Decimal失败，因为实例化值为空

我有什么TRIED：

double cafeSales = db.InvoiceLines
    .Where(x =>
        x.UserId == user.UserId &&
        x.DateCharged >= dateStart &&
        x.DateCharged <= dateEnd)
    .DefaultIfEmpty()
    .Sum(x => x.Quantity * x.Price);

And：

double? cafeSales = db.InvoiceLines
    .Where(x =>
        x.UserId == user.UserId &&
        x.DateCharged >= dateStart &&
        x.DateCharged <= dateEnd)
    .Sum(x => x.Quantity * x.Price);

这两个都不工作。我知道这个问题的原因是该表中没有对我进入的UserId的行。在这种情况下，我宁愿Sum（）刚刚返回一个0给我。任何想法？

Neither of these work. I know the cause of the problem is that there are no rows in that table for the UserId I am passing in. In that case, I would prefer Sum() just returned a 0 to me. Any ideas?

推荐答案

最佳解决方案

double cafeSales = db.InvoiceLines
                     .Where(x =>
                                x.UserId == user.UserId &&
                                x.DateCharged >= dateStart &&
                                x.DateCharged <= dateEnd)
                     .Sum(x => (double?)(x.Quantity * x.Price)) ?? 0;

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