c# - 转换为值类型 'Double' 失败，因为具体化值为 null

Question

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，因为具体化值为 null。结果类型的通用参数或查询必须使用可空类型。

我已经看到的:

The cast to value type 'Int32' failed because the materialized value is null

The cast to value type 'Decimal' failed because the materialized value is null

我尝试了什么:

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

和:

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

这些都不起作用。我知道问题的原因是该表中没有我传入的 UserId 的行。在那种情况下，我宁愿 Sum() 只是向我返回 0。有什么想法吗？

Answer 1

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;