c# – 转换为值类型“Double”失败,因为物化值为null

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.

我已经看到了什么：

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

我有什么：

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.有任何想法吗？

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