内容简介：http://stackoverflow.com/questions/15297925/the-cast-to-value-type-double-failed-because-the-materialized-value-is-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？
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;
http://stackoverflow.com/questions/15297925/the-cast-to-value-type-double-failed-because-the-materialized-value-is-null
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