c# – 轉換為值型別“Double”失敗,因為物化值為null

碼：

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

錯誤：

轉換為值型別“Double”失敗,因為物化值為null.結果型別的通用引數或查詢必須使用可空型別.

我已經看到了什麼：

ofollow,noindex" target="_blank">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

我有什麼：

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);

這兩個都不工作.我知道這個問題的原因是該表中沒有對我進入的UserId的行.在這種情況下,我寧願Sum()剛剛返回一個0給我.有任何想法嗎？

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