c# - LINQ:失败，因为物化值为 null

Question

我试图在下面的代码中使用 sum 但出现错误:

The cast to value type 'System.Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

Product_Order:
    ---------------- ----------- ---------
    |   ProductId   | OrderId   | Quantity |
    ---------------- ----------- ---------

我在“let quantity”处收到错误

  var fullInfo = (from product in allProdcts
                 let quantity = db.Product_Order.Where(x=> x.ProductId == product.ID).Sum(x => x.Quantity)
                select new ReportVm
                    {                          
                        ProductId = product.ID,
                        ProductName = product.Name,
                        AmountProduct = quantity,
                        TotPrice = (quantity)*(product.Price)
                    }).ToList();

这是我的Product_Order 表(M-M 关系):

Product_Order:
    ---------------- ----------- ---------
    |   ProductId   | OrderId   | Quantity |
    ---------------- ----------- ---------

知道如何解决这个问题吗？

Answer 1

您需要允许可空的数量，您可以使用?? 表达式实现它并在使用 时转换为int? Sum().

.Sum(x => (int?)x.Quantity)??0

你的查询应该是这样的

var fullInfo = (from product in allProdcts
            let quantity = db.Product_Order.Where(x => x.ProductId == product.ID).Sum(x => (int?)x.Quantity)??0
            select new ReportVm
            {
                ProductId = product.ID,
                ProductName = product.Name,
                AmountProduct = quantity,
                TotPrice = (quantity)*(product.Price)
            }).ToList();