请帮助我解决该错误 The cast to value type 'System.Decimal' failed because the materialized value is null.

Question

我有错误：转换为值类型“System.Decimal”失败，因为具体化值为空。

结果类型的泛型参数或查询必须使用可为 null 的类型。

请帮助我解决该错误。

如果值为空，则会发生错误.

public partial class Warehouse
    {
        public int WarehouseId { get; set; }
        public bool Operation { get; set; }
        public DateTime DateOperation { get; set; }
        public int NomenclatureId { get; set; }
        public decimal Quantity { get; set; }
        public string Notes { get; set; }
        public virtual Nomenclature Nomenclatures { get; set; }
    }

using (var db = new ContextTest())
            {

                var creditBy = from row in db.Warehouses
                               group row by row.Nomenclatures.NomenclatureName into byNomenclature
                               select new { Date = byNomenclature.Key, CreditTotal = byNomenclature.Where(o => o.Operation == true).Sum(o => o.Quantity) };

                var expenseBy = from row in db.Warehouses
                                group row by row.Nomenclatures.NomenclatureName into byNomenclature
                                select new { Date = byNomenclature.Key, ExpenseTotal = byNomenclature.Where(o => o.Operation == false).Sum(o => o.Quantity) };

                var q = from o in creditBy
                        join e in expenseBy on o.Date equals e.Date
                        select new
                        {
                            o.Date,
                            o.CreditTotal,
                            e.ExpenseTotal,
                            Total = o.CreditTotal - e.ExpenseTotal
                        };
                grid.DataSource = q.ToList();
            }

Note：此问题总结整理于:Please help me get around the error.

Answer 1

你好，问题是 Quantity 属性类型是不可为空的，当源为 null 时，EF 需要能够存储一个可为 null 的值。
因此，您可以使 Quantity l 属性可为 null 来修复它。

public partial class Warehouse  
{  
   public decimal? Quantity { get; set; }  
}

如果回复有帮助，请单击“接受答案”并投赞成票。 注意：如果您想接收此线程的相关电子邮件通知，请按照我们文档中的步骤启用电子邮件通知。