c# - LINQ to Entities 查询不支持转换为十进制

Question

我有一个数据库表 Transaction (transactionID, LocalAmount...)。其中 Localamount 属性的数据类型是 float。在 UI 上，我试图在按钮单击事件的一行中返回列 (Localamount) 的 SUM。

我使用了decimal 而不是float

但是，我在转换为decimal

的代码中遇到错误

System.NotSupportedException was unhandled by user code
Message=Casting to Decimal is not supported in LINQ to Entities queries, because the required precision and scale information cannot be inferred.

 public static IEnumerable<TransactionTotalForProfitcenter> GetTotalTransactionsForProfitcenter(int profitcenterID)
    {
        List<TransactionTotalForProfitcenter> transactions = new List<TransactionTotalForProfitcenter>();
        using (var context = new CostReportEntities())
        {
          transactions = (from t in context.Transactions
                            join comp in context.Companies on t.CompanyID equals comp.CompanyID
                            join c in context.Countries on comp.CountryID equals c.CountryID
                            where c.CountryID.Equals(comp.CountryID) && t.CompanyID == comp.CompanyID 
                             
                            join acc in context.Accounts
                                 on t.AccountID equals acc.AccountID
                            join pc in context.Profitcenters
                                on t.ProfitcenterID equals pc.ProfitcenterID
                            group t by pc.ProfitcenterCode into tProfitcenter
                            
                            select new TransactionTotalForProfitcenter
                            {
                                ProfitcenterCode = tProfitcenter.Key,
                    //the error is occurring on the following line           
                                TotalTransactionAmount = (decimal)tProfitcenter.Sum(t => t.LocalAmount),  
                   //the error is occurring on the following line       
                                TotalTransactionAmountInEUR = (decimal)tProfitcenter.Sum(t => t.AmountInEUR) //the error is occurring on this line 
                            }
                            ).ToList();

        }
        return transactions;

    }

我尝试了以下帖子中的几个选项，但没有成功。

• LINQ to SQL strange float behavirou
• Casting from Decimal to Float

谁能指出我可以尝试的其他选项。如果它太琐碎，请原谅我对 LINQ 的了解。

Answer 1

Entity Framework 表明它不支持您想要的转换。一种解决方法是尽可能多地执行数据库中的工作，然后在内存中完成该过程。在您的情况下，您可以计算其 native 类型的总和，将结果作为匿名类型拉入内存，然后在构建实际需要的类型时执行转换。要进行原始查询，您可以进行以下更改:

select new // anonymous type from DB
{
    ProfitcenterCode = tProfitcenter.Key,
    // notice there are no conversions for these sums
    TotalTransactionAmount = tProfitcenter.Sum(t => t.LocalAmount),       
    TotalTransactionAmountInEUR = tProfitcenter.Sum(t => t.AmountInEUR)
})
.AsEnumerable() // perform rest of work in memory
.Select(item =>
     // construct your proper type outside of DB
    new TransactionTotalForProfitcenter
    {
        ProfitcenterCode = item.ProfitcenterCode,
        TotalTransactionAmount = (decimal)item.TotalTransactionAmount
        TotalTransactionAmountInEUR = (decimal)item.TotalTransactionAmountInEUR
    }
).ToList();