使用LINQ转换为Int

Question

下面的LINQ有几个Convert.ToInt32方法。但它不起作用。参考互联网，它使用int.Parse而不是转换。仍然给出错误。请告诉我一个方向来做到这一点。

在下面查询tody的数据类型是DateTime

var ds = (from a in dbSetInvHeader 
      join b in dbSetCustomer on a.BusinessEntityID equals b.Id 
      join c in dbSetFinancialInfo on b.Id equals c.Id 
      where (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount) > 0 
         && DbFunctions.AddDays(a.InvoiceDate, Convert.ToInt32(c.CreditPeriod)) >= tody 
      select new OverDueInvoices 
         { 
          CustomerName = b.Name, 
          InvoiceNo = a.InvoiceNo, 
          InvoiceAmount = a.TotalAmount - a.ApplyToInvoiceCreditAmount, 
          DueAmount = (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount), 
          CreditAmount = a.ApplyToInvoiceCreditAmount, 
          NoOfDays = Convert.ToInt32(DbFunctions.DiffDays(tody, a.InvoiceDate)) 
         }).ToList(); 

更新：

的代码使用实体框架上面LINQ引发错误：

当使用Convert.ToInt32：

“LINQ to Entities不识别方法'Int32 ToInt32（System.Decimal）'方法，并且此方法不能转换为存储表达式。”

当使用int.Parse：

“LINQ实体无法识别方法‘的Int32解析（System.String）’方法，和这种方法不能被翻译成表达店”。

Answer 1

SQL不知道Convert.ToInt32函数。在Entity Framework中似乎没有将字符串转换为int的解决方法。你可以做的是另一列添加到您反对NoOfDaysString

public string NoOfDaysString {get; set;} 
public string NoOfDays {get { return Convert.ToInt32(NoOfDaysString); } ;} 

和重写查询这样

var ds = (from a in dbSetInvHeader 
        join b in dbSetCustomer on a.BusinessEntityID equals b.Id 
        join c in dbSetFinancialInfo on b.Id equals c.Id 
        where (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount) > 0 
        && DbFunctions.AddDays(a.InvoiceDate, Convert.ToInt32(c.CreditPeriod)) >= tody 
        select new OverDueInvoices 
        { 
         CustomerName = b.Name, 
         InvoiceNo = a.InvoiceNo, 
         InvoiceAmount = a.TotalAmount - a.ApplyToInvoiceCreditAmount, 
         DueAmount = (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount), 
         CreditAmount = a.ApplyToInvoiceCreditAmount, 
         NoOfDaysString = Convert.ToInt32(DbFunctions.DiffDays(tody, a.InvoiceDate)) 

        }).ToList(); 

这是唯一的解决方法，我可以在这里声明想到的。添加CreditPeriod在你的对象，并执行其中后的对象保存在内存中

var ds = (from a in dbSetInvHeader 
       join b in dbSetCustomer on a.BusinessEntityID equals b.Id 
       join c in dbSetFinancialInfo on b.Id equals c.Id 
       where (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount) > 0 
       select new OverDueInvoices 
       { 
        CustomerName = b.Name, 
        InvoiceNo = a.InvoiceNo, 
        InvoiceAmount = a.TotalAmount - a.ApplyToInvoiceCreditAmount, 
        DueAmount = (a.TotalAmount - a.AppliedAmount - a.ApplyToInvoiceCreditAmount), 
        CreditAmount = a.ApplyToInvoiceCreditAmount, 
        NoOfDaysString = Convert.ToInt32(DbFunctions.DiffDays(tody, a.InvoiceDate)) 
        CreditPeriod = c.CreditPeriod 
       })ToList().Where(t=>Convert.ToInt32(t.CreditPeriod) >= NoOfDays).ToList(); 

Answer 2

DbFunctions.DiffDays(tody, a.InvoiceDate)返回一个Nullable<int>，所以不是Convert.ToInt32尝试以下

DbFunctions.DiffDays(tody, a.InvoiceDate).GetValueOrDefault(); 
DbFunctions.DiffDays(tody, a.InvoiceDate) ?? someIntegerDefaultValue; 
DbFunctions.DiffDays(tody, a.InvoiceDate).Value; 

第一个是一个如果您知道结果可能是null，并且您希望获得0而不是此案例。第二个是当你想要一个不同的默认值或者计算一些更复杂的替换时，第三个是你知道结果永远不会为空的情况，如果它为空则可以得到异常。

Answer 3

由于@LiviuBoboia指出，SQL不知道这两个Convert.ToInt32和int.Parse方法什么，EF不能正确转换调用此方法为SQL。取而代之的是，你可以做简单的转换：

NoOfDaysString = (int)DbFunctions.DiffDays(tody, a.InvoiceDate) 

这个转换将被转换为SQL作为SQL函数CONVERT，这应该工作良好。

虽然DbFunctions.DiffDays返回Nullable<int>或int?所以这将是更好地避免铸造(int)，你可以得到InvalidCastException试图null转换为int