如何处理LINQ子查询中的空值?

斯图亚特

我将数据库中的price字段更改为不允许空值,得到了与您相同的错误:

"Operator '??' cannot be applied to operands of type 'decimal' and 'int'". 

正如您指出的,当price设置为不允许在数据库中为空时,空合并运算符将不再工作,因为它期望看到可为空的类型decimal:

decimal?

如果移除空合并运算符并运行不包含价格的测试用例,我将得到:

"The null value cannot be assigned to a member with type System.Decimal which is a non-nullable value type.."

这是有效的代码。我将子查询结果强制转换为十进制?在应用空合并运算符之前。

public class Class1
{

    DataClasses1DataContext dc = new DataClasses1DataContext();

    public decimal test(int stockID)
    {

        var q = from s in dc.Stocks
                where s.StockID == stockID
                select new
                {
                    StockID = s.StockID,
                    memberPrice = ((decimal?)(from mp in dc.StockPrices
                                   where mp.StockID == s.StockID
                                   select mp.Price).FirstOrDefault()) ?? 0
                };

        return q.FirstOrDefault().memberPrice;
    }
}

斯图尔特,试试这个:

Dim q = From s In dc.STOCKs _
    Select s.ID, MemberPrice = _
      if((From mp In dc.STOCKPRICEs Where mp.NUMBER = s.NUMBER _
      Order By dc.date Descending _
      Select mp.PRICE).FirstOrDefault),0)

空合并运算符将强制memberprice的空值为零。

DefaultifEmpty扩展方法是否执行您要查找的操作?

斯图亚特

这就是我让它在我的机器上工作的方法。我为它在C里道歉,我用VB已经太久了。

注意“select”语句中“new”运算符的用法,以及在firstOrDefault()之后使用空合并运算符。

public class Class1
{

    DataClasses1DataContext dc = new DataClasses1DataContext();

    public decimal MemberPrice(int stockID)
    {

        var q = from s in dc.Stocks
                where s.StockID == stockID
                select new
                {
                    StockID = s.StockID,
                    memberPrice = (from mp in dc.StockPrices
                                   where mp.StockID == s.StockID
                                   select mp.Price).FirstOrDefault() ?? 0
                };

        return q.FirstOrDefault().memberPrice;
    }
}