LINQ查询抛出Null参考异常

Question

我正在使用LINQ查询来填充数据网格。 作为新增强功能的一部分，我必须通过在WHERE子句中包含另一个条件来更改现有的LINQ查询。 看了这么多帖子，觉得堆叠WHERE子句条件很简单。 早先的查询返回简单的对象类型（匿名）并且工作正常。 现在，我将查询分为两部分。 在第一部分中，我尝试返回已知类型，并尝试在下一部分中堆叠where子句。 但是有些方法是行不通的，没有取得任何结果。 它在Grid上引发了NULL参考异常。DataBind正在引发异常（Null参考异常。）在此处发布我的代码。

Using db As New ProjectDataContext
        Dim orderLines As IEnumerable(Of orderline)
        Dim customOrderLines As Object
        Try
            If VATSearch = 1 Then
                ' Show only VAT orders
                'orderlines = (From O In db.orderlines Where O.order.order_date > MinTime And O.order.order_date < MaxTime And O.order.order_status_fk > 1 And (O.ol_vat_free Is Nothing OrElse O.ol_vat_free = 0) Order By O.order.order_date _
                '        Select OrderLineID = O.ol_id, Ref = O.order.order_ref, Email = CStr(IIf(O.order.User Is Nothing, O.order.order_billing_email, O.order.User.user_email)), Code = O.StockItem.productsbycolor.product.product_code & O.StockItem.productsbycolor.color.color_code, Size = O.size.size_code, Qty = O.ol_qty, Price = O.ol_product_price, LineTotal = O.ol_lineprice, Delivery = (O.order.order_delivery_total / O.order.orderlines.Count), NonVAT = O.order.order_vat_free _
                '            )
                orderLines = (From o In db.orderlines Where o.order.order_date > MinTime And o.order.order_date < MaxTime And o.order.order_status_fk > 1 And (o.ol_vat_free Is Nothing OrElse o.ol_vat_free = 0) Order By o.order.order_date _
                Select o)


            ElseIf VATSearch = 2 Then
                ' Show only non-VAT orders
                'orderlines = (From O In db.orderlines Where O.order.order_date > MinTime And O.order.order_date < MaxTime And O.order.order_status_fk > 1 And (Not O.ol_vat_free Is Nothing) AndAlso O.ol_vat_free = 1 Order By O.order.order_date _
                '    Select OrderLineID = O.ol_id, Ref = O.order.order_ref, Email = CStr(IIf(O.order.User Is Nothing, O.order.order_billing_email, O.order.User.user_email)), Code = O.StockItem.productsbycolor.product.product_code & O.StockItem.productsbycolor.color.color_code, Size = O.size.size_code, Qty = O.ol_qty, Price = O.ol_product_price, LineTotal = O.ol_lineprice, Delivery = (O.order.order_delivery_total / O.order.orderlines.Count), NonVAT = O.order.order_vat_free _
                '                )
                orderLines = (From O In db.orderlines Where O.order.order_date > MinTime And O.order.order_date < MaxTime And O.order.order_status_fk > 1 And (Not O.ol_vat_free Is Nothing) AndAlso O.ol_vat_free = 1 Order By O.order.order_date _
                        Select O)
            Else
                ' Show both VAT and non-VAT orders
                'orderlines = (From O In db.orderlines Where O.order.order_date > MinTime And O.order.order_date < MaxTime And O.order.order_status_fk > 1 Order By O.order.order_date _
                '   Select OrderLineID = O.ol_id, Ref = O.order.order_ref, Email = CStr(IIf(O.order.User Is Nothing, O.order.order_billing_email, O.order.User.user_email)), Code = O.StockItem.productsbycolor.product.product_code & O.StockItem.productsbycolor.color.color_code, Size = O.size.size_code, Qty = O.ol_qty, Price = O.ol_product_price, LineTotal = O.ol_lineprice, Delivery = (O.order.order_delivery_total / O.order.orderlines.Count), NonVAT = O.order.order_vat_free _
                '            )
                orderLines = (From o In db.orderlines Where o.order.order_date > MinTime And o.order.order_date < MaxTime And o.order.order_status_fk > 1 Order By o.order.order_date _
                        Select o)
            End If

            customOrderLines = (From o In orderLines
                        Select orderLineID = o.ol_id, ref = o.order.order_ref, email = CStr(IIf(o.order.User Is Nothing, o.order.order_billing_email, o.order.User.user_email)),
                        code = o.StockItem.productsbycolor.product.product_code & o.StockItem.productsbycolor.color.color_code,
                        size = o.size.size_code, qty = o.ol_qty, price = o.ol_product_price, lineTotal = o.ol_lineprice,
                        delivery = (o.order.order_delivery_total / o.order.orderlines.Count), nonVAT = o.order.order_vat_free, orderPaymentType = o.order.order_google_order_number)

            results.DataSource = customOrderLines
            results.DataBind()
            results.Visible = True
            btnExportButton.Visible = True

        Catch ex As Exception

        End Try
    End Using

Answer 1

选择后使用new创建匿名类型并将其括在大括号中。

customOrderLines = (From o In orderLines
                    Select new { orderLineID = o.ol_id, ref = o.order.order_ref, email = CStr(IIf(o.order.User Is Nothing, o.order.order_billing_email, o.order.User.user_email)),
                    code = o.StockItem.productsbycolor.product.product_code & o.StockItem.productsbycolor.color.color_code,
                    size = o.size.size_code, qty = o.ol_qty, price = o.ol_product_price, lineTotal = o.ol_lineprice,
                    delivery = (o.order.order_delivery_total / o.order.orderlines.Count), nonVAT = o.order.order_vat_free, orderPaymentType = o.order.order_google_order_number})

我希望这将有所帮助。

Answer 2

您收到的错误表明您尝试运行的LINQ没有返回任何值，因此customOrderLines作为空集合返回。 由于LINQ包含很多过滤器，因此我建议删除Dim customOrderLines As Object并改为执行以下操作：

Dim customOrderLines = (From o In orderLines 
  Select orderLineID = o.ol_id, ref = o.order.order_ref, 
  email = CStr(IIf(o.order.User Is Nothing, o.order.order_billing_email, o.order.User.user_email)), 
  code = o.StockItem.productsbycolor.product.product_code & o.StockItem.productsbycolor.color.color_code, 
  size = o.size.size_code, qty = o.ol_qty, price = o.ol_product_price, lineTotal = o.ol_lineprice, 
  delivery = (o.order.order_delivery_total / o.order.orderlines.Count), nonVAT = o.order.order_vat_free, 
  orderPaymentType = o.order.order_google_order_number)

if customOrderLines isnot nothing andalso customOrderLines.Any then
  results.DataSource = customOrderLines 
  results.DataBind() 
  results.Visible = True 
  btnExportButton.Visible = True
else
  results.DataSource = nothing
  results.DataBind()
  results.Visible = False
  btnExportButton.Visible = False
end if

这将确保您不会尝试绑定空集合，并且如果LINQ不返回任何内容，它将清除您的datagrid 。

但是主要问题仍然是您的LINQ正在沿行的某个地方返回一个空集合。 我主要建议返回并确保LINQ使用LINQpad或类似的其他实用工具真正恢复了您的期望。

您可以在此处获取LinqPad。

Answer 3

我收到错误消息是因为我使用IIF进行空检查。

有时它返回null，查询抛出NULL引用异常。

现在，我使用的是IF而不是IIF，它工作正常。

• 纳雷什

Answer 4

我在此页面上是因为我对此有一个NullReferenceException ：

var allHeights = MyCities.SelectMany(city => 
    city.Houses.SelectMany(h => 
        h.Storeys.Values.Select(s => new { City = city, House = h, Storey = s })))
   .GroupBy(g => g.Storey.Height)
   .OrderBy(heightGroup => heightGroup.Key)
   .ToList();
int i = 0;
foreach (var heightGroup in allHeights)
{
    if (i > 1)
    {
        var previousHeightGroup = allHeights[i-1].ToList();
        var previousPartIDs = previousHeightGroup.Select(g => g?.City?.Name).ToList();
        //.... <do stuff in case of i > 0>
    }
    // do other stuff
    i++;
}

“ previousHeightGroup.Select(g => g?.City?.Name).ToList() ”引发了NullReferenceException ，即使：

• 该数据结构的所有元素都不为空
• previousHeightGroup不为空
• “名称”是City对象上的字符串属性，并且是非空的
• Storeys对象上的Values集合是非空的
• 无论如何，在NullReferenceException ，应该已经通过ToList()调用对allHeights List进行了完全评估，并且解析了Values
• 显然，所有意外的空成员访问都受到Elvis运算符的保护，即使所访问的成员实际上不为空

而且，我重构了这样的代码：

.Select(g => {
   var group = g;
   var city = g?.City;
   var cityName = city?.Name;
   return cityName;
})

...即，我分解了Select中引用的数据结构，并在调试器中单步调试它，一切正常。 尽管如此， Select()调用仍然具有NullReferenceException ，据称在我自己的代码中。

解决此问题的唯一方法是稍微向下重构代码，关闭Visual Studio（2017）和整个计算机，重新启动所有内容，并且该错误不再重现。

这意味着，LINQ有时确实会到处乱扔。

这是.Net 4.6.1。