C# LINQ查询引发空引用异常

我正在使用LINQ查询填充数据网格。作为新增强的一部分，我必须通过在WHERE子句中包含更多的条件来更改现有的LINQ查询。 看了这么多的帖子，觉得条款条件很简单。 之前的查询返回简单对象类型（匿名），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，并在下一部分中尝试堆栈where子句。 但有些人认为这是行不通的，也没有取得任何结果。它正在网格上引发NULL引用异常。DataBind正在引发异常（NULL引用异常） 在这里发布我的代码

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

使用db作为新的ProjectDataContext
将订单行标注为IEnumerable（订单行的）
将自定义医嘱行作为对象
尝试
如果VATSearch=1，则
'仅显示增值税订单
'orderlines=（从db.orderlines中的O开始，其中O.order.order\u date>MinTime和O.order.order\u date1和（O.ol\u vat\u free为零或O.ol\u vat\u free=0）按O.order.order\u date订购_
'选择OrderLineID=O.ol\U id，Ref=O.order.order\U Ref，Email=CStr（IIf（O.order.User什么都不是，O.order.order\u billing\u Email，O.order.User.User\u Email）），Code=O.StockItem.productsbycolor.product.product\u Code&O.StockItem.productsbycolor.color.color\u Code，Size=O.Size.Size\u Code，Qty=O.ol\u Qty，Price=O.ol\u product\u Price，LineTotal=O.ol\u lineprice，Delivery=（O.order.order\u Delivery\u total/O.orderlines.Count），NOVAT=O.order.order\u免增值税_
'            )
orderLines=（从db.orderLines中的o开始，其中o.order.order\u date>MinTime和o.order.order\u date1和（o.ol\u vat\u free为零或o.ol\u vat\u free=0）按o.order.order\u date订购_
选择o）
ElseIf VATSearch=2，则
'仅显示非增值税订单
'orderlines=（从db.orderlines中的O开始，其中O.order.order\u date>MinTime和O.order.order\u date1和（非O.ol\u vat\u free不算什么）以及O.ol\u vat\u free=1按O.order.order\u date订购_
'选择OrderLineID=O.ol\U id，Ref=O.order.order\U Ref，Email=CStr（IIf（O.order.User什么都不是，O.order.order\u billing\u Email，O.order.User.User\u Email）），Code=O.StockItem.productsbycolor.product.product\u Code&O.StockItem.productsbycolor.color.color\u Code，Size=O.Size.Size\u Code，Qty=O.ol\u Qty，Price=O.ol\u product\u Price，LineTotal=O.ol\u lineprice，Delivery=（O.order.order\u Delivery\u total/O.orderlines.Count），NOVAT=O.order.order\u免增值税_
'                )
orderLines=（从db.orderLines中的O开始，其中O.order.order\u date>MinTime和O.order.order\u date1和（非O.ol\u vat\u free为零）以及O.ol\u vat\u free=1个订单，按O.order.order\u date_
选择O）
其他的
'显示增值税和非增值税订单
'orderlines=（从db.orderlines中的O开始，其中O.order.order\u date>MinTime和O.order.order\u date1 order By O.order\u date_
'选择OrderLineID=O.ol\U id，Ref=O.order.order\U Ref，Email=CStr（IIf（O.order.User什么都不是，O.order.order\u billing\u Email，O.order.User.User\u Email）），Code=O.StockItem.productsbycolor.product.product\u Code&O.StockItem.productsbycolor.color.color\u Code，Size=O.Size.Size\u Code，Qty=O.ol\u Qty，Price=O.ol\u product\u Price，LineTotal=O.ol\u lineprice，Delivery=（O.order.order\u Delivery\u total/O.orderlines.Count），NOVAT=O.order.order\u免增值税_
'            )
orderLines=（从db.orderLines中的o开始，其中o.order.order\u date>MinTime和o.order.order\u date1 order By o.order.order\u date_
选择o）
如果结束
customOrderLines=（从订单行中的o开始）
选择orderLineID=o.ol\u id，ref=o.order.order\u ref，email=CStr（IIf（o.order.User什么都不是，o.order.order\u billing\u email，o.order.User.User\u email）），
代码=o.StockItem.productsbycolor.product.product\u代码&o.StockItem.productsbycolor.color.color\u代码，
尺寸=o.size.size\U代码，数量=o.ol\U数量，价格=o.ol\U产品价格，lineTotal=o.ol\U lineprice，
交货=（o.order.order\u delivery\u total/o.order.orderlines.Count），NOVAT=o.order.order\u vat\u free，orderPaymentType=o.order.order\u google\u order\u number）
results.DataSource=customOrderLines
results.DataBind（）
results.Visible=True
btnExportButton.Visible=True
特例
结束尝试
终端使用

选择后使用new after select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})

---

我希望这会有所帮助。

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

将作为空集合返回。由于

LINQ

包含许多过滤器，我建议删除

Dim customOrderLines As Object

，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，而且如果

LINQ

没有返回任何内容，它也会清除您的

datagrid

但主要问题仍然是，您的

LINQ

正在返回一个空集合。我主要建议返回，并确保

LINQ

实际上是使用LINQpad或其他类似的实用程序返回您所期望的

---

您可以获取LinqPad。

我收到错误，因为我使用IIF进行空检查

有时它会返回nul

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++;
}

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