Problem using Lambda .Where()

Question

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I've got a weird problem with some code. If I use this code, it crashes on there custParts.Where(Function(cv), with the error:
Delegate to an instance method cannot have null 'this'

VB

Dim custParts As CustomerPartNumbers = New CustomerPartNumbers() ' Inherits ComponentModel.BindingList(Of CustomerPartNumber), which is just another data class
Dim queryString As String = "Wallpaper"
custParts.Load("DummyCustomer")
If Not custParts Is Nothing Then
  Dim qS As String = queryString
  Dim cp As CustomerPartNumber
  For Each cp In custParts.Where(Function(cv) cv.CustomerPartNumber = qS)
    Dim cpPN As String = cp.PartNumber
    If prodResults.Where(Function(z) z.PartNumber = cpPN).Count = 0 Then
      prodResults.Add(New Hathaway.Library.Part(cpPN))
    End If
  Next
End If

However, if I put the lambda into a variable, it seems to work without a problem.

VB

Dim custParts As CustomerPartNumbers = New CustomerPartNumbers() ' Inherits ComponentModel.BindingList(Of CustomerPartNumber), which is just another data class
Dim queryString As String = "Wallpaper"
custParts.Load("DummyCustomer")
If Not custParts Is Nothing Then
  Dim qS As String = queryString
  Dim xxx As Object = custParts.Where(Function(cv) cv.CustomerPartNumber = qS)
  Dim cp As CustomerPartNumber
  For Each cp In xxx
    Dim cpPN As String = cp.PartNumber
  Next
End If

Does anybody have any ideas why it's doing this? and why I can't just for Ror Each against my Lambda??

Thanks

Edit:
Bizarrely, this works:

VB

Dim zxcustParts As CustomerPartNumbers = New CustomerPartNumbers()
zxcustParts.Load("DummyCustomer")
Dim zxqueryString As String = "Wallpaper"
For Each zxcustPart In zxcustParts.Where(Function(zxcp) zxcp.CustomerPartNumber = zxqueryString)
  Dim cpPN As String = zxcustPart.PartNumber
Next

Edit 2:
Okay, I've actually figured out what this is... My code paste of the failing code actually missed the bit which was causing the error (though I'm sure I just C/P'd straight from the source). So I've updated that.

The problem is trying to do a second ".Where" inside a For Each of the first.

Posted 11-Jun-12 2:12am

MarqW

Updated 23-Jun-20 3:26am

v3

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2 solutions

Solution 2

The problem is that the item passed by the lambda expression is null... think about this

ITEM1
name:Guillermo
age:14

ITEM2
name:null
age:null

ITEM3
name:Jordan
age:18

now if I do my where:

var x = mylist.Where(w => w.name.Contains("Jor"))

the error will show, because the item name is null... I resolved it by using a test first as follow:

var x = mylist.Where(w => w.name != null && w.name.Contains("Jor"))

hope this helps!

Posted 23-Jun-20 3:28am

Guillermo Perez

Comments

Richard MacCutchan 23-Jun-20 10:41am

Already answered eight years ago.

Guillermo Perez 23-Jun-20 10:47am

yes, but the answer is wrong, the solution 1 says you can't use where inside a for loop, which is incorrect...

Richard MacCutchan 23-Jun-20 11:16am

There are plenty of current questions that are waiting for answers, rather than resurrecting dead ones.

Guillermo Perez 23-Jun-20 11:24am

dead ones? nobody replied this correctly (not even you!) and did happen to me yesterday... what's going on?

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

MarqW · Accepted Answer · 2012-06-11T02:51:00

Solution 1

See Edit 2 - You apparently can't call a .WHERE inside a "For Each In .Where" loop

Posted 11-Jun-12 2:51am

MarqW

Problem using Lambda .Where()

2 solutions

Solution 1

Solution 2

Add your solution here

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Problem using Lambda .Where()

2 solutions

Solution 1

Solution 2

Add your solution here

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