How do I handle a NULL XmlElement?

28 Dec 2006 4:11 AM

Paulers

Hello all,

I am trying to parse an XML document and populate objects to store in
an ArrayList but I am having issues when an element is NULL. I cant
seem to figure out how to handle it. I keep getting a null pointer
exception if a 'name' element is missing from the xml. Here is my code.
Any help is greatly appreciated.

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Load
        Dim menuFile As String = Application.StartupPath & "\" &
"vars.xml"
        Dim doc As XmlDocument = New XmlDocument()

        doc.Load(menuFile)

        Dim myVars As XmlNodeList =
doc.DocumentElement.SelectNodes("/Root/Row")

        Console.WriteLine(myVars.Count)
        Dim element As XmlElement

        For Each element In myVars
            Dim type = element.ChildNodes.Item(0).InnerText
            Dim variable = element.ChildNodes.Item(1).InnerText
            Dim startByte = element.ChildNodes.Item(2).InnerText
            Dim endByte = element.ChildNodes.Item(3).InnerText
            Dim totalBytes = element.ChildNodes.Item(4).InnerText
            Dim name = element.ChildNodes.Item(5).InnerText
        Next
    End Sub

28 Dec 2006 2:58 PM

_AnonCoward

Show quote Hide quote

"Paulers" <SuperG***@gmail.com> wrote in message
news:1167279074.753962.209980@79g2000cws.googlegroups.com...
: Hello all,
:
: I am trying to parse an XML document and populate objects to store
: in an ArrayList but I am having issues when an element is NULL. I
: cant seem to figure out how to handle it. I keep getting a null
: pointer exception if a 'name' element is missing from the xml. Here
: is my code.
: Any help is greatly appreciated.
:
:
:    Private Sub Form1_Load(ByVal sender As Object, ByVal e As
: System.EventArgs) Handles Me.Load
:        Dim menuFile As String = Application.StartupPath & "\" &
: "vars.xml"
:        Dim doc As XmlDocument = New XmlDocument()
:
:        doc.Load(menuFile)
:
:        Dim myVars As XmlNodeList =
: doc.DocumentElement.SelectNodes("/Root/Row")
:
:        Console.WriteLine(myVars.Count)
:        Dim element As XmlElement
:
:        For Each element In myVars
:            Dim type = element.ChildNodes.Item(0).InnerText
:            Dim variable = element.ChildNodes.Item(1).InnerText
:            Dim startByte = element.ChildNodes.Item(2).InnerText
:            Dim endByte = element.ChildNodes.Item(3).InnerText
:            Dim totalBytes = element.ChildNodes.Item(4).InnerText
:            Dim name = element.ChildNodes.Item(5).InnerText
:        Next
:    End Sub

What's happening is your source xml doesn't have a consistent
structure. As a result, you code bombs when it attempts to read the
sixth child node of a given "Row" element. Here's one recommendataion
for dealing with this:

=====================================================
[...]

For Each element In myVars
Dim type As String = GetValue(element, 0)
Dim variable As String = GetValue(element, 1)
Dim startByte As String = GetValue(element, 2)
Dim endByte As String = GetValue(element, 3)
Dim totalBytes As String = GetValue(element, 4)
Dim name As String = GetValue(element, 5)
Next

[...]

Private Function GetValue(ByVal element As XmlElement, _
                          ByVal ndx As Integer) As String
If ndx >= element.childNodes.count Then
    Return ""
Else
    Return element.ChildNodes.item(ndx).InnerText
End If
End Function
=====================================================

By the way, I encourage you to use Option Strict unless you absolutely
can't and always declare your variable types.

Ralf

--
--
----------------------------------------------------------
*             ^~^                   ^~^                  *
*          _ {~ ~}                 {~ ~} _               *
*         /_``>*<                   >*<''_\              *
*        (\--_)++)                 (++(_--/)             *
----------------------------------------------------------
There are no advanced students in Aikido - there are only
competent beginners. There are no advanced techniques -
only the correct application of basic principles.

28 Dec 2006 3:50 PM

Paulers

Ralf,

Thank you very much for your assistance. I appreciate the feedback and
will start using the strict option as you suggested. Also thank you for
taking the time to write that function, it helped me to better
understand how to handle null elements.

_AnonCoward wrote:
Show quoteHide quote

> "Paulers" <SuperG***@gmail.com> wrote in message
> news:1167279074.753962.209980@79g2000cws.googlegroups.com...
> : Hello all,
> :
> : I am trying to parse an XML document and populate objects to store
> : in an ArrayList but I am having issues when an element is NULL. I
> : cant seem to figure out how to handle it. I keep getting a null
> : pointer exception if a 'name' element is missing from the xml. Here
> : is my code.
> : Any help is greatly appreciated.
> :
> :
> :    Private Sub Form1_Load(ByVal sender As Object, ByVal e As
> : System.EventArgs) Handles Me.Load
> :        Dim menuFile As String = Application.StartupPath & "\" &
> : "vars.xml"
> :        Dim doc As XmlDocument = New XmlDocument()
> :
> :        doc.Load(menuFile)
> :
> :        Dim myVars As XmlNodeList =
> : doc.DocumentElement.SelectNodes("/Root/Row")
> :
> :        Console.WriteLine(myVars.Count)
> :        Dim element As XmlElement
> :
> :        For Each element In myVars
> :            Dim type = element.ChildNodes.Item(0).InnerText
> :            Dim variable = element.ChildNodes.Item(1).InnerText
> :            Dim startByte = element.ChildNodes.Item(2).InnerText
> :            Dim endByte = element.ChildNodes.Item(3).InnerText
> :            Dim totalBytes = element.ChildNodes.Item(4).InnerText
> :            Dim name = element.ChildNodes.Item(5).InnerText
> :        Next
> :    End Sub
>
>
> What's happening is your source xml doesn't have a consistent
> structure. As a result, you code bombs when it attempts to read the
> sixth child node of a given "Row" element. Here's one recommendataion
> for dealing with this:
>
> =====================================================
> [...]
>
> For Each element In myVars
>   Dim type As String = GetValue(element, 0)
>   Dim variable As String = GetValue(element, 1)
>   Dim startByte As String = GetValue(element, 2)
>   Dim endByte As String = GetValue(element, 3)
>   Dim totalBytes As String = GetValue(element, 4)
>   Dim name As String = GetValue(element, 5)
> Next
>
> [...]
>
> Private Function GetValue(ByVal element As XmlElement, _
>                           ByVal ndx As Integer) As String
>   If ndx >= element.childNodes.count Then
>     Return ""
>   Else
>     Return element.ChildNodes.item(ndx).InnerText
>   End If
> End Function
> =====================================================
>
> By the way, I encourage you to use Option Strict unless you absolutely
> can't and always declare your variable types.
>
> Ralf
> --
> --
> ----------------------------------------------------------
> *             ^~^                   ^~^                  *
> *          _ {~ ~}                 {~ ~} _               *
> *         /_``>*<                   >*<''_\              *
> *        (\--_)++)                 (++(_--/)             *
> ----------------------------------------------------------
> There are no advanced students in Aikido - there are only
> competent beginners. There are no advanced techniques -
> only the correct application of basic principles.